Comparison of the different diagnostic criteria in normal aging, mild cognitive impairment and dementia (according to DSM-IV and DSM-5).
\r\n\t1) Modeling in engineering - with more details on models and less (but some) on computer simulations;
\r\n\t2) Computer simulations in engineering - with more details on computer simulations and less on models;
\r\n\t3) Large scale simulations - parallel computing including vectorizations;
\r\n\t4) Engineering computational mathematics - with the explanation of basic or advanced numerical methods.
\r\n\tThe main aim of contributions is a clear description of studied models together with details of computational techniques.
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\r\n\tAuthors are advised to use either engineering of mathematical language in their formulations. High-quality pictures, graphs, and tables are expected as outputs of simulation algorithms.
Population aging is a global reality that is happening in a gradual and unavoidable manner, as a result of the low birth rate and mortality in the population, and at the same time, due to the increase in life expectancy. However, aging is not only a population phenomenon but also an individual reality [1], which involves a series of changes in people at biological, psychological and social levels. In the psychological field, changes in the domains related to personality, affectivity, emotions, emotional control, and interpersonal relationships have been reported [2].
Regarding cognitive functioning, the changes that occur during aging are of increasing interest for gerontology because of the implications they could have in case they finally appear in their most pathological form: dementia.
Historically, the research of cognitive functions has its epistemological origin in the studies carried out by the philosopher Galenus, who argued that in the ventricles of the brain, the consciousness of the human being was found as a set of different capacities: perception, intellect, and memory. From the philosophy of Rene Descartes (1596–1650) arises the neurophysiological theory, which defined the relationship between body and spirit and tried to find the explanation of mental function in the ventricles as the basis of psychic functions, later setting the pineal gland as related to mental disorders.
Later, Flourens (1794–1867) argued that all neural tissues are involved in the different cognitive functions. But it was until Gall’s studies (1758–1828) with his Frenology theory, that on one side, cognitive functions were associated with structures by examining the skull, and on the other side, the role of the cerebral cortex in relation to cognitive functions was presented. It was until the nineteenth century, with the establishment of the neuropsychology, when the correlation of anatomo-clinical structures with the alterations in cognitive functions was clearly set up [3, 4].
During the nineteenth century, the first stage of neuropsychology was established. Its study object is the relationship between the cerebral organization and the behavior in its broadest sense: actions, emotions, motivations, and social relations. The unit of analysis of neuropsychology is the individual, including his personal history, and his social and cultural environment. The founders of this approach are Luria, Vygotsky, and Leontiev, with the concern of locating psychological functions within circumscribed parts of the brain, defined higher mental human functions as complex reflex-like processes of social origin whose functioning is both conscious and voluntary and are possible due to their structure and functioning [5]. Later, in 1981, Luria proposed that cognitive functioning analysis should be done by looking for what is located outside the individual, the place where the origins of conscious activity are found. He also developed the idea that several macroanatomical areas and brain regions help each other to ensure control of the so-called human cognitive functions [6]. The cognitive psychology perspective studies the cognitive functioning as the way to know the world, through the construction of reality guided by experience. From there, the cognitive structure is formed and the concept of a cognitive scheme arises [7].
Piaget’s theory can be found under this perspective, where the study of structures is left aside to focus on the development of cognitive functioning and its schemes, from a constructive approach of knowledge that at the same time disproves empiricists and innatists theories, based on a psychogenetic perspective [8].
Neuropsychology is a discipline with an integrative view, which today contributes decisively to our knowledge about how the brain and the alterations of its functioning work, focused on the cognitive development in relation to sociocultural factors.
The conceptualization of cognitive functioning functions had several meanings.
Cognitive functioning has been defined as an evolutionary process in which individuals are immersed, which begins in fertilization and ends in death. In this process, both the organism in general and the nervous system in particular experience a series of changes that, in interaction with the environment, enable the development and maturation of both the nervous system itself and the behavior [9].
A more integrative view of mind-brain relationships defines the cognitive functions as functional interactions within and among cortical networks, which in turn are distributed throughout the cerebral cortex as memory, attention, perception, language, and intelligence; all sharing the same structure [4].
From another perspective, cognitive functions come from the information processing activity in neural networks distributed along the cortex and represent past and future schemes of action. This perspective suggests that temporal organization affects perceptual processes, action, and cognition within a sequence designed to achieve a goal [10].
From a psychopedagogical framework, complex cognitive functions consist of the organizing and sequencing of plans, the ability to respond to various stimuli at the same time, cognitive flexibility, the ability to respond according to the context, resistance to distraction, and inhibition of inappropriate behaviors [11].
From Piaget’s theoretical position, cognitive functions are considered as the mechanisms of information processing, which main function is to transform the internal and external stimuli into inputs for development and, in addition, to provide the individual with tools to face the positive entropy, and also the trend to exhibit states of thermodynamic equilibrium [12].
From the point of view of the structural cognitive modifiability theory, the cognitive functions are classified as perceptual thinking (basic functions), strategic (executive functions), analogical (educational functions), and reflexive (meta-cognitive functions) according to the last generation of the constructivism paradigm [13].
Finally, from the neuropsychology perspective, the different components of cognitive functions are defined as the abilities developed by brain structures that allow them to work with the information that is acquired from the environment. These cognitive abilities are divided into two groups: those known as basic cognitive functions such as sensation, perception, memory, attention and concentration; and higher cognitive functions such as thought, language, and intelligence, which are considered complex systems and also group different functions [14].
During the last decades, several scientific efforts have been focused on the study of normal cognitive aging. This has resulted in agreements, as well as numerous discrepancies around the topic, mostly regarding the use of different research methodologies, as well as the little control of other variables that are considered to be closely related to cognitive functioning.
In addition, finding differences between normal cognitive aging and a cognitive impairment involving pathology is clinically difficult, since the limits of diagnosis are not precise. This task becomes even more complicated if these differences are also associated with other variables such as age, schooling, and other population differences [15].
The concept of cognitive functioning in normal aging has been defined as “the functioning of the cognitive system, either in adaptation or alteration, which can generate a regression or successful management of the functions of daily life in older adults” [16].
The study of the changes that occur in the cognitive domains has found a close relationship between the physiological and social aspects. On one hand, research focused on the study of the human brain through different techniques (brain mapping, electroencephalogram and cerebral magnetic resonance among others) has reported that the mechanism behind successful cognitive aging may be the preservation of the hippocampal function combined with a high responsiveness in the frontal area [17].
Likewise, studies developed with electroencephalogram and neuropsychological tests found a reduction in age-dependent cerebral electrical power in cortical areas such as the parietal, temporal, and occipital lobes, causing a decline in functions such as memory, attention, visuospatial skills, and processing speed, concluding that the physiological aging of the brain is characterized by a loss of synaptic contacts and neuronal apoptosis that causes a dependent decline in sensory aspects, processing, motor performance, and some cognitive functions.
On the other hand, Steffener et al. [18] conducted a study which reported that cognitive changes during normal aging are due to the slow decrease across different ages of cerebral blood flow and the gray matter volume, mainly in areas such as the prefrontal cortex and the temporal convolutions of the putamen and occipital regions. On the other hand, the social aspects that have been described in different longitudinal studies and were related to the changes of the cognitive functioning in older adults are the schooling, the good health, the social participation, the lifestyle, and the genetic factors [17, 18].
It should be pointed out that socioenvironmental variables can contribute to an individual’s cerebral aging and therefore modify his cognitive and behavioral profile. This causes that while some of these factors can affect negatively, precipitating cognitive deterioration in normal aging, others can soften or even slow their effects.
To recognize which cognitive functions normally decline in older adults and when they occur is a complicated task, however, research has agreed that the domains generally involved in it are attention, verbal memory, visuospatial and visuoconstructive skills, processing speed and some of the executive functions such as inhibition, working memory and mental flexibility, while functions such as semantic memory and language are preserved, and even the latter can improve over the years [19, 20, 21].
Attention is a complex, dynamic, multimodal, and hierarchical functional system that makes easier the processing of information, selecting the relevant stimuli to perform a certain sensory, cognitive, or motor activity [21]. According to data, cognitive changes are particularly difficult for older adults, mainly in activities that involve orienting them between several elements or constantly changing between different successive testing options, due to the decrease in selective visual attention, which in part is due to the degradation of sensory processing.
It is important to emphasize that attention control is related to other cognitive functions such as processing speed, which suggests that older adults are less involved in tasks of anticipatory attentional resources due to the slower reaction time during aging [22].
Changes associated with age have been studied from the different domains of cognitive functioning.
Memory is a neurocognitive function that allows us to record, encode, consolidate, retain, store, retrieve, and evoke information [21]. This cognitive function has a sequence of three types of memory, from sensory to short-term (which is a transitory, fragile and sensitive storage to interfering agents) to long term memory (responsible for the more permanent storage of information and involves a process of consolidation); each have their own particular mode of operation but they all cooperate in the process of memorization and can be seen as three necessary steps in forming the lasting memory. There are also three main processes involved in the human memory: encoding, storage and recall (retrival) [23].
Memory is one of the most studied cognitive domains because it is a frequent complaint that older adults make during normal aging. Kral’s research since 1962 has led to evidence of the existence of a slowly progressing memory loss characterized by the inability to remember, sometimes relatively unimportant parts of the experiences of the past. The affectation of this domain in its processes of acquisition, consolidation, and spontaneous evocation is related to the cerebral biological functioning that will depend on variables such as quality of life.
Regarding the different types of memory, aging has a significant effect, on one hand, on the decline of immediate and episodic memory rather than on semantics and, on the other hand, on evocation rather than consolidation. Aging also affects the codification of new information, especially when strategic processing is needed [24].
Perception is the mental capacity that allows us to integrate and recognize through our senses. It allows us to recognize those objects to which we pay attention and to create our own knowledge patterns. In that sense, there must be an encounter between the sensorial information and the memory files that leads to the perception or interpretation of reality.
It is often difficult to dissociate spatial skills from constructional ones, being the latter defined as the ability to integrate elements into an organized whole (examples of these skill are copying geometric figures and the construction with cubes), since it requires the handling of space. According to the Pan-American Health Organization, changes in these cognitive functions in aging are due to the decline of visual acuity and processing, which causes problems of sensitivity to illumination and vision difficulties in poorly lit places, problems to distinguish colors, to focus at different distances and deficits related to spatial perception in general.
The executive functions (or meta-cognitive processes) would be those processes involved in the planning and supervision of cognitive processing. The term “executive” encompasses a series of cognitive processes, including updating and tracking information and inhibiting responses [24].
This kind of functions could be understood as a set of high-level operations that sequence and control the basic operations and, at the same time, make decisions in the moments of choosing among alternatives. Because they are linked with other cognitive functions, it is difficult to evaluate them in a specific way. At the same time, it is more complicated to find tasks that refer only to the performance of each one of them.
Some of the tasks that have been considered as executive functions are the working memory, the majority of everyday cognitive tasks that require the establishment of goals, the implementation and follow-up of the operations to reach those goals, and both the checkup of each one of these operations and of the fulfillment of the final purpose; their relevance could be used as evidence of the importance of executive functions in the lives of people [25]. In normal aging, it has been found that changes in executive functions are mainly observed in: working memory, when keeping information available for a short period of time; in inhibition, because over the years, more problems to concentrate on relevant information are experienced and inhibit attention to irrelevant aspects, in addition inhibitory processes are less efficient to allow the initial entry of information into the operational memory and in mental flexibility [26].
Processing speed has been defined as the reaction time that produces a global effect on cognition [27]. It is one of the functions in which a decline has been found as part of normal aging, and it has even been associated with the cause of cognitive changes in other domains such as care and executive functioning.
Moreover, as a cognitive task becomes more complex, older adults may not have the necessary resources of mental operations to carry out the later phases of it because cognitive functioning is slower and sometimes does not allow them to complete some mental operations that are needed for a correct final task performance [28]. Other studies compared two groups, one of young adults and other of older adults, and applied neuropsychological tasks to measure executive functioning and found a lower performance in inhibitory control, abstraction, and working memory but not the rest of this kind of functions [29].
The subjective perception of adults about their cognitive functioning (also called meta-memory) is another factor that significantly influences the activities of daily living (ADL) during aging, a recent study showed that a third of the evaluated population reported memory problems, thinking skills, and their ability to reason, all of them associated with their overall health [30].
Finally, it is important to note that the cognitive changes that occur in normal aging are presented as a slight decline and do not interfere with the level of independence during aging; if these changes appear in the opposite way, it is possible to suspect deterioration or cognitive change related to a pathology.
In cognitive aging, there is a decline which is considered normal. Some cognitive functions remain stable while others decline as part of normal aging. These cognitive changes associated with age occur to people who do not have pathologies that affect memory or cognitive abilities, and these changes do not interfere with the ability to participate in everyday activities. However, cognitive changes in aging can have a wide range, from those that are normal to those that are pathological, and between these there may be a series of intermediate changes. This transition state is known as Mild Cognitive Impairment (MCI) [31].
The construct of Mild Cognitive Impairment (MCI) has been extensively used worldwide, both in clinical and in research settings, to define the gray area between intact cognitive functioning and clinical. The MCI intends to identify this intermediate stage of cognitive impairment that is often, but not always, a transitional phase from cognitive changes in normal aging to those typically found in dementia [32]; in this sense, MCI is considered a pre-demential syndrome [33].
In 2013, the American Psychiatric Association (APA) proposed new criteria for dementia in the fifth edition of the Diagnostic and Statistical Manual for Mental Disorders (DSM-5) and recognizes the predementia stage of cognitive impairment [34]. The condition, which has many of the features of MCI, is known as mild neurocognitive disorder (NCD). Mild NCD recognizes subtle features of cognitive impairment that are different from aging but do not represent dementia. Furthermore, mild NCD focuses on the initial phases of cognitive disorders and precedes major NCD that is analogous to the previous diagnosis of dementia.
There are several subtypes of MCI, which differ according to the type and number of impaired cognitive abilities, the most common is the amnesic which mainly involves memory problems, while in the nonamnesic, memory operation is not compromised. Likewise, when only one dimension of cognitive functioning is affected, it is called DCL of a domain or multidomain if more than one cognitive ability (e.g., memory, reasoning, executive functions, etc.) is affected [32]. These MCI subtypes are usually related to different pathological processes, for example, it has been found that people with amnestic DCL are more likely to progress to Alzheimer’s disease (AD) [35, 36], while people with nonamnestic MCI are more likely to develop Lewy Body Dementia [36].
According to this definition, MCI is operationalized based on clinical data of changes in cognitive abilities (see Table 1). The subjective cognitive complaint needs to be confirmed by objective cognitive measures, such as neuropsychological test batteries. Objective cognitive impairment is defined as a poor performance in one or more cognitive measures, which suggests deficits in one or more cognitive areas or domains. There is no gold standard to specify which neuropsychological test battery to use, but it is important that all the main cognitive areas are examined. Typically, executive functions, attention, language, memory, and visuospatial skills are taken into account. Functional abilities are investigated by means of a thorough interview with the person and with the next of kin and registered in terms of activities of daily living (ADL) and instrumental activities of daily living (IADL) scales [32].
Normal aging [31] | Mild cognitive impairment [32] | Dementia DSM-IV [41] | Major neurocognitive disorder DSM-5 [34] | |
---|---|---|---|---|
Memory | Absence or presence or memory complaints | Subjective cognitive complaint, raised by the patient or an informant, or observations made by the clinician | A1. Memory impairment | A. Evidence of significant cognitive decline from a previous level of performance in one or more cognitive domains:
|
Normal objective memory according to age. | ||||
Memory problems are gradual, do not worsen suddenly | ||||
Other cognitive functions | Normal cognitive functioning according to age | Objective cognitive impairment in one or more cognitive domains preferably relative to appropriate normative data for that individual | The course of deterioration is characterized by a gradual onset and a continuous cognitive impairment. | Evidence of decline is based on: Concern of the individual, a knowledgeable informant, or the clinician that there has been a significant decline in cognitive function; and a substantial impairment in cognitive performance, preferably documented by standardized neuropsychological testing or, in its absence, another quantified clinical assessment. |
A2. At least one of the following:
| ||||
Activities of daily living (ADL) | Preservation of functional independence | Preservation of functional independence | B. The cognitive deficits in A1 and A2 each cause significant impairment in social or occupational functioning and represent a significant decline from a previous level of functioning | B. The cognitive deficits interfere with independence in everyday activities. At a minimum, assistance should be required with complex instrumental activities of daily living |
Associated pathologies | No dementia | No dementia | C. The cognitive deficits do not occur exclusively during the course of delirium | C. The cognitive deficits do not occur exclusively in the context of a delirium |
D. The cognitive deficits are not better explained by another mental disorder |
Comparison of the different diagnostic criteria in normal aging, mild cognitive impairment and dementia (according to DSM-IV and DSM-5).
According to this definition, MCI is operationalized based on clinical data of changes in cognitive abilities (see Table 1). The subjective cognitive complaint needs to be confirmed by objective cognitive measures, such as neuropsychological test batteries. Objective cognitive impairment is defined as a poor performance in one or more cognitive measures, which suggests deficits in one or more cognitive areas or domains. There is no gold standard to specify which neuropsychological test battery to use, but it is important that all the main cognitive areas are examined. Typically, executive functions, attention, language, memory, and visuospatial skills are taken into account. Functional abilities are investigated by means of an in-depth interview with the person and with the person’s next of kin and registered in terms of both activities of daily living (ADL) and instrumental activities of daily living (IADL) scales [32].
It has been shown that a significant proportion of people with MCI progresses to dementia in periods of 1–2 years and approximately 50% progresses toward dementia over a 5-year period [37].
Dementia is a NCD that usually begins gradually and has a progressive course. It can be variable, and there is often a long period of time between the occurrence of the first signs of cognitive impairment and the moment they meet the criteria for the dementia diagnosis [38].
The American Psychiatric Association (APA) introduced in 2013 the term “Major neurocognitive disorder” replacing the term “dementia,” defined as a decline in mental ability severe enough to interfere with independence and daily life [34]. However, not all the care professionals and organizations are likely to use the new term. Currently, the Alzheimer’s Association, for example, uses the term dementia instead of neurocognitive disorder. See criteria in Table 1.
Globally, around 47 million people have dementia, with nearly 60% living in low- and middle-income countries, and there are 9.9 million new cases every year; Alzheimer’s disease is the most common cause of dementia and may contribute to 60–70% of cases. The estimated proportion of the general population aged 60 years and over with dementia at a given time is between 5 and 8 per 100 people. The total number of people with dementia is projected to near 75 million in 2030 and almost triple by 2050 to 132 million. Much of this increase is attributable to the rising numbers of people with dementia living in low- and middle-income countries [39]. The most common forms of dementia are Alzheimer’s disease and vascular dementia (VD) [40]. Table 1 shows a comparison of diagnostic criteria in normal aging, mild cognitive impairment, and dementia.
Science has gradually shown which risk factors (RF) for MCI and dementia can be currently considered. The knowledge of RF for these pathological processes plays an important role in its prevention. Ideally, prevention strategies should target people who are not even symptomatic [42]. Prevention of dementia is a public health priority [43].
In the health sciences field, a RF is the probability of suffering a certain disease, having a complication or dying [44]. In this paper, we will present some of the most recognized RF, classifying them according to their origin in social, biological, and psychological and by their nature in modifiable and nonmodifiable (see Table 2).
Risk factors | Modifiable | Nonmodifiables |
---|---|---|
(A) Biological | Vascular disorders Metabolic disorders: diabetes mellitus | Genetic Brain injuries |
(B) Psychological | Depression | — |
(C) Social | Education Intellectual commitment | Age Sex |
Risk factors for MCI and dementia.
Regarding blood pressure (BP), both high and low BP have been linked to cognitive impairment and dementia [45]. The role of cerebral blood vessels in the wide spectrum of pathologies underlying cognitive impairment highlights the importance of vascular structure and function in brain health [46]. The pathophysiology of the relationship between BP and cognition is unclear, but hypoperfusion and neurodegeneration have emerged as potential underlying mechanisms [45, 47]. Results from a longitudinal study as part of the Kungsholmen Project [48] showed that low diastolic pressure predicted the risk of dementia among very old people. In the study, blood pressure showed a substantial decrease for approximately 3 years before the dementia syndrome became clinically evident [45].
In contrast, cohort studies have found that elevated blood pressure levels in the middle age may increase the risk of dementia in advanced age. As a result, the exposure to four risk factors related to BP: smoking, hypertension, high cholesterol, and diabetes in the middle age increased the risk of dementia in old age compared to only having one of the risk factors [49]. This relationship between blood pressure and the risk of dementia may depend on the age of patients when blood pressure is measured, as well as the time interval between blood pressure and dementia assessments [50, 51].
Diabetes mellitus (DM) is associated with a dementia risk of 1.5–2.5 times higher among old adults in the community. DM is a significant risk factor not only for vascular dementia but also for Alzheimer’s Disease. The mechanisms that support this association are unclear but may be multifactorial in nature, such as cardiovascular risk factors, glucose toxicity, changes in insulin metabolism, and inflammation [52].
Both hyperglycemia and hyperinsulinemia, as part of the metabolic process leading to DM type 2 (DM2), are associated with cognitive dysfunction and dementia due to stroke. This is often accompanied by other mental function disorders, such as depression or anxiety [53]. An epidemiological study showed that the incidence rates of hospitalization for VD in adult aged 70 years and over were twice as high in patients with DM2 as in those who did not presented it [54].
Genetics clearly plays a role in AD, both in early and late onset. Early-onset AD or beginning before age 65 years can be caused by one of the more than 200 sequence variants in the genes of the beta amyloid precursor protein, presenilin 1 (PSEN1), or presenilin 2 (PSEN2) [55, 56]. Despite the consistent genetic basis for AD, significant variability in onset age has been observed, suggesting an important role of environmental factors or genetic modifiers in determining the onset age [56]. Late-onset AD is also heavily influenced by genetics, although the Mendelian pattern of inheritance is often unclear. There are several factors that could explain this, even if causal mutations exist [57]. Late-onset AD is complex, and apolipoprotein E is the only genetic risk factor unanimously accepted for its development. Several genes involved in AD have been identified using advanced genetic technologies; however, there are many additional genes that have not been identified [58].
Related to this, a long research that analyzed the Genealogical Index of Familiarity up to 14 generations showed that the pairs of people with family ties who died of AD were significantly related. The relative risk for AD death among the relatives of individuals who died of AD increased significantly for close and distant relatives [57].
A new area of interest involves understanding the effect that head trauma has on the behavior and cognitive abilities of brain aging. This issue becomes even more important as the geriatric population grows [59].
Traumatic brain injury (TBI) is an injury in which effects could be devastating often resulting in lifetime cognitive deficits [60]. More than 70% of people with TBI report memory deficits [61]. Contact sports are a source of recurrent TBI. Athletes whose last concussion was in early adulthood (more than 30 years before examination) were reported to have poorer episodic memory and poorer response inhibition, as well as significantly reduced movement speeds in neuropsychological tests, when compared with same-age athletes without a history of concussion [62]. Regarding the cognitive aging process, the evidence showed that cognition problems exhibited by young adults after severe TBI are similar to many cognitive weaknesses in attention deficit and poor working memory of an elderly population with no neurological history. There is evidence that TBI can result in decreased cognitive reserve that can accelerate the cognitive decline normal process, leading to premature aging, potentially increasing the risk of dementia [63].
Depression can affect cognitive functions and may emulate cognitive impairment. It can be considered comorbidity, a prodromal factor or a consequence of vascular cognitive impairment, more than a factor that specifically alters vascular physiology or neural health, leading to cognitive impairment [64]. Some studies have concluded that depressive symptoms are associated with cognitive impairment; however, the mechanisms underlying the association between these two common conditions need further exploration. It is unclear whether cognitive impairment over time can be explained by depression or it is just a sign of an incipient dementia [65].
Through studies results, the age and sex of the individuals have been considered as risk factors of mild cognitive impairment and dementia. Some studies have reported that the prevalence of dementia increases exponentially with age [66] and doubles every 5 years after the age of 65 years. Several studies showed an increasing prevalence among the older age groups [67, 68]. In higher income countries, prevalence is 5–10% among those over 65 years [68]. Regarding sex, there are results in which dementia is higher in women than in men [68, 69]. One possible explanation for this is that women live longer than men [68]. However, recently, another cohort study reported that both the prevalence and the incidence were higher in men [69, 70].
Dr. James A. Mortimer was one of the first to propose a relationship between years of formal education and risk of dementia. He suggested that education can be a protective factor against dementia, raising the level of “intellectual reserve.” Regarding this, a systematic review of the literature on the relationship between education and dementia in the last 25 years concluded that lower education was associated with an increased risk of dementia in many but not all studies.
Education associated with the risk of dementia showed different results according to the population, and the years of education did not uniformly reduce the risk of dementia. It seems that a more consistent relationship with dementia occurred when the years of education reflected cognitive ability, suggesting that the effect of education on the risk of dementia can be better assessed in the context of a life development model [71].
In addition to this, occupations performed during lifetime that did not require complex cognitive processes or stimulants seem to be associated with an increased risk of dementia. For example, when studying a group of nuns (average 54 years of age), a strong association was found between low educational and occupational levels with dementia. The risk of dementia increased in those participants with poor education, without professional training and who had never been in charge of a leadership position. These findings support the hypothesis of the benefits of having a cognitive reserve capacity against the consequences of brain diseases [72]. In this sense, it was reported that university preparation represented a lower risk of dementia among five categories, where illiterates showed the highest proportion of individuals with dementia, while the lowest proportion was found in university students [73].
Broadly speaking, research on cognitive aging shows a gradual decline scenario, which may or may not be normative, and is associated with age and previously identified risk factors. The progression from normality to pathology is a concern in the health sciences field due to the negative implications that mild cognitive impairment and dementia have on people’s lives.
This is why gerontology has focused on the study of nonpharmacological intervention techniques that promote the improvement or maintenance of cognitive functioning at a level that allows people to lead a functional and disability-free life associated with cognitive pathologies. The main conceptual basis for nonpharmacological intervention on cognitive functioning in aging focuses mainly on the concepts of brain plasticity, brain reserve, and cognitive reserve.
Under the concept of brain plasticity [74], in the last 25 years, evidence has been presented to support the idea that the brain is far more flexible in structure and function than it was previously believed. Brain plasticity refers to the extraordinary ability of the brain to modify its own structure and function following changes within the body or in the external environment. Although it is stronger during childhood, it remains the fundamental and significant lifelong property of the brain during aging. Brain plasticity is implicated in learning abilities and plays a fundamental role in degenerative brain disorders. Recent research suggests that the pathology of the Alzheimer’s disease, for example, is associated with the loss of plasticity.
The brain reserve is related to neurobiological aspects and it has a more passive approach, since it refers to the size and number of neurons that a person has after a brain injury.
Finally, the cognitive reserve has been defined as the adaptation of the brain to an injury situation using pre-existing cognitive processing resources or compensation resources through the activation of neural networks [75]. The cognitive reserve allows better tolerance of the effects of the disease associated with dementia, supporting a greater amount of neuropathologies before reaching the symptoms of the disease. The cognitive reserve influences the manifestation of the symptoms of cognitive impairment and, at least partially, in its development toward dementia [76]. People with MCI and low reserves show a steeper decline early in the process of deterioration, compared to the high level of reserve this marked deterioration would have at the end of the process, due to the protective role of this reserve [77].
The intervention for the optimization of cognitive functions is based on these concepts to implement nonpharmacological treatments, in order to overcome the challenges of cognitive changes associated with aging, prevent pathologies such as MCI and dementia, and, finally, if it is necessary, alleviate their effects.
According to the British Psychological Society [78], there are a variety of nonpharmacological treatments and interventions which can help people to maintain good mental health, especially after diagnosis of MCI or dementia. Psychosocial interventions can help the diagnosis of dementia, reducing stress and improving mood (such as anxiety or depression), improving and maintaining cognitive functioning, and promoting quality of life in general. Specifically, treatments for improving and maintaining cognitive functioning in aging are Cognitive Training, Cognitive Stimulation Therapy, and Cognitive Rehabilitation that have significant differences in terms of their purpose, target population, duration, and management.
The Cognitive Training, also called Brain Training, involves specific aspects of memory and other cognitive skills. Since it is not personally tailored, regular pastimes such as crosswords, Sudoku, games, or exercises on a computer would also count as cognitive training. Cognitive training is for anyone who wants to keep his brain active and enjoys brain training games and puzzles, including people living with dementia. Exercises are designed to train specific functions, such as memory of words, logic and reasoning, attention, problem solving, and mathematics. Training could be a regular activity done continuously and can be self-administered [78].
Cognitive Stimulation Therapy (CST) is a group therapy that is used to help strengthen personal communications skills, thinking, and memory. CST groups run for a limited number of sessions (usually 12–14, one or two per week). As a complement, the maintenance cognitive stimulation therapy (MCST) groups continue indefinitely and aim to maintain the benefits that CST groups provide. CST and MCST are suitable for people with diagnosis of mild cognitive impairment or dementia in mild-to-moderate stages. A typical CST session lasts for 1 hour and may involve games, singing, applying reminiscence therapies, sharing stories, discussing current events, practicing arts, and making crafts. CST has shown to be beneficial for cognition and quality of life, and it is also cost-effective. Additionally, if CST is followed by MCST, it offers a significant improvement in cognitive function providing long-term benefits [79].
On the other side, cognitive rehabilitation is an approach to manage the impact that dementia-related difficulties, such as problems with thinking and memory, can have on everyday life. It is recommended for people who have early-onset dementia. Cognitive rehabilitation is not about curing or reducing dementia-related difficulties with thinking and memory, instead it is about learning ways of compensating these difficulties or managing them better. Many cognitive rehabilitation programs could involve families and careers. Usually, it is implemented by gerontologists, occupational therapists, clinical psychologists or clinical neuropsychologists [78]. Cognitive rehabilitation mainly focuses on identifying and addressing individual needs and goals, which may require strategies for taking in new information or compensatory methods such as memory aids, and has provided preliminary indications of its potential benefits in improving activities of daily living in people with mild Alzheimer\'s disease [80].
Any kind of cognitive intervention should be based on a previous diagnosis, including two types of assessment. The first should be a screening (usually with the Mini-Mental State Examination), and the second is an in-depth evaluation (with standardized tests in the sociocultural context, according to age and schooling) of the performance of the individual in different cognitive tasks. From the diagnosis results if the person shows a “normal” or intact performance, meaning that he preserves his cognitive functions as expected to his age and schooling in their context; or it presents a significantly inferior performance that can be classified as slight cognitive impairment and in case of suspected dementia. This previous evaluation is needed to take the decision of whether an intervention is necessary and what kind is required, what aspects should be developed on and what capacities should be promoted [81].
The objectives of intervention programs based on training and/or cognitive stimulation are generally set out in terms of “improving, maintaining, strengthening, and restoring.” While in programs based on cognitive rehabilitation, the objectives are defined in terms of “compensate.”
Once the type and purpose of the treatment have been selected, during the planning of the cognitive intervention, basic methodological aspects must be considered, in order to systematize the steps involved in the process. These guidelines include [82]: (1) Systematic organization of the session and its activities, (2) progression, starting with easy and continue with difficult activities, (3) intensity, with a suitable and adapted rhythm, (4) logic and sense, with meaning and actual sequence, (5) the activities should be interesting, (6) motivation, curiosity, and desire to learn, (7) the activities should be gratifying, (8) personal and emotional involvement, the elements of the process should have a pleasant and emotional sense, (9) the elements of the process should promote the interpersonal relationships of people with their environment.
As a basic guideline during the intervention work, it is recommended to maintain a routine through a structured session, in this sense, as part of the training and/or cognitive stimulation a session scheme is proposed. It includes the following elements, not necessarily in this order:
Orientation to reality (personal, spatial and temporal) [81].
Attention/concentration technique.
Relaxation technique.
Psycho-educational technique, knowledge and theoretical information promote the improvement of the perception of memory.
Practical training in the use of mnemonic strategies adapted to the needs of the person (see Table 3).
Feedback and closure. Always ask: How does this help me in everyday life?
Strategies | Technique | Definition and examples |
---|---|---|
Internal | Organization/categorization | It consists of establishing categories of data or information grouping it based on their common characteristics. (e.g., grocery lists according to the type of food, color, location in the kitchen) |
Visualization | Based on the ability to recreate visual mental images. (Ex: visually imagine a photo or movie where all the elements that want to be remembered are found) | |
Mental associations | Relate items that want to be remembered (e.g., associate the name of a person with a physical characteristic) | |
Mental hooks | Associate elements linked to the imagination and location, which can mentally link data that can be easily located in the mind | |
Story technique | Organize a story with data from a list of items or events that want to be remembered (e.g., to create a story that includes the planned activities during the day) | |
Itinerary method | It is about making mental associations of an image in a specific place. To achieve this, a mental journey or an itinerary should be made, setting in certain places the elements to remember (e.g., in the different rooms of the house) | |
Mental maps | It involves creating a panoramic view of a situation in order to remember both general and specific data | |
External | Memory aids | These are aids located in the context or near the person’s environment. In this situation a person or object promotes the memory (e.g., change the ring from one finger to another, carrying a schedule, diary, calendar, etc. Ask a person to remind me of an activity) |
Mnemonic strategies and techniques that can be used as part of training and cognitive stimulation programs.
Mnemonic strategies are used for improving memory processes and with it ensuring that important information is available when needed in our daily lives. Memory strategies can be distinguished according to their origin, whether they are external or internal. The first involves using aids that are outside our body to help us remember things, while internal strategies are mental activities that engage the person in remembering information [31] (Table 3). Both types of strategies are effective ways of learning and retaining information and are widely used as part of training and cognitive stimulation programs in the aging process.
On the other hand, interventions based on cognitive rehabilitation, designed for people with mild to severe dementia, should be highly personalized to fulfill the requirements regarding both to the potential and deterioration of the person, so it is difficult to design sessions with rigid schemes. However, this does not imply that the work should not be systematized. In a review of interventions targeting people with Alzheimer\'s disease or related dementia, a diversity in the types of interventions was found which consisted mainly of memory training, reminiscence therapy, validation therapy, and life review techniques [83].
Cognitive changes associated with aging can range from subtle to severe, those related to normal aging are generally mild and do not interfere with the ability to participate in normal daily activities. On the other hand, cognitive pathologies, such as dementia, affect a person’s ability to live independently and are overwhelming for the families of affected people. Physical, emotional, and economic pressures can cause great stress to families, and support is required from the health, social, financial, and legal systems [39]. Mild cognitive impairment falls between these extremes. In MCI, cognitive changes are more substantial than those seen in normal aging but not severe enough to cause disability. Both MCI and dementia are pathological conditions, caused by underlying brain disorders or conditions that are not part of the normal aging process [31].
In the study of the age-associated changes, declines in memory, attention, perception, speed processing, and some executive functions have been reported; however, there is considerable inconsistency in the results. Limitations of the studies should be analyzed in order to identify bias associated with methodology, differences in the assessment tools, and diagnostic and performance criteria. The optimal approach to study the age-related cognitive decline involves the longitudinal examination of population-based aging cohorts [84]. Despite this, researching on cognitive decline in normal aging is very relevant in the gerontology field, due to the possibility that it may represent a less severe but similar process to that in dementia [85]. Moreover, as decline in cognitive functioning and the onset of dementia are associated with older age, the study of social, environmental, and individual risk factors is also needed.
Estimating the burden of the disease and its proportion due to the major risk factors of mild cognitive impairment and dementia allows effective preventive measures to be taken, especially against those risk factors that are modifiable and highly dependent on lifestyles. The cardiovascular and DM2 risk decrease with healthy eating, physical exercise, and therapeutic control. On the other hand, continuous learning that stimulates lifelong cognitive training and leisure activities that represent intellectual challenges can also reduce the risk of cognitive impairment; also, depression symptoms could be successfully treated.
Besides the study of cognitive change in aging, the progress toward the pathologies and risk factors, the field of study of the gerontology involves the challenge to develop effective intervention programs for promoting cognitive health in aging and old age. In this sense, it has largely shown that loss of function in cognitive domains is partly preventable and controllable, since it is susceptible to training through strategies of cognitive stimulation and rehabilitation. Despite the heterogeneity and variety in interventions and outcomes, that limit generalizability, the role of nonpharmacological interventions targeting MCI is promising, and must studies found a benefit with the intervention [86].
Finally, population aging coincides with other converging and interdependent global trends that are shaping our collective future, regarding the epidemiological transitions the past decades have witnessed a major transformation in the profile of diseases that are the principal causes of disability and mortality. Today, chronic, noncommunicable diseases are the major cause of death and disability, and the rates are rising. The vast majority of older people have chronic conditions, and many have multiple conditions [87]. Mental diseases related to cognitive functioning are in the spotlight, specifically the dementia, considered as a public health priority [39]. The full impact of the pathologies in cognitive aging, mean mild cognitive impairment, or dementia is resonating throughout society. The economic costs of these pathologies impact families, health-care systems, businesses, and social structures. The emotional, psychological, and physical burdens of cognitive pathologies in aging impact individuals, his/her family, as well the formal support networks that provide assistance [83].
From Gerontology, the challenge entails rethinking the life course, to make aging a positive and disability-free individual experience. In this sense, the World Health Organization has proposed as a key element the active aging [88], also called “successful” [89] or “healthy” [90]. In any case, this type of ideal aging requires that the person can maintain an autonomous cognitive ability, which allows the functionality and control of his own life, for which it is necessary to preserve healthy cognitive functions.
Funded by the National Council of Science and Technology (CONACYT), México. Project: 256589.
Petrochemical-based plastics are being replaced by biobased materials because of being widely eco-friendly. In the last decades, the biobased films have been investigated due to their biodegradability and for being suitable, generally obtainable, and less expensive materials in the industry. The plastics produced from petrochemical sources (e.g., polyesters and polyolefins) have been commonly used in the packaging industry due to their potential features. They are obtainable in large quantities and at low cost, displaying advantageous properties (i.e., good tensile strength, enriched barrier properties, and heat sealing) and applicability in the industry [1, 2]. However, these plastics are totally nonbiodegradable and expose a serious ecological problem due to hydrophobic properties and very low water vapor transmission rate [2, 3]. The growing public interest on the environment is induced by a growing research on biohybrid films (i.e., biobased films) as alternatives to traditional nondegradable plastics due to the harmful effect of petroleum-based plastic packaging [4]. The eco-friendly polymeric resources can be categorized into three main groups depending on the raw material used: renewable natural, biodegradable synthetic polymers, and microbially produced biopolymers. The renewable natural polymers can be obtained from several sources such as starch, cellulose, chitosan, etc. [5], while biodegradable synthetic polymers such as polyvinyl alcohol, polycaprolactone, polylactic acid, polybutylene succinate, and copolymers are produced by using natural or petroleum-based monomers. On the other hand, microbially produced biobased polymers (e.g., polyhydroxybutyrate and valerate copolymer) are manufactured via various microorganisms.
\nStarch among all natural biopolymers has been believed to be one of the most suitable biopolymer resources due to its biodegradable, regularly available, and inexpensive features. There are two major polymers of starch: amylose and amylopectin. Amylose is a linear molecule with a spiral structure unlike a branched structure of amylopectin. Moreover, the molecular weight of amylose is commonly a smaller molecule (1–1.5 million), while amylopectin is a large molecule (50–500 million) [6]. Various starches have been used in the biohybrid films because of changing amylose/amylopectin ratio. And these are classified by amylopectin content [7]. In most studies, biobased films have been manufactured from starch of corn, wheat, rice, potato, tapioca, and cassava [8]. Various starch types have been also used to the biobased films because of changing amylose and amylopectin ratio [7, 8, 9, 10].
\nHowever, the biobased films obtained from starch have demonstrated some disadvantages such as brittleness, low processability, high water sensitivity (i.e., low moisture resistance and hydrophilic character), and poor mechanical properties compared to petroleum-based conventional polymers [37]. Moreover, the starches are not thermoplastic biopolymers due to the intra- and intermolecular hydrogen bonds, because the degradation temperature of starch exceeds the melting temperature [11]. These make them inadequate for some packaging purposes limiting their widespread industrial applications [12, 13, 14]. Therefore, physicochemical and biological properties of the starch should be improved. Several modification techniques are needed to improve the mechanical and physical properties to overcome the inadequate features of the starch-based biobased films [15]. Thus, several efforts have been made to improve thermal properties of starch by blending with a plasticizer for its stability, elasticity, and edibility. Despite the above-mentioned modifications and applications, the biodegradable films produced from starch are still limited due to poor mechanical and hydrophilic properties along with susceptibility to the biological attacks. Accordingly, polyvinyl alcohol (PVA) known as synthetic biodegradable polymer and thermoplastic starch was utilized together to obtain excellent compatibility [16]. In several studies, starch was used with polyvinyl alcohol (PVA). As an example, various starches such as corn, potato, rice, and tapioca have been studied in combination with PVA polymers [9, 10].
\nPVA is an important polymer having superior gas barrier properties along with higher strength, tear, and flexibility than those of natural biobased polymers. Nevertheless, it has weak dimensional stability owing to high water uptake. Furthermore, PVA has relatively high manufacturing cost in comparison with the other commercial polymers in the market. Thus, if PVA is blended with renewable and abundant natural sources like starch, the manufacturing costs can be reduced. This method also resulted in improved moisture resistance and rapid biodegradation [17, 18].
\nOver the last 10 years, hundreds of studies about PVA/starch biobased films have been carried out on the topic using various production techniques. The researchers used the PVA and starch together for the purpose of exposing their superior properties and eliminating the poor properties. The main aim of this chapter was to investigate the effects of additives and modifications on the several properties of various novel additives on the PVA/starch-based biodegradable hybrid films. Moreover, the review of the studies was explained in a molecular chemistry point of view in the specific subheadings. So, the results obtained from the literature have been evaluated based on the effect of different novel additives on the several properties of PVA/starch-based biodegradable films.
\nPVA/starch-based biodegradable formulations are produced from polyvinyl alcohol (PVA) and starch known as main compositions with different additives such as plasticizer, cross-linkers, and filing materials. Until now, PVA/starch blend biobased films have been prepared using casting (sol-gel or mixing) and thermal (extruder or extrusion) methods by many researchers in the literature.
\nIn casting method, PVA is dissolved in hot water with the gelatinized starch in order to form intermolecular interaction. The obtained mixture is then stirred in a mechanical high-speed mixer for homogenization [19] over 1000 rpm/min and at 85–95°C temperature, and then some of the additives are added into the mixture under continuous stirring. After removing the bubbles formed during the preparation of biobased films by an aspirator, it is dried at room temperature [20]. The hybrid films are generally heated in an oven at 80–95°C for 1 h to induce the cross-linking reactions [21]. Another approach for the fabrication of PVA/starch blend biobased films is using a single or twin screw extruders. Primarily, the plasticizing starch is mechanically mixed with PVA, and then, PVA/starch blend granules are obtained following the extrusion process under the optimum conditions at various temperature and screw speeds. After that, the biobased films are obtained from the prepared granules by using the blown film extrusion or hot-press molding [4, 22].
\nIn casting method, considerable amounts of water are evaporated from aqueous solutions or suspensions with a high energy-consuming process to obtain PVA/starch hybrid films [23]. The PVA/starch blends are generally produced via solution casting method. Nevertheless, this method has several deficiencies such as low solution density, low manufacturing yield, high energy consumption, etc., which limit its industrial practices [19]. On the other hand, the extrusion films have a great importance due to energy-efficiency process, high productivity, and continuous industrial production possibilities. However, the solution casting method has gained much more attention compared to extrusion process for the production of PVA/starch blend films in scientific publications due to its easy applicability in laboratories [4, 23, 24].
\nVarious additives such as plasticizers (e.g., glycerol and sorbitol), cross-linkers (e.g., glutaraldehyde and epichlorohydrin), fillers (e.g., silicium dioxide and calcium carbonate), and natural raw materials (e.g., cellulose and chitosan) as well as thermoplastic starch have been used to improve the mechanical, thermal, and morphological properties of PVA/starch-based biodegradable hybrid films. Furthermore, a variety of methods including esterification, oxidation, and etherification were applied in order to modify the starch [25]. These novel additives and the modifications have been discussed under the following subheadings as plasticizers, cross-linkers, fillers, and chemical and physical modifications.
\nPlasticizers are additives that increase the elasticity. These are the ingredients for nonthermoplastic starch, which are added in order to alter their physical properties. Plasticization takes place in the amorphous zone, which has a higher molecular dynamism. The type and the amount of plasticizer have an important influence on the ability to hinder hydrogen bonding along the polymer chains. The major gain obtained from utilization of plasticizers is that the tensile strength (TS) is decreased, while the elongation at break (E%) increases as well as they become more flexible [26]. One of the most important properties of an efficient plasticizer is to be compatible with the polymer matrix. The plasticizers such as glycerol, polyethylene glycol, urea, ascorbic acid, sorbitol, citric acid, and tartaric acid are usually used [19]. However, the plasticizers in the biobased films cause an increase in permeability to moisture, oxygen, and aromatic compounds [27]. Table 1 presents the various plasticizers and used methods along with the effects of plasticization on some properties of PVA/starch films.
\nPlasticizers added | \nCharacteristics of PVA/starch film and obtained improvement | \nProcessing method | \nReference | \n
---|---|---|---|
Water | \nA large decrease in tensile strength in all the tested films was recorded when the storage relative humidity increased from 15% to 33%. | \nCasting | \n[8] | \n
Glycerol | \nE% increased while TS decreased. TS, E%, swelling behavior, and degree of the compatibility with PVA and starch were lower compared to sorbitol and citric acid. However, solubility was higher. | \nCasting | \n[8] | \n
Urea | \nIt had a good interaction, homogeneity, and sensitivity to the water with PVA and starch in comparison with glycerol and sorbitol. | \nCasting | \n[28] | \n
Formamide | \nIt was not a good plasticizer and could not improve the compatibility and flexibility of the blend. It exhibited synergistic effects and the compatibility, especially E%, with simultaneously added urea in the blend. | \nCasting | \n[19] | \n
Effect of plasticization on some characteristics of PVA/starch films along with obtained improvements.
Water is accepted as a basic plasticizer for PVA/starch biofilms. Physicochemical properties of films could differ based on the changing water content. At the same time, the water is also compatible with other plasticizers. When the plasticizers are added into biofilm formulations, the physical properties are affected due to the increasing relative moisture because of compatibility with water. Possibly, water is absorbed because of polarity compliance to the solubility of other plasticizers added as additives. The plasticizer effect of water could usually be effective when it is used also together or not with an above-mentioned plasticizer. Furthermore, glycerol, sorbitol, and citric acid are generally favored as an efficient plasticizer for PVA/starch films. According to previous studies, the E% increased while TS decreased because of increasing glycerol, sorbitol, and citric acid ratio from 10 to 50% in the biobased films. It was also reported that E% and TS of sorbitol or citric acid–added films were higher than those of the glycerol-added films. However, the water absorption property of the biobased film decreased with the increasing glycerol ratio due to its hydrophobicity [29]. Moreover, the swelling behavior of the film containing glycerol was the lowest compared to the sorbitol- and citric acid-added films due to weaker hydrogen bonding capabilities, unlike the solubility value due to weaker hydrogen bonding capable [8]. The homogeneity of PVA/starch biobased films could also be enhanced with the addition of urea, like glycerol. However, urea as a plasticizer showed stronger interactions with starch and PVA in biofilms than those of glycerol and sorbitol [11, 28]. Consequently, urea was considered a better plasticizer to improve the flexibility of PVA/starch films [19]. Furthermore, the crystallinity of biobased films was also decreased by the addition of urea and formamide. These agents could penetrate into the crystallization zone of PVA/starch biobased films during the process forming new hydrogen bonds with starch and PVA molecules, which damage the crystal region of PVA in the biobased films. However, formamide is not a good plasticizer and could not develop the compatibility and flexibility of the biobased films, while the compatibility was improved when it combined with urea. When the additives containing both urea and formamide are simultaneously used, their synergistic effects and the compatibility could occur in the blend. Besides, while TS and young modulus of biobased films were significantly decreased, the E% was substantially improved. With the increasing amount of urea in the biobased films, the sensitivity to water increased, while the melting point of blends decreased. It was likely due to the facilitation of molecular ability of both urea and formamide as a plasticizer [19].
\nPhysicochemical properties of blend films are substantially affected by the functional groups of plasticizers used in PVA/starch biobased films. The total number of both carboxyl and hydroxyl groups in plasticizers were given in Table 2 along with their behaviors in films. For instance, as regards the hydroxyl and carboxyl groups of glycerol (H.3, C.0) and succinic acid (H.0, C.2), the E% of the glycerol-added film has shown a high enhancement than that of the films containing succinic acid, contrarily to the TS behavior. However, when malic acid (H.1, C.2) in the same carboxyl number with succinic acid (H.0, C.2) was added to the film, the TS and E% were improved compared to glycerol (H.3, C.0) and sorbitol (H.6, C.0) because of the presence of two functional groups. Depending on the increasing functional groups of plasticizer, TS and E% of tartaric acid (H.2, C.2) added biobased films with two same functional groups were greater than those of malic acid, glycerol, and sorbitol [6, 30]. Furthermore, the biobased films containing citric acid (H.1, C.3) were stronger and more flexible than that of containing glycerol [7, 30] and xylitol (H.5, C.0) [31]. On the other hand, when the glycerol and xylitol added films were compared, it was found that xylitol-added biobased films had a higher strength and more elasticity than glycerol-added biobased films due to its 5 hydroxyl groups [31]. Even a few xylitol molecules can play an extra role in plasticizer than others [32]. Similarly, the comparison of glycerol- and sorbitol-added films showed that TS and E% of sorbitol-added film were greater than glycerol [7, 30]. Consequently, E% value increases while TS decreases with an increase in the total functional groups and the amount of these plasticizers in blend films.
\nEffect of functional group type and number on the plasticization in PVA/starch films.
The concept of plasticization could be understood with the analysis of different properties such as elongation at break (mentioned above) or glass transition point (Tg). For instance, Aydin et al. reported that the addition of plasticizers reduced the Tg point clearly and the change of plasticizing performances could be observed by increasing Tg point. Apart from the above-mentioned plasticizers, 1,4-Butanediol (H.2, C.0), 1,2,6-Hexanetriol (H.3, C.0), pentaerythritol (H.4, C.0), xylitol (H.5, C.0), and mannitol (H.6, C.0) from 2 to 6 hydroxyl groups have also been investigated based on the changes in Tg point. Among the investigated plasticizers, 1,4-butanediol demonstrated the maximum plasticizing effect for starch and PVA due to small molecular size and geometry [32]. Table 2 shows the effects of the various plasticizers with different functional groups and number on the properties of PVA/starch films.
\nThe different plasticizing effect of xylitol and mannitol was attributed to lower penetration capability. Due to larger molecular geometry and size of 1,2,6-Hexanetriol and pentaerythritol, further penetration into the chain fragment of starch and PVA was prevented. Moreover, the plasticizer efficiency of pentaerythritol was generally lower than that of 1,4-butanediol, 1,2,6-Hexanetriol, xylitol, and mannitol. Consequently, the increase in hydroxyl groups and molecular size of the plasticizers such as mannitol caused an improvement in the thermomechanical stability on the contrary of xylitol. For example, the maximum amount of mannitol (due to more hydroxyl number) in the films tends to interact more with the blend, on the contrary, with lower hydroxyl number plasticizers [32]. Based on the literature data obtained, it could be expressed that the molecular structure and geometry of plasticizers could inhibit or support their penetration into the molecular chain segments and reduce or increase inter- and intramolecular interactions, although the number of hydroxyl groups of plasticizers is hydrogen bonding quarters for starch and PVA.
\nThe presence of two type of functional groups could also significantly influence other properties of biobased films. For instance, the citric acid could improve the water stability and inhibit degradation of starch molecules [15]. Due to the very strong interaction of water with glycerol and sorbitol, the solubility values were higher than the plasticizer with carboxyl groups. While the solubility of tartaric and citric acids was easy in water, their solubility value was lower than that of glycerol and sorbitol [7]. Eventually, the degree of swelling and mechanical properties of biofilms could decrease or increase slightly with the increasing content of plasticizer depending on functional groups [30]. However, the degree of swelling of the films without plasticizer was higher than that of films containing additives, while the solubility of films without plasticizer was lower [7].
\nWhen the plasticizers having both hydroxyl and carboxyl groups were simultaneously added into the biobased films, their physicomechanical properties were better than those of plasticizers with only hydroxyl group-containing agents. Yoon et al. reported that as the additives containing both hydroxyl and carboxyl groups were simultaneously added into the formulation, the TS and %E were enhanced compared to the glycerol, having only hydroxyl groups. For instance, %E of the glycerol-succinic acid–added films increased, while only succinic acid–added film showed inadequate potential. The usage of the plasticizer composed of both hydroxyl and carboxyl groups could enhance the flexibility and strength. Moreover, the degree of swelling and solubility values of the comalic acid-/tartaric acid-added films were higher than those of coglycerol-/succinic acid-added films. [6]. This was because the hydroxyl (-OH) group number of malic acid-tartaric acid (7 hydroxyl number) was higher (i.e., having a more hydrophilic character) than glycerol-succinic acid mixture (5 hydroxyl number). %E of the glycerol or sorbitol-succinic acid–added films increased, while %E of succinic acid–added film decreased with increasing amount of additives. Nevertheless, TS of glycerol or sorbitol-succinic acid–added films decreased, while TS of succinic acid–added film diminished with increasing content of additive. The results of TS and %E mentioned above showed that using cohydroxyl/carboxyl group as a functional group increased physicochemical and mechanical properties of films [30]. Eventually, when the plasticizers having both hydroxyl and carboxyl groups were used simultaneously, TS and %E of the films were found to be better than the films containing plasticizer having only the hydroxyl group [7].
\nCross-linking modification method is an effective and frequently applied approach to enhance the physicochemical and mechanical properties of PVA and starch [33]. Cross-linking can be carried out via treatment of granular starch using functional or multifunctional materials, which generated stable ether (R-O-R) or ester (R-CO-OR) linkages with the hydroxyl groups (-OH) in starch [9, 34]. Some of these multifunctional compounds are monosodium phosphate, sodium trimetaphosphate, sodium tripolyphosphate, epichlorohydrin, phosphoryl chloride, a mixture of adipic and acetic anhydrides, and a mixture of succinic anhydride and vinyl acetate. Cross-linker starch showed better compatibility and interaction with PVA than those of unmodified starch; such as, water absorption and TS of starch cross-linked films with sodium trimetaphosphate were higher than those of uncross-linked starch films, unlike E%. Moreover, weight loss in the soil of uncross-linked starch films was higher than that of the cross-linked starch films. Since the weight loss of starch under the soil is related to the amount of moisture, the use of cross-linked starch improves the water resistance of the biobased films [9].
\nWhen epichlorohydrin was used for cross-linking, the TS and %E of starch/PVA blend films increased. Thermal degradation of biofilms has been diminished by the cross-linker epichlorohydrin [35]. If sodium carbonate and sodium hexametaphosphate as the other cross-linkers are used, the equilibrium moisture content of the biofilms is significantly reduced by lowering their hydrophilic characteristic. Furthermore, these modifications increase the TS and modulus of elasticity of biofilms, unlike elongation at break [33].
\nIn a study, the usage of sodium trimetaphosphate and sodium tripolyphosphate as the cross-linker enhanced the physicochemical properties such as swelling behavior compared to the uncross-linked starch [8]. Likewise, the swelling of the biofilms was intensely reduced after utilization of cross-linker epoxidized natural rubber owing to the interaction between the mixtures. Thus, the hydrophilicity of the blend film decreases due to the reduction of the number of free hydroxyl groups in PVA and starch molecule. And, %E of film improves with the addition of cross-linker in blend polymer [36]. Singha and Kapoor have reported that the TS of PVA/starch cross-linked with glutaraldehyde has shown improvement. Moreover, modification with glutaraldehyde also improved the thermal stability of films. Moreover, their antibacterial activities against Gram-positive bacteria compared to Gram-negative bacteria indicated good resistance [15]. Additionally, borax can also be used as a cross-linker for starch and PVA. The enthalpy and crystallinity slightly decreased with increasing concentration of borax due to increasing cross-linking. Also, it improved the TS and %E of biobased films compared to the biobased films without cross-linker. Citric acid as another cross-linker can also be preferred for biobased films. This cross-linker decreases the water absorption of biobased films. So, citric acid also acts not only as a plasticizer but also as a cross-linker [29].
\nFilled PVA/starch biobased films are the high-potential class of hybrid materials composed of filler incorporated into a biobased matrix [37]. With the aim to attain synergic effects, such a collaboration between environmental biopolymers and fillers is one of the most impressive ways to improve the features of this bioblends [38]. Because of the nature and the geometry of the filler, the properties of biobased films such as gas barrier, mechanical stiffness, transparency, and thermal stability have been enhanced [37, 39].
\nIn a study, the use of silica as a filler has increased the TS of biobased films [40]. With the increase of silica amounts in blend film, the water absorption and water vapor transmission of starch have been decreased. This was due to the complex structure designed by links between silica and hydroxyl groups of starch and PVA. This phenomenon prevented the water molecules from dissolving and developed the water resistance of the biofilm. Furthermore, silica has also improved the compatibility between PVA and starch and formed a rigid structure. Even, according to SEM results, the low amount of silica has provided excellent diffusion and interaction between starch and PVA. On the other hand, filler silica has shown less effect on the biodegradability of the films because of decreasing microorganism penetration rate [9].
\nNano-calcitine was preferred as a filler for PVA/starch film because of its positive effects on the physicochemical properties of blend films. As an example, the addition of nano-calcitine into blend film reduced the crystallinity, water solubility, biodegradability, and oxygen permeability. At the same time, it increased TS, limiting oxygen index, decomposition temperature, and water absorption [41]. Simultaneously, addition of nano-SiO2/TiO2/CaCO3 into PVA/starch blends increased the TS of biofilms enhancing the interfacial adhesion through inter- and intramolecular interactions. With nano-TiO2, an increase in clearness of biofilm was noticeably observed. However, water vapor permeability of biofilms containing nano-SiO2 was lower than that of biofilms containing nano-TiO2/CaCO3 [4, 42]. Therewithal, TS and Young’s modulus of biobased films were also increased with filler TiO2 unlike E% [43].
\nZirconium phosphate as another filling material had an attractive effect in the biobased films because of composing new hydrogen links. The addition of zirconium phosphate decreased the moisture uptake, while the degradation temperatures of biobased films increased [44]. The filler clay had an important effect on biobased films due to its hydrophilicity. The use of clay in biofilm increased TS and heat resistance, enhanced the barrier properties to water vapor, and lowered glass transition temperature [24].
\nIn PVA/starch biohybrid film, natural raw materials were also added as fillers such as cellulose nanofibers, chitosan, and feather keratin. In investigations, cellulose nanofibers blocked the recrystallization of starch by decreasing the mobility of polymer chains. Hence, the physicomechanical properties and crystal structure of blend film were significantly enhanced. In relation to this, storage conditions of biobased films improved. Cellulose nanofibers significantly enriched also the stiffness and strength of blend films by the storage conditions [34]. Similarly, the storage conditions of biobased films in natural weathering could be also enhanced by added graphene into PVA and starch [45]. Moreover, when the chitosan known as a natural filler was added to biofilms, their physico-chemical properties such as TS, E%, water vapor permeability, and oxygen transmission rate improved. Also, water vapor and oxygen permeability, water uptake, and hydrophobic character of the chitosan-added bioblend film were better than biofilm without chitosan due to its incorporation [46]. As different inorganic salts are used in the biobased films, their crystalline [47], thermal, water vapor barrier, and mechanical properties can be significantly affected via strong hydrogen bonds. For this reason, Jiang et al. have reported that LiCl, MgCl2,6H2O, CaCl2, and AlCl3,6H2O salts have provided a good compatibility with PVA and starch [48, 49]. Moreover, the ZnO added biobased films have shown good dispersion, homogeneity, mechanical properties, and water resistance [49]. Another filler salt, AlCl3.6H2O, can show compatibility with PVA and starch. Hence, these salts have presented great destroying effect on the crystalline and good mechanical properties [50].
\nThe chemical modifications applied to the biohybrid films produced from PVA and starch have improved their physical-biological-chemical properties because of the changing molecular structure of blend. For instance, the carboxyl group of PVA and starch has occurred in bioblend films after oxidation of starch with H2SO4 and KMnO4. After the increase of polar carboxyl groups by oxidation, the hydrogen bonds in blend molecules were stronger than those of nonmodified ones. At the same time, their TS and E% have improved [44]. After modification, hydroxypropyl distarch phosphate converted from starch has shown highest TS and capability of retarding evaporation of water due to being compatible with fillers [4]. In another modification, PVA and starch blend grafted with methylmethacrylate had a higher E% and water desorption. For this reason, polysaccharide chain of starch and OH- groups of PVA are mostly occupied with monomers [51]. However, Yoon et al. depicted that TS increased on the contrary E% after using to blend film modified methylmethacrylate with acrylamide [31].
\nThe plasma and irradiation treatment known as novel modification were also applied to PVA and starch blend films. These treatments can influence physicochemical properties of biohybrid films. Therefore, the treatment can cause a chemical bonding or graft functional groups on the PVA and starch backbone without any additives [52, 53]. Hence, the carbonyl groups of biofilms are improved with plasma treatment by using rotary argon plasma equipment. In addition, while E% of blend films can tolerate, its TS could also be lower. The plasma or irradiating pretreated with PVA and starch exhibited better thermal, processing, and mechanical (tensile) properties and toughness due to the induction of the cross-linking reaction [53]. With irradiated or plasma modifications, biofilms could prolong the storage conditions up to 15 days [54].
\nPhysical modification of PVA and starch could be safely used in biohybrid films. In generally, gelatinized, ungelatinized, fast and slow drying, varying amylose contents of raw material starch, changing of PVA and starch ratio, and impregnation of antioxidants were preferred in blend films for this modification. Applying physical modifications to biobased films affects significantly their physicomechanical properties. For example, gelatinized starch-polyvinyl alcohol blend films illustrate their uniformity of morphologies than the ungelatinized films, which corresponds well with the intensity of newly formed hydrogen bonds between starch, polyvinyl alcohol, and plasticizer. After the starch was gelatinized, the melting point of blend film decreased because of forming stronger hydrogen bonding interactions at an elevated preparation temperature [11]. At the same time, the gelatinized procedure is believed as a useful way to eliminate the crystalline structure [55]. In the fast (at 50°C) and slow (at 5°C) drying modification of the blend films, solubility, TS, E%, and degree of swelling values of the biobased films are preferable at slow drying than those of fast drying owing to the hydrogen bonding interaction forming at low temperature [56].
\nWith increasing amylopectin contents of starch in blend film, the %E and Tg increased while TS decreased. The linear structure of amylose improved the tensile property of films (especially, the amount of elongation) and the degree of crystallinity. However, because of the amorphous structure of amylopectin, %E of blend film was lower. Increasing amylose ratio in blend film significantly increased %E values as it plays an important role in cross-linking [7]. Moreover, changing rates of PVA and starch illustrated important role in blend films. The TS of the film decreased with increasing starch content in PVA compared to pure PVA film [5]. At the same time, the crystallinity of PVA in blend film decreased importantly compared to pure PVA film. On the other hand, the water absorption of blend film increased with the increasing starch ratio, because the water absorption of PVA is weaker than that of starch [29].
\nThe impregnation of antioxidants into the biofilm is another physical modification technique bringing antioxidative effect in biofilms. For instance, PVA/starch biofilms impregnated with catechin showed antioxidant and antimicrobial properties, while TS and %E of films decreased. Moreover, the biofilm containing catechin hinders lipid oxidation and microbial growth on raw meat during storage condition without substantial change in redness compared with commercial polyethylene pack [57].
\nPVA/starch biohybrid films are widely becoming an eco-friendly alternative to petrochemical-based plastics due to their biodegradability and for being suitable, generally obtainable, and less expensive materials. These biohybrid films have been obtained by using casting (sol-gel or mixing) and thermal processing (extruder or extrusion) methods. A great number of components in PVA/starch biobased films have been added to the matrix in order to improve physicochemical and mechanical properties. Moreover, various additives such as plasticizers, cross-linkers, fillers, and natural raw materials as well as thermoplastic starch have been used to improve the mechanical, thermal, and morphological properties of PVA/starch-based biodegradable hybrid films.
\nPlasticization in starch and PVA involves place in the amorphous area for higher molecular dynamism as well as their flexibility. Elasticity and other properties of biobased films are significantly affected by the functional groups (carboxyl and hydroxyl groups) of plasticizers. The using of cohydroxyl/carboxyl group as a functional group increases the flexibility and physicochemical and mechanical properties of films. Cross-linking modifications in biobased films increase amorphous zone in molecular structure. This formation is effective to enhance the physicochemical and mechanical properties. These modifications improve the TS [2], modulus of elasticity [33], water resistance [49], thermal resistance [16], swelling behavior [8], and antibacterial activity of biofilms [15], unlike %E [2]. The filler in PVA/starch biobased films has a high potential class. Nature and the geometry of the filler-added biobased films have enhanced their properties such as gas barrier, mechanical stiffness, transparency, and thermal stability. The chemical modifications occur in the carboxyl group in molecular structure of PVA and starch because of oxidation. With increasing of carboxyl groups, the hydrogen bonds in biobased films were stronger than those of nonmodified ones. This stronger hydrogen bonding has improved physical-biological-chemical properties of biobased films because of the changing molecular structure of blend. Moreover, applied physical modifications to biobased films also significantly affect their physicomechanical properties. Consequently, these modifications applied to starch and PVA cause the esterification, etherification, hydrogen bonding, and oxidation in their molecular structure.
\nIntechOpen's Authorship Policy is based on ICMJE criteria for authorship. An Author, one must:
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