Nanocomposite Biomaterials for Tissue Engineering and Regenerative Medicine Applications

Abstract

Nanocomposites are materials that are usually created by introducing appropriate nanoparticles into a macroscopic matrix, enabling the resulting bulk nanocomposites remarkable characteristics in electrical, thermal conductivity, mechanical, optical, magnetic properties, and so on. Such nanocomposite materials are of great importance for biomedical applications, particularly promising for tissue engineering scaffolds. Recent trends in the nanocomposites field show bio-based/environmentally friendly materials to be among the components in these nanocomposite materials. Particular attention has been paid to the use of bio-based/biodegradable polymers as a matrix component in nanocomposite applications, because of their great widespread potential and advantages over other traditional synthetic materials. In this chapter, we focus on the current research trends of the tissue engineering scaffolds based on nanocomposite materials and mainly introduce the properties, types, manufacturing techniques, and tissue engineering applications of various nanocomposite biomaterials. Besides, challenges and prospects associated with nanocomposite biomaterials for the tissue engineering field were discussed. We believe that this chapter provides a new envision for building functional nanocomposite materials for broad biomedical applications.

1. Introduction

Nanomaterials are becoming a new strategy to control and manipulate the nano and macro properties of hydrogels without hindering the exchange of nutrients with the surrounding environment [1]. Nanomaterials refer to synthetic or natural materials whose size does not exceed 100 nm, but for better medical applications, a diameter in the range of 10–100 nm is more required. Materials with a diameter of less than 10 nm are reactive and toxic due to their large specific surface area and activity and materials larger than 100 nm can even cause embolism. In addition, it has been shown that nanotopology rather than microtopology is the main influence factor on cell structure and arrangement [2]. Usually, natural tissues also contain nano-scale substances, such as proteins, which can directly interact with cells. The nanocomposite hydrogel designed with a biomimetic structure is more suitable for host cell colonization. Therefore, nanomaterials have high biological activity (for example, cell-binding motifs), which are conducive to protein adsorption and cell adhesion and proliferation [3]. The properties of the obtained nanocomposite hydrogel can be the sum of the functions of the nanomaterial and the hydrogel, or the result of better properties, such as improved stiffness, stretchability, and even higher cell compatibility, which may be derived from synergistic interactions [4]. Liu et al. introduced hydroxyapatite (HAp) into the hydrogel network, which improved the modulus and toughness of the hydrogel, and had a qualitative impact on the differentiation behavior of mesenchymal stem cells (MSCs) [5]. Nanoparticles (NPs) can be cross-linked agents with polymers both chemically and physically to achieve dynamic system behavior and form shear-thinning hydrogels. The resulting hydrogels exhibit a temporarily reduced viscosity under shear stress and can be injected through a plug flow system, and then return to their original viscosity under low shear owing to the electrostatic interactions [3]. Because of these characteristics, they can be further applied to 3D bioprinting. Moreover, due to their structural characteristics, nanocomposite hydrogels can easily present a gradient biomolecules delivery by (porous) NPs. A nanocomposite hydrogel composed of a poly(amide amine) network and mesoporous silica NPs capable of releasing chemokines has been reported. They have been observed to play an important role in tissue regeneration in vivo through their influence on (MSCs) (Figure 1) [7].

Figure 1.

Various topographical architecture of nanocomposite scaffolds affects the binding and spreading of seed cells [6].

Nanocomposites are defined as a combination of materials or phases, where one or more components is more concentrated and provide support, and other components enhance the performance of the prepared composite by adding extra characteristics [8]. In recent years, there have been more and more research toward nanocomposites by closely mimicking the biological environment and natural matrix. Especially, the properties of nanocomposites can be modified to meet the functional requirements of each tissue, which makes it an excellent choice for tissue engineering applications [9, 10]. Nanoscale scaffolds are needed to provide a suitable niche for interactions between cells and the extracellular matrix (ECM) and guide cell behaviors [11]. The emergence of nanocomposite materials provides more meaningful characteristics, combinations, and unique design possibilities for scaffolds, and finally provides a revolutionary platform for tissue engineering.

Nanocomposites involving biodegradable and bioactive properties have been regarded as strategies for tissue engineering and regeneration medicine [12]. Nanoscale fillers can greatly change the physical properties of the polymer matrix, thereby achieving improved engineering design of biomaterials. Compared with traditional ones, nanoparticles with a larger surface area, can form a tighter interface with the polymer matrix, provide better mechanical properties, while outstanding mechanical conductivity and biocompatibility of the filler, thereby affecting protein adsorption, cell adhesion, proliferation, and differentiation [13].

The components used to prepare the nanocomposite can be of natural or synthetic origin. The components obtained from natural sources include polysaccharides from microorganisms, such as chitin/chitosan, starch, alginate, and cellulose, while biopolymers from animal proteins include wool, silk, gelatin, and collagen. These polymers have been widely used in biomedical applications for their physical and chemical similarities to natural ECM [12]. These polymers provide favorable support for living systems, including outstanding biological properties, adjustable degradation rates, and faster tissue regeneration [14, 15]. However, some disadvantages, including the risk of pathogen transmission, allergic reactions, poor mechanical strength, and high cost have also restricted their applications [16]. In recent years, researchers have begun to overcome these problems related to natural nanocomposite scaffolds. In tissue engineering, the developed scaffold can be used as a substitute for ECM. Scaffolds formulated with biodegradable polymers, cells, growth factors, and appropriate biochemical signals can repair or replace damaged tissues by providing the environment and conditions that enable cells to secrete their own new natural ECM [17, 18].

Tissue engineering and regenerative medicine (TERM) is a multidisciplinary research field that uses the principles of chemistry, biology, and engineering science to study the regeneration process of damaged tissues or organs [19]. Tissue engineering requires a combination of molecular biology and materials design for the urgent need of providing temporary artificial substrates for cell seeding. In general, the scaffold should exhibit high porosity, appropriate pore size, biocompatibility, and appropriate degradation rate [20]. Besides, the scaffold also needs to provide sufficient mechanical support to maintain the stress and load generated during the regeneration process in vitro or in vivo. For the purpose of improving mechanical properties and cell adhesion and proliferation, the incorporation of nanoparticles (such as apatite components, carbon nanostructures, and metal nanoparticles) has been extensively studied. Polymer/layered nanocomposites such as HAp, carbon nanotubes, and layered silicates have become the focus of attention in academia and industry [21, 22]. The introduction of a small amount of high-aspect-ratio silicate nanoparticles can significantly improve the mechanical and physical properties of the polymer matrix.

This chapter mainly introduces the current research trends of nanocomposite materials used in tissue engineering, the properties, types, manufacturing techniques, and applications of ideal nanocomposite scaffolds in various tissue engineering fields.

2. Properties and types of nanocomposite materials for tissue engineering

2.1 Properties and selection criteria for ideal nanocomposite scaffolds for tissue engineering

In the past research, various nanocomposite scaffolds have been prepared using different manufacturing techniques. In a broad sense, the ideal nanocomposite scaffold should meet the following demands:

1. Biocompatible: It provides an ECM-like environment for cells’ physiological behavior, such as proliferation, migration, differentiation. Its degradation must be nontoxic and easily adsorbed or eliminated from the body.

2. Biodegradability: It must possess a controllable rate of degradation to maintain the balance between newly formed tissues or organs and the degradation (resorption) of the nanocomposite scaffold.

3. Nutrient delivery: Scaffolds should contain a large number of interconnected pores to facilitate the transport of nutrients and regulatory factors.

4. Mechanical factor: It must have sufficient mechanical integrity to adapt to the surgical process and meet the corresponding tissue tolerance. There should be a balance between the porosity, stiffness, and hardness. Stiffness determines the direction of cell differentiation to a certain extent and avoids insufficient vascularization caused by excessive porosity.

5. Pore geometry: The pore distribution of the scaffold should be conducive to the morphology of the cells. Pore size must be in the critical range larger than the lower limit so that cells can migrate easily and smaller than the upper limit to have enough surface area or binding sites.

6. Surface topography: It must have adjustable surface topography, hydrophilicity, surface energy, etc. This allows the control of cell adhesion, migration, intracellular signaling, as well as in vivo cell recruitment.

Of course, the final criteria for choosing a nanocomposite scaffold depends on the type of tissue to be treated. These nanocomposite scaffolds must be resistant to damage during the implantation process, as damage can lead to necrosis and inflammation. For example, nanofibers scaffolds for tendon tissue engineering should be arranged in parallel to imitate the natural tissues arrangement structure. In cardiac tissue engineering, nanocomposite scaffold materials should have a certain electrical conductivity, and gradually promote the repair and regeneration of myocardial infarction damaged tissue under the conditions of satisfying electrophysiology [23]. For neural tissue engineering, scaffolds with higher flexibility are needed to allow cells to adhere, migrate and differentiate, and they need to mimic the geometry of natural nerves [24]. The scaffold required for bone tissue engineering should have the characteristics of providing multiple growth factors for different stages of bone tissue regeneration, corrosion resistance, osteoconductivity, and shape controllability [25, 26]. It also needs to create multiscale layered structures at the nanometer level to add growth factors that promote vascularization and provide a surface for stem cells to initiate bone repair and regeneration [15].

According to the type of tissue to be repaired, degradation rate and surface morphology are also important criteria for stent selection. There should be coordination between the degradation time of the scaffold and the time required for natural tissue replacement, it can be implanted for a long time without multiple operations and avoid subsequent surgical removal. The degradation rate can be controlled by introducing nanoparticles into the scaffold. On the other hand, the scaffolds used for skin and heart repair usually require planar scaffolds, the scaffolds used for bone tissue repair are usually cube-shaped or disc-shaped, while the tissue regeneration of nerves, blood vessels, and trachea require tubular scaffolds [27].

2.2 Types of nanocomposite scaffolds

2.2.1 Hydrogel

Hydrogel, a highly hydrated 3D network, has a similar structure and composition with ECM and become preferred tissue engineering scaffolds due to their adjustable network structure, outstanding biocompatibility, effective mass transfer, and the ability to encapsulate cells and biological factors [28]. These properties are affected by the degree of crosslinking of polymer chains, molecular arrangement, and the amount of water they absorb [29]. Hydrogels typically exhibit a hydrophilic network porous structure with interconnected pores (>10 μm) to allow gas penetration, nutrient delivery and provide sufficient space for cell attachment and interaction. In addition, the porosity of the hydrogel can be controlled by preparation methods (phase separation, gas foaming, solvent evaporation, etc.), types of raw materials, and the polymers concentrations [30]. The hydrogel nanocomposite scaffold can be designed in different sizes and shapes, such as patches, microspheres, sheets, hollow tubes, to satisfy their unique practical application [31]. From the perspective of regeneration medicine, hydrogels can be used as the cell niche and provide stimuli to accelerate tissue regeneration, and able to act as the delivery vehicle for biologically active molecules.

Nanocomposite hydrogels were first reported in 2002 when Haraguchi et al. introduced exfoliated clay flakes into poly(N-isopropylacrylamide) (PNIPAM) to form a unique organic/inorganic network [32]. Compared to conventional ones, nanocomposite hydrogels have enhanced chemical, physical, electrical, and biological properties, which are mainly attributed to the improved interaction between the polymer network and nanoparticles. However, nanoparticles themselves lack some essential properties, such as biodegradation and stimulated reactivity, which can be ameliorated by incorporating multiphase in nanocomposite hydrogels (Figure 2).

Figure 2.

Hydrogels for different applications such as tissue engineering and other biomedical researches [33].

2.2.1.1 Inorganic nanomaterial based nanocomposite hydrogels

Over the last decades, significant progress has been made in the study of the impact of introducing inorganic nanomaterials into tissue engineering applications in natural and synthetic polymer networks. Inorganic nanomaterials, such as HAp, metal oxide nanoparticles, bioglass, and calcium phosphate NPs, are based on materials found in biological tissues and show significant biological activity. NPs confer and enhance signal (mechanics, bioelectricity, etc.) conductivity, while polymers provide flexibility and stability to nanocomposite structures. Surface chemistry favorable for protein adsorption and enhanced matrix stiffness makes these nanocomposites ideal for tissue engineering scaffolds.

2.2.1.2 Organic nanomaterial based nanocomposite hydrogels

Organic nanomaterials, such as dendrimers, liposomes, hyperbranched polymers, and micelles, are some typical polymer NPs that have a wide range of applications in tissue engineering, drug delivery, and cancer immunotherapy. For example, the properties of polymer NPs (such as conductivity) can be adjusted to achieve the repair of myocardial infarction. Particle size and shape are the most widely studied and reported properties, as they affect blood circulation time, cellular uptake, antigen presentation, and T-cell immunity. In vivo studies of nanocomposite hydrogels have shown that changing the shape of nanoparticles can lead to changes in the immune response. Compared with larger sizes, smaller size nanoparticles show better immune responses due to their larger specific surface area and higher reactivity (Figure 3).

Figure 3.

Design and fabrication of organohydrogel fibers [34]. (a) Molecular design of hybrid crosslinked polymeric network in organohydrogel fibers and schematic of the wet-spinning process and molecular evolution of hydrogel fibers. (b) Schematic of the preparation of organohydrogel fibers from hydrogel fibers by displacement solvent. (c) Photograph of a long single fiber collected on a continuously winding drum spool. (d) Schematic and photograph of an organohydrogel-fiber knitted textile.

2.2.2 Fibers

Fibrous scaffolds, a considerable option of mimicking the nanostructure of natural ECM, possess a more favorable morphology compared to porous scaffolds and hydrogels [35]. The nanofibers show similarity to the collagen fiber network, whose diameter distributes in the range of 50–500 nm. Besides, nanoscale fiber scaffolds with well-controlled pattern structures have received special attention in enhancing cell functions, such as cell adhesion, migration, proliferation, and differentiation, for their isotropic structure, uniform fiber size, and pore distribution, which also decide their mechanical properties [36]. Nanofiber scaffolds have been used in tissue engineering for heart, bone, cartilage, ligament, skeletal muscle, skin, and nerve tissue engineering, and as a carrier for the controlled delivery of drugs, proteins, and even DNA [37].

Nanocomposite fiber scaffolds can be obtained from polymers and nano-sized phases by molecular self-assembly, phase separation, and electrospinning manufacturing technology. Compared to the other two methods, electrospinning methods can produce fibers with controllable pore size, fiber size, and stiffness, making it the most widely studied technique [38]. Moreover, incorporating nanoparticles into electrospun fibers can improve biomimetic scaffolds because cell-matrix interactions are strongly affected by the presence of chemical cues that support cell attachment, proliferation, and differentiation.

2.3 Preparation of tissue engineering nanocomposite scaffold materials

The processing method of the above-mentioned nanocomposite scaffold is summarized as follows:

2.3.1 Blending

This mixing method is the simplest preparation method and has many advantages. We can obtain NPs with specialized size by varying the stirring speed, material concentration, and preparation period. Additionally, the preparation process is easy for the separate preparation and cross-linking of NPs. Filippi et al. mixed PEG-functionalized iron oxide (II, III) nanoparticles with an average particle size of 15 nm and cells in sterile Tris-buffered saline to form a nanocomposite hydrogel network [39]. The obtained hydrogel has a smooth texture, possesses excellent mechanical properties, such as stress relaxation and high elastic modulus. However, the asymmetric distribution of nanoparticles in nanocomposite hydrogels and their diffusion behavior in solution needs to be further studied.

2.3.2 Solvent casting/particulate leaching

A combination of solvent casting and particle leaching methods has been widely used to successfully manufacture 3D porous scaffolds [40]. It is a process of dispersing salts in polymers that are dissolved in organic solvents. The solvent is removed and then the salt crystals are leached out with water to form the porous scaffold. Utilizing this process, people can make a scaffold with a porosity value of up to 93% and an average pore diameter of about 500 μm by changing the size of the salt crystals and salt/polymer ratio [41]. However, this method cannot control certain key variables, such as pore shape and pore interconnectivity (Figure 4).

Figure 4.

Solvent casting/particulate leaching process [40].

2.3.3 Electrospinning

Electrospinning is a special form of electrostatic atomization of polymer fluids. The biggest difference between the two is that the electrostatic spray uses a low-viscosity fluid, and the product is in the form of nanoparticles, which are mainly used for surface spraying. Electrospinning is a fluid with a higher viscosity, and the product is a nanofiber membrane, which is mainly used to prepare materials with three-dimensional shapes. Electrospinning is essentially a stretching process to generate long nanofibers of uniform diameter from a polymer solution in the electric field [42]. As the electrostatic interaction between the positively charged polymer and the collector increases, the droplets of the polymer solution become finer and stretch further, resulting in an ejection of fibers out of the solution.

This technique allows the production of bio-sized fibers with higher porosity and high surface area-to-volume ratio, making them promising candidates for tissue engineering and drug delivery applications. The increased cell surface area provides more cell attachment sites and allows for effective cell adhesion. Compared with synthetic polymers, natural polymers are generally less suitable for spinning. Therefore, for natural polymers, the polymer and solvent concentration optimization must be carefully optimized (Figure 5) [43, 44].

Figure 5.

Schematic diagram of set up of electrospinning apparatus (a) typical vertical set up and (b) horizontal set up of electrospinning apparatus and (bottom column) SEM micrographs of representative electrospinning fibrous materials [ 42, 43].

Electrospinning technology has been used to develop tissue engineering scaffolds. For example, hepatocyte-like cells from human mesenchymal stem cells (hMSCs) were observed to aggregate on PLLA co-PCL collagen (PLACL/collagen) nanofiber scaffolds to form functional liver spheres. The results indicate that the bioengineered PLACL/collagen nanofiber scaffold may be a promising candidate for the treatment of damaged hepatocytes in advanced liver failure [45]. Researchers have also used electrospinning to manufacture different types of nanocomposite scaffolds for tissue engineering applications, such as polyurethane/cellulose fiber scaffolds, and polyethylene terephthalate (PET) scaffolds [46, 47, 48]. The main advantage of nanofibers prepared by electrospinning technology is that it allows easy transport of nutrients and waste across the scaffold. However, there are still some limitations, the use of cytotoxic solvents and a wide range of optimization parameters, such as applied voltage, flow rate, and travel distance to obtain ordered nanofibers, urgently need to be resolved.

2.3.4 Freeze drying/emulsification

The principle of freeze-drying is actually the sublimation process and a promising technique for preparing scaffolds, in which the frozen water is directly transformed from solid to gas without liquefaction. Thus, obtained highly porous interconnected polymer structure is used as a scaffold for tissue engineering [49]. The obtained scaffold usually has a high porosity, but it can be further adjusted as needed by varying the freezing method, the amount of water, the polymer concentration, the size of the ice (solvent) crystals, and the pH value of the solution as needed. Freeze drying is widely used to prepare nanocomposite scaffolds, but some of the limitations associated with this process include long preparation time, high-energy consumption, and the possible formation of closed cells by gas foaming. The obtained scaffold usually has a high porosity, but it can be further adjusted as needed by varying changing the freezing method, the amount of water, the polymer concentration, the size of the ice (solvent) crystals, and the pH value of the solution as needed. Freeze drying is widely used to prepare nanocomposite scaffolds, but some of the limitations associated with this process include long time, high-energy consumption, the use of cytotoxic solvents, and the formation of closed cells by gas foaming (Figure 6) [51].

Figure 6.

Schematic representation of freeze-drying process [50].

The freeze-drying technique has been successfully used in many studies related to tissue engineering, such as chitin-chitosan/zirconium oxide (ZrO₂) and chitosan/gelatin/nanosilicon dioxide (nSiO₂)) composite material [52]. In one such study, freeze-drying methods were used to produce a chitosan 3D scaffold. The results obtained by dispersing nHA and fucoidan in a chitosan matrix were found to be suitable for cell growth and nutritional supplementation. In vitro results show that mesenchymal stem cells (PMSCs) derived from periosteum grow well in nanocomposites, which implies the potential for tissue engineering.

2.3.5 3D printing

Traditional manufacturing techniques, such as electrospinning, freeze-drying, and solvent casting, have certain defects, including limited control over pore size, fiber arrangement, and pore interconnection, which can lead to poor nutrient transport or reduced cell survival and migration rates. 3D printing is one of the new methods to obtain highly ordered scaffolds. It provides a highly controllable and precise design for the internal structure and surface of the bracket. It can deposit cells and biological materials in a way that mimics the structure of biological tissues [53]. The principle of printing is based on a polymer solution containing cells or cell aggregates “biological ink,” which is deposited layer by layer on a substrate to generate 3D structures, such as tissues or organs (Figure 7).

Figure 7.

Integrated tissue-organ printer (ITOP) system [54]. (a) The ITOP system consists of three major units: (i) 3-axis stage/controller, (ii) dispensing module including multi-cartridge and pneumatic pressure controller and (iii) a closed acrylic chamber with temperature controller and humidifier. (b) Illustration of basic patterning of 3D architecture including multiple cell-laden hydrogels and supporting PCL polymer. (c) CAD/CAM process for automated printing of 3D shape imitating target tissue or organ. A 3D CAD model developed from medical image data generates a visualized motion program, which includes instructions for XYZ stage movements and actuating pneumatic pressure to achieve 3D printing.

3D printing has been used to prepare a nanocomposite scaffold containing nano-HA and chondrogenic transforming growth factor-β1 (TGF-β1) in the highly porous subchondral bone layer. In the cartilage layer, the prepared scaffolds showed osteogenic differentiation, high biocompatibility with hMSC, and mechanical properties required for osteochondral tissue regeneration [55]. 3D printing is promising, but the application of this technique in tissue engineering still needs to be explored due to high device costs, limited available materials, and low mechanical strength.

2.3.6 Self-assembly/self-organizing

It is an effective method to generate well-organized and stable supramolecular structures through the spontaneously rearranging of molecules under thermodynamic equilibrium conditions. Self-assembly can occur spontaneously in nature through hydrogen and ionic bonds, hydrophobicity, etc., like the self-assembly of phospholipid bilayer membranes in cells. Molecular self-assembly is a useful method for generating supramolecular structures that rely relies on the potential of molecules to spontaneously rearrange themselves into well-organized and stable structures under thermodynamic equilibrium conditions. Such molecular interaction is non-covalent and can occur through hydrogen and ionic bonds, hydrophobicity, van der Waals forces, metal coordination, and electromagnetic interactions [56]. Self-assembly can occur spontaneously in nature, just like the self-assembly of lipid bilayer membranes in cells. Molecular self-assembly is a strategy to develop nanofiber materials with tissue engineering potential [57]. At the molecular scale, the precise and controlled application of intermolecular forces can lead to new and previously unachievable nanostructures. Thus, with proper design, the mechanical properties and release characteristics of the assembled material can be tailored specifically for its intended use through appropriate design (Figure 8 and Table 1).

Figure 8.

Schematic representation of self-assembly/self-organizing [58].

Materials	Preparation methods	Applications	Advantages	Disadvantages	Refs
Hydrogel, fibrous materials	Blending or doping	Controlled drug delivery, tissue engineering, cell induction	Stress relaxation and high elastic modulus	Asymmetric distribution	[39, 59]
Nanofiber membrane	Electrospinning	Controlled drug delivery, enzyme immobilization, wound dressings, tissue engineering, biosensors, filtration	Tunable porosity, high surface area-to-volume ratio	Cytotoxicity, diversified parameters	[43, 60]
3D scaffold, hydrogel	3D printing	Tissue engineering, organ transplantation, biomaterial, growth factor delivery	High precise internal and surface structure	High device costs, limited available materials , low mechanical strength	[61, 62]
Hydrogel, nano/micro-particle	Microfluidics	Precise drug delivery, microfluidic biochip, growth factor delivery	High throughput production rate, ultrahigh drug loading degree	Clogging and limited architectural features, low-volume production	[63, 64, 65, 66]
Nanofiber	Self-assembly	Tissue engineering , biomaterial	Enhanced mechanical properties and biocompatibility, controlled intermolecular forces	Structure restricted, few biocompatible and biodegradable characteristics	[67, 68]
Hydrogel	Cross-linking/binding	Tissue engineering, shape memory, controlled drug delivery	Excellent biocompatibility, higher intensity and willfulness	Lower modulus	[69]
Nano/micro-particle	Ink-jet or electrospray	Tissue regeneration , controlled drug delivery, biosensors	Higher biological activity, convenient delivery	Limited thickness	[70, 71]
3D scaffold	Freeze-drying/emulsification	Tissue engineering, regenerative medicine	High and tunable porosity	Long time, high-energy consumption, cytotoxic solvents	[52]

Table 1.

Nanocomposites biomedical materials for tissue engineering applications.

3. Application for tissue engineering

See Figures 9 and 10.

Figure 9.

Different types of damaged body tissues repaired by applications of nanocomposites biomaterial-based tissue engineering [72].

Figure 10.

Various nanocomposite hydrogels were designed and prepared for different types of tissue engineering [73].

3.1 Cardiovascular tissue engineering

Nowadays, cardiovascular disease has become a major issue affecting the health of countless people. Myocardial infarction (MI) is the leading cause of morbidity and mortality worldwide [74]. MI is caused by local obstruction of myocardial blood flow, which leads to permanent damage to the heart pump function and gradually develops into chronic heart failure. Due to the limited regeneration capacity of myocardial tissue, heart transplantation is the only solution for patients with advanced heart failure to restore heart function. However, the shortage of heart donors and the high cost of surgery procedures make it out of reach for many patients (Figure 11).

Figure 11.

Schematic illustrating of various nanocomposite conductive hydrogel and their applications in myocardial infarction repair [75, 76, 77].

Cardiac tissue engineering is a promising approach to effectively replace or repair damaged myocardium [78]. The approach of cardiac tissue engineering is to create potential 3D scaffolds that mimic ECM to reconstruct injured myocardium.

Cardiac tissue engineering should meet some essential requirements, such as biodegradability, porosity, and mechanical/conductive properties, to match healthy cardiac tissue. Based on these characteristics, nanocomposite hydrogels containing conductive nanomaterials are considered attractive strategies for cardiacregeneration [79]. There are many reports on the use of electroactive nanocomposite hydrogels for cardiac tissue engineering.

Polypyrrole (Ppy) is widely used in cardiac tissue engineering due to its outstanding electrical conductivity and biocompatibility. Qiu et al. prepared a variety of conductive hydrogel myocardial patches containing Ppy NPs and used them to repair myocardial defects. Some animal experiments results showed that the fractional shortening and ejection fraction are elevated by about 50% and that the infarct size is reduced by 42.6% [77]. Metal-based nanomaterials are also widely used in cardiac tissue engineering, Chen et al. prepared a hyaluronic acid-based injectable hydrogel containing Au NPs to repair myocardial defects by loading human induced pluripotent stem cells [80]. Additionally, Dong et al. introduced Au NPs into silk-based hydrogel (SF/ECM) for cell proliferation and expansion of cardiomyocytes [81]. The uniform distribution of Au NPs in the matrix can provide favorable electrical conductivity and biological effects for cardiac repair. In addition to Au NPs, Liu et al. prepared an injectable PEGylated chitosan hydrogel scaffold for cardiac repair by introducing titanium dioxide (TiO₂) nanoparticles [82]. The TiO₂ nanocomposite hydrogel significantly enhances the functionalization of the cardiomyocytes, resulting in excellent synchronous contraction by increasing the expression of α-actinin and connexin CX-43. Results of cardiac markers confirmed the formation of interconnected cardiac layers within these nanocomposite hydrogels and the formation of cell-hydrogel matrix interactions. These nanocomposites are well suited for cardiac regeneration and provide a new platform for cardiac tissue engineering.

Mineral-based bioactive nanomaterials, such as SiO₂, P₂O_5, and CaO, can also be used to effectively improve composite hydrogels for cardiac tissue engineering. It has been proven that these materials can stimulate cells to secrete a large number of angiogenic factors, thereby promoting the formation of blood vessels in engineering scaffold [83, 84, 85]. To address the lack of functional blood vessels in engineered tissue and low survival rates of injected cells in cardiac tissue engineering, Barabadi et al. [86] introduced bioactive glass nanoparticles into the gelatin-collagen hydrogel to improve the biological properties. In addition, in vivo evaluation experiments confirmed that this nanocomposite hydrogel scaffold can effectively promote the formation of capillaries, reduce scar area, and improve cardiac function.

Although electroactive nanocomposite hydrogels can achieve satisfying biocompatibility and durability for the generation of cardiac microtissues in vitro, there is still space for improvement in terms of mechanical properties and electrical conductivity [87]. In addition, the goals of high-level cell attachment, viability, endothelialization, and recruitment of cardiac progenitor cells have not yet been fully achieved. Therefore, future research should aim to address these issues.

3.2 Bone tissue engineering

Among all tissues in the human body, cartilage and bone are some of the most extensively researched tissues in tissue engineering because of their high regeneration potential. Bone graft materials, due to their osteoinductive and osteoconductive properties, have been used to repair fractures and other defects [88]. However, there may be risks of disease metastasis, infection, chronic pain, immunogenicity, and inadequate supply (Figure 12).

Figure 12.

Cryotropic gelation scaffold for bone tissue engineering [89].

Bone tissues, connective tissues, that are composed primarily of cells and extracellular matrix. These tissues are hard tissues that can withstand repeated mechanical stimulation without harm or loss of human functions. Bone tissue is closely related to the movement of the human body. Once damaged, it will cause great inconvenience to the patient’s life. Based on this, the key goal of bone tissue engineering is to develop a tough and natural 3D microenvironment with the physiological environment required to form the target tissue [6, 90]. In recent years, more and more nanocomposite hydrogels have been developed for bone tissue engineering, especially bone and cartilage tissue engineering.

Inspired by the nature of bones, there is a need for three-dimensional hierarchical structures and nanocomposites that can contain multiple levels of tissues, that is, from the macroscopic tissue arrangement to the molecular arrangement of proteins [91]. These nanostructured materials can provide enhanced mechanical properties and allow the proper transduction of mechanical stimuli to the cellular level. Bioabsorbable β-TCP can improve the clinical application of pure HAp to achieve better bone regeneration. The main attraction of these materials is that they can bind well with host tissues to form a robust interface. However, these materials are limited to non-load-bearing applications due to their poor mechanical properties.

The mineral composition of bone is similar to HAp, but it contains other ions in composition, which can better prepare biological materials. Yazaki et al. developed the incorporation of carbonate or fluoride into the DNA-fibronectin-apatite composite layer for tissue engineering to adjust the solubility of the layer [92]. The incorporation of carbonate increases the effect of gene transfer on the efficiency layer, while fluoride reduces the efficiency and delays the time of gene transfer in a dose-dependent manner. In addition, manganese (M_n), detected as a minor component of teeth and bones, modulates bone remodeling.

The scaffold is designed to mimic the 3D support structure of the ECM in the surrounding bone tissue and offers the following advantages—(i) porous interconnected structure, that afford the transport of quality, nutrition, and regulatory factors to allow cell survival, proliferation and differentiation, (ii) sufficient mechanical properties for the support of cells, (iii) controllable degradation and minimal inflammation or toxicity to the body [93]. Besides, the scaffold has the desirable properties of transferring cells to the defect site, limiting cell loss, and even recruiting the body’s own cells, rather than simply injecting cells into the defect site [94].

The challenge is to ensure good compatibility of the scaffold while maintaining the porous structure and mechanical properties. It is important to achieve a uniform distribution of NPs within the scaffold. Methods for maximizing the distribution of NPs include precipitating nanoparticles in situ in a polymer matrix or using dispersants. For example, the in situ synthesis method is used to prepare nanocomposite scaffolds of SF and CaPs. Phosphate ions are added to the calcium chloride solution in which SF is dissolved, and then the diammonium hydrogen phosphate solution is added by salt immersion/freeze drying technology [95]. The scaffold exhibits highly interconnected macropores with a size of approximately 500 μm and a micropore size range of 10–100 μm under SEM.

3.3 Skin tissue engineering

The skin is the first line of defense against infection and its injuries (such as burns) may cause serious health problems. Skin tissue engineering is a promising method of skin regeneration. In addition to the proper mechanical properties, the scaffold material for skin tissue engineering should also possess antibacterial and anti-infective properties, and can play a role in drug delivery to promote wound repair. To achieve this goal, nanocomposite hydrogels are widely used in skin tissue engineering. Studies have shown that zinc oxide (ZnO) NPs exhibit strong antibacterial activity and have no side effects on normal tissue. Rakhshaei et al. introduced ZnO nanoparticles into chitosan/gelatin hydrogel (CS-gel/ZnO) to endow the hydrogel with additional antibacterial and drug delivery properties [96]. The ZnO NPs on the surface of the scaffold extremely enhance the flexibility of the scaffold. These nanocomposite hydrogels can effectively destroy the cell walls of gram-negative bacteria to achieve antibacterial properties, which is due to ROS and Zn²⁺ in the nanocomposite hydrogel can attack the negatively charged bacterial cell wall, causing bacterial death (Figure 13).

Figure 13.

Skin healing process and mechanism using the nanocomposite wound dressing [97].

In addition to ZnO nanoparticles, graphene-based nanomaterials are also used as fillers to prepare nanocomposite hydrogels for skin tissue engineering. Narayanan et al. mixed rGO nanosheets with PAAm to form nanocomposite hydrogels which have antibacterial activity and can promote the formation of biofilms [98]. Besides, it is confirmed that the ginsenoside molecules in the nanocomposite scaffold can be released slowly to achieve antioxidant effects. Xu et al. introduced ginsenoside Rg3 (GS-Rg3) into an electrospun fiber of polyglutamic acid, this nanocomposite hydrogel exhibits fast tissue repair and inhibits the excessive scar formation [99]. The results show that this nanocomposite hydrogel can be used as a wound dressing in skin tissue engineering.

3.4 Other tissue engineering applications

In addition to the aforementioned fields, nanocomposite scaffolds also have a wide range of applications in other areas, such as muscle tissue and nerve tissue.

3.4.1 Skeletal muscle tissue engineering

Skeletal muscle is one of the most abundant tissues in the body, possessing a complex structure. In addition to supporting, connecting, nourishing, and protecting muscle tissue, skeletal muscle also has the function of regulating the activity of muscle fiber groups. Extensive skeletal muscle defects caused by trauma or tumor ablation can cause movement disorders and organ dysfunction, which can lead to pain in patients [100]. Restoration of the original function of skeletal muscle is limited and fibrosis and scarring may occur for serious injuries of a mass loss of more than 20% [101].

The purpose of skeletal muscle tissue engineering is to replicate the natural structure and function of muscle in vitro and to transplant this tissue to the damaged area, since the behavior of myogenic cells is regulated by the flexibility and strength of the scaffold, such scaffold systems in skeletal muscle tissue engineering should have mechanical elasticity. Nanocomposite hydrogels with conductive components have proven to be good candidates as 3D biomaterials for skeletal muscle tissue engineering. In the field of skeletal muscle regeneration, graphene and carbon nanotubes are the two most widely used carbon nanomaterials in skeletal muscle regeneration. To further optimize the mechanical properties of the scaffold, Patel et al. combined graphene with chitosan and gellan gum to develop a graphene-polysaccharide nanocomposite hydrogel scaffold [102]. This nanocomposite hydrogel can positively affect myoblasts, and as an ideal multifunctional biomaterial, it has the mechanical properties of natural skeletal muscle tissue and the ideal conductivity for transmitting electrical signals to cells.

Metal oxides can also be used in skeletal muscle tissue engineering as nano-reinforcing materials. For example, Tognato et al. introduced iron oxide (Fe₂O₃) nanoparticles into gelatin methacrylic acid (GelMA) [103]. In an external magnetic field, the magnetic Fe₂O₃ nanoparticles can be aligned, which then caused the seed cells to line up in the same direction. In particular, in the absence of differentiation media, the skeletal myoblasts in the nanocomposite hydrogel can differentiate into myotubes. One of the challenges of current research in vitro skeletal muscle regeneration is the lack of functional vascular structure, which also exists in most areas of tissue engineering today. Therefore, the direction of skeletal muscle tissue engineering in the future is to overcome the challenge above to promote the formation of muscle-like intravascular structures.

3.4.2 Nervous tissue engineering

The function of the nervous system is to receive information from cells in different parts of the body, process the received information, and send signals to other cells and organs to elicit appropriate responses [104]. Nervous system damage caused by ischemia, chemical, mechanical or thermal factors is very common in our current society. Surgical strategies, such as nerve repair and autologous nerve transplantation, are widely used in nervous system injuries to achieve optimal recovery. However, the prognosis is not ideal. Recent studies have shown that only 40–50% of patients recover their motor function after receiving autologous nerve transplantation [105]. Therefore, tissue engineering technology is considered an ideal method to repair nerve damage.

The goal of nervous tissue engineering is to manufacture nerve graft substitutes for the treatment of nerve damage and achieve long-term functional recovery. Among them, graphene-based nanocomposite hydrogels can be used as a viable option to promote nerve regeneration. Huang et al. prepared a nanocomposite hydrogel based on graphene and polyurethane [106]. This polyurethane hydrogel can improve the growth of neural stem cells and the differentiation of neurons. Qiao and his colleagues introduced GO into poly(acrylic acid) (PAA) and formed an electrically responsive nanocomposite hydrogel through in situ polymerization [107]. In particular, scientists also proved that carbon nanotube (CNT)-based polyethylene glycol (PEG) nanocomposite hydrogels can increase total neurite growth and average neurite length through electrical stimulation.

However, the influence of these nanocomposite hydrogels on the activity of neural stem cells needs to be further studied.

4. Conclusion

Currently, these reported nanocomposite hydrogels can mimic certain characteristics of natural tissues to a certain extent. However, there are still many challenges in applying them as tissue engineering scaffold biomaterials in clinical practice. Although some new functional nanomaterials have been introduced into nanocomposite scaffolds, the relatively poor interfacial interaction between polymer chains and nanomaterials and the uneven dispersion of nanomaterials in the matrix significantly limit their application. At present, processing technologies, such as 3D printing, electrospinning, and microfluidic reactors, have been rapidly developed, providing a great number of opportunities to quickly obtain nanocomposite scaffolds with anisotropic structures.

5. Future perspectives

We expect that the discussion in this chapter on the effects of different nanomaterials on the physical and chemical properties of nanocomposite hydrogels and their eventual application in different types of tissue engineering will be useful for the design, fabrication, and production of new generation nanocomposite hydrogel materials. Nanocomposite hydrogels are a new generation of 3D biomaterials for many different applications, including kinds of tissue engineering. The introduction of different nanomaterials in a polymer hydrogel network leads to the formation of nanocomposite systems with different functions. For instance, the addition of conductive nanomaterials into the polymer hydrogel matrix, such as Ppy, carbon nanotubes, graphene, and metal-based nanoparticles, not only provides mechanically strong hydrogels but also imparts conductivity to the hydrogel. These nanocomposite hydrogels can be used for the regeneration and repair of electroactive tissues, such as nerve, cardiac, and skeletal muscle tissues. Additionally, the introduction of rigid nanomaterials, such as nHAp, tricalcium phosphate, bioglass into the hydrogel matrix, can enhance its mechanical properties and promote the occurrence of mineralization inside the scaffolds. These nanocomposite hydrogels can be used for bone or cartilage tissues repair. In addition to these, some nanocomposite hydrogels can even be used in skin and vascular tissue engineering.

We believe that high-performance nanocomposite hydrogels with excellent mechanical properties, ordered structures, and novel functions will soon be used in tissue engineering applications.