Parameters of the impact device: primary and secondary electrostatic transducers
Energy harvesting from mechanical vibrations is one among several alternatives currently considered as power sources for wireless devices. The main motivation is the prospect of eliminating maintenance or increasing maintenance intervals by providing a means for recharging batteries or replacing batteries altogether. Energy harvesting can also be an enabling technology for applications where operating conditions, e.g. temperature, inhibit use of batteries. The prospect of reducing system size can also be a factor of interest. Vibration energy harvesting is therefore a topic of great interest in the scientific community [1-6], especially regarding miniaturized devices. For macro scale devices, commercial products have already emerged [7].
A vibration energy harvester is usually a spring-mass-damper system with a transducer that is continuously driven by the relative motion of the mass with respect to a device frame. The transducers are typically one of the three main types: electromagnetic, piezoelectric or electrostatic [1-2, 8-14]. For small scale systems, vibration energy harvesters face at least two fundamental obstacles. Reduced size necessarily means reduced mass, meaning reduced output power in an inertially driven device. Furthermore, the smaller harvesters have smaller space available for proof mass motion which again limits the distance over which work can be done.
In practical generators, mechanical end-stops are intentionally designed in order to confine the displacement of the inertial mass to the finite die dimension and to avoid spring fracture or degradation of material properties. When the acceleration amplitude is sufficient for proof-mass impacts on end-stops to occur, non-linear effects such as the jump phenomenon in the displacement vs. frequency response appear. Even though this behavior can be exploited to extend device bandwidth, operating a conventional harvester in this regime has the considerable disadvantage that the output power saturates at high excitation levels and therefore the effectiveness of the device decreases. This saturation is quite generic and has been reported for a variety of devices [15-20].
This chapter is concerned with the extent to which the internal impacts on these end-stops can be exploited by making transducing end-stops. Several prototypes utilizing impact principles in macroscale piezoelectric devices have been presented [21-28]. Here we consider microscale electrostatic energy harvesters with two types of transducers, main transducer and secondary end-stop transducers. At sufficiently strong excitations, the impact of the proof-mass onto the end-stops actuates the secondary transducer and thereby harvests the excess kinetic energy of the proof mass. Therefore, the device provides power through two states as excitation strength increases, a first stage with only primary transducer output, and a second stage with output from all transducers.
For a velocity damped generator, the end-stop limit is reached when AQ=ZLω02 where A is the package acceleration, Q is the total (loaded) quality factor, ω0 is the angular resonant frequency and ZL is the maximum displacement. In a previous work [29], we demonstrated the concept on an open device with a relatively high mechanical Q of about 200. Here, we demonstrate the concept on an encapsulated device with a rather low Q of about 4. As in [29], we compare to a reference device of the same die dimensions.
Section 2 of this chapter details the motivation and working principle for the impact-based electrostatic device. The MEMS-implementation of the concept, made in the Tronics MPW foundry process [31], is described in detail in Section 3, modelled in Section 4 and characterized in Section 5.
A schematic model of a traditional harvester is shown in Figure 1. With such a design, the typical behavior in frequency sweeps is a clipping of the resonance peak and the occurrence of a jump phenomenon on the high frequency side of the clipped peak. With increasing amplitude, the output power eventually saturates, at least approximately. These effects have been observed in several devices, e.g. in a mesoscale electromagnetic harvester by [16], a mesoscale piezoelectric harvester [18-19] and a microscale electrostatic harvester [17]. Some examples of measured and simulated characteristics of a microscale electrostatic energy harvester from [30] are shown in Figure 2 which displays “clipping“ of the response and extended up-sweep bandwidth, and Figure 3 which displays saturation.
The clipping of the response in Figure 2 and the saturation in Figure 3 are direct negative consequences of the displacement limit. Whether the end-stop impacts are elastic or give loss of kinetic energy, is not significant for the output power when the vibrations are sinusoidal and at the resonant frequency [19]. Loss at end-stop impacts mainly affects the phase relationship between the driving force and the displacement. This has consequences for the value of the jump-down frequency in the up-sweep (at about 1450 Hz in Figure 2) and the details of displacement waveform. The displacement waveform may even show period doubling or chaotic-like behaviour without significant deviation from the saturation characteristic in Figure 3, see [32]. If we are mainly concerned with vibrations at the resonant frequency, we are then free to design the end-stop with any degree of loss that we deem suitable without compromising the output power performance.
A schematic illustration of typical energy harvesters including a spring-dashpot mass system with use of mechanical end-stops to limit mass motion
Up-sweep frequency response of RMS output voltage for different acceleration levels at bias voltage Vb=30V. From [30].
Output power versus acceleration amplitude for different bias voltages. From [30].
The observation that end-stop loss is not important suggests that it can be beneficial to design end-stops that are also transducers. The concept is illustrated by velocity damped end-stops in Figure 4. If these secondary transducers can scavenge significant proof-mass kinetic energy at each impact and convert it to electrical energy, we obtain power in addition to that already available from a primary transducer that will be present and associated with the proof-mass motion anyway. The questions are then how these transducing end-stops can be made and, since some chip real estate must be allocated for them, if this approach has any advantages over using the entire area for a conventional device.
A schematic illustration of device concept with use of end-stops as additional transducers
Here we consider a MEMS realisation of a device based on internal impacts as motivated in the previous section.
Figure 5 shows an impact-device design. There are three independently suspended structures that constitute the device: one primary mass with its electrostatic transducer (ET1) and two secondary electrostatic structures with their own transducers (ET2) acting as end-stops to prevent the ET1 proof mass motion. The ET1 is an ordinary comb-drive structure driven by a movable proof mass mp attached to four linear-springs with total stiffness kp and a corresponding damping coefficient bp. For in-plane motion of the primary mass, the output power is scavenged by the ET1 transducer which has two overlap-varying capacitances with opposite phase. The ET1 transducer is characterised by a capacitor finger length lp, a capacitor finger width wp, gap between fingers gp, an nominal finger overlap x0p and Np fingers on each side.
Schematic layout of the impact device with additional secondary electrostatic transducers functioning as end-stops for the primary mass
A total view of the impact device with primary and secondary electrostatic transducers (Photograph: Tronics Microsystems S.A.)
A close-up view of the primary and secondary transducers of the impact device (Photograph: Tronics Microsystems S.A.)
Secondary spring and gap-closing transducer of the secondary structure (Photograph: Tronics Microsystems S.A.)
The ET2 transducer has a mass ms and a mechanical damping bs. The ET2 suspension is designed as two single beams with a total spring stiffness of ks, giving much larger resonant frequency than that of the ET1. The ET2 uses a gap varying capacitance with capacitor finger length ls, finger width ws, finger overlap x0s and a nominal capacitor gap gs. The number of fingers is Ns for each ET2 electrode. Rigid end-stops are used to limit the ET2 motion under overload conditions. The gap varying transducer was chosen for ET2 in order to obtain a large capacitance variation for a small displacement.
The ET1 and ET2 are accelerated in the same direction. Assuming negligible inertial actuation of the secondary structure, the impact between the primary and secondary masses occurs when the displacement amplitude of the primary mass reaches the limit dps, exciting the secondary transducers to generate extra output power. The maximum displacement amplitude of the secondary mass is dss. The impact is a contact of bumps on flat surface designed on the mass shape. The cylindrical bumps have semi-cylindrical geometry with radius R. The modelling of this kind of structures was investigated in [30].
The die is 8×4mm2 and is fabricated in the Tronics MPW (multi-project wafer) service with high aspect ratio micromachining of the 60µm thick device layer of Silicon-on-Insulator (SOI) wafers [31]. Figure 6 shows the full view of the device. The ET2 mass ms is significantly smaller than the ET1 mass mp, mp=14ms. Effort is made to utilize the available area. Placing the supports within the area of the proof mass, makes sure a minimum area is wasted so the proof mass can be as large as possible while leaving the entire length of proof mass available for the comb-drives.
Figure 7 shows a close-up view of the primary and secondary masses. The ET1 proof mass is attached to four springs. The springs in this device are designed as folded flexures with released stress in the axial direction, resulting in the linear beams for transverse motion. The ET2 spring design makes use of two single beams separated by a distance of 2.5mm, giving linear behavior within the ET2-structure travel length. In order to secure predictable beam widths, protection beams oriented in parallel with the spring beams are included to reduce over-etching of the spring beam during fabrication. With this counter measure, we expect the beam cross-section to be closer to the ideal rectangular shape, and therefore its stiffness to be close to the design value. The measured resonance frequency deviates approximately 1.5% from the design value.
There are four metal pads on anchors: two placed on the ET1’s anchors and two deposited on the ET2’s anchors. They connect to voltage sources used for external biasing in the experiments. Four remaining metal pads are placed on the fixed electrodes to connect the external load resistances.
Figure 8 presents details of the ET2. We see the gap-closing transducer and the bump geometry for the contact regions between the ET1 and ET2 structures. The spring anchors of the ET2 structure also function as rigid end-stops that restrict maximum displacement of both ET1 and ET2 structures to avoid contact between fixed and counter electrodes. All of the device parameters for the ET1 and the ET2 are listed in Table 1.
Parameters | Primary structure | Secondary structure |
Die dimensions | 8×4mm2 | |
Device thickness, t | 60µm | |
Length of capacitor fingers, lp, ls | 25µm | 30 µm |
Width of capacitor fingers, w | 4µm | 4µm |
Gap between capacitor fingers, gp, gs | 3.0µm | 3.5µm |
Number of capacitor fingers on each electrode, Np, Ns | 416 | 225 |
Nominal capacitor finger overlap, x0p, x0s | 10µm | 25 µm |
Length of spring Width of spring | 500µm 6.5µm | 350µm 5.2µm |
Distance between primary and secondary masses, dps | 4.0µm | |
Distance between secondary mass and rigid end stops, dss | 3.0µm | |
Bump radius, R | 30µm |
Parameters of the impact device: primary and secondary electrostatic transducers
Figure 8 shows a view of the reference device with its in-plane overlap-varying transducer. The transducer is similar to the primary transducer of the impact device. Both device prototypes have the same chip dimension. The reference transducer has a larger area for the proof mass and a slightly higher transducer capacitance than the ET1. This is due to more space being available within the same chip real-estate when there are no transducing end-stops. The reference proof mass is suspended by four folded flexure beams. The beams are connected to fours anchors acting as rigid end-stops to confine maximum displacement of the proof mass. This device was described in detail in [30] where end-stop modeling was studied.
A view of the reference device with the same die dimension [30] (Photograph: Tronics Microsystems S.A.)
The reference transducer is also biased externally. The output voltage is simply connected to load resistance via the metal pads deposited on the fixed electrodes. Further details of the reference-device geometry can be found in [30].
As a check that the device operates according to our understanding, we will compare measurements to simulation. At the lumped-model level, the device dynamics is governed by a few nonlinear differential equations that can be solved by a variety of numerical tools. We prefer to use a circuit simulator as a solver, i.e. LT-SPICE, and therefore need to formulate the dynamics as an equivalent circuit. The overall scheme of the modelling is the same as we previously used for our previous MEMS devices [30]. Special features here are that there are 3 mechanical degrees of freedom in the impact device, the proof mass position and the position of each of the two secondary structures, and that the impacts are very crucial for the operation.
Lumped model of the impact device: a) primary electrostatic transducer (ET1) and b) secondary electrostatic transducer (ET2) in one port
The device is modeled as showed in Figure 10 which gives equivalent circuits for the mechanical and electrical parts of the primary structure and of a secondary structure. The proof mass displacement of the ET1 and ET2 are characterized by two variables xp and xs giving the displacement from the nominal position. An impact between the primary and secondary masses takes place for the relative displacement beyond the limit of dps. Similar to the reference device, the ET2 design also has its own rigid end-stops to prevent its proof mass motion from extremely high acceleration which probably causes collapsing effects. The rigid end-stops are engaged for secondary displacements larger than a maximum distance dss. The force Fss between the ET2 and the rigid end-stops are modeled using behavioral voltage sources as described in [30]. All model parameters of the impact device are listed in Table 2. Both ET1 and ET2 have the same bias voltage Vb. Due to the similar design of the reference device and the primary ET1, their optimal load resistances are almost equal. For simplicity the ET2s are also connected to the same load resistance RL as the reference device and ET1, giving a straightforward later comparison between the outputs of the two devices.
Parameters | Primary ET1 | Secondary ET2 |
Inertial proof mass, mp, ms | 2.1mg | 0.15mg |
Spring stiffness, kp, ks | 115.1N/m | 29.5N/m |
Damping coefficient, bp, bs | 4.0e-3Ns/m | 0.2e-3Ns/m |
Nominal variable capacitance, C0p,C0s | 1.3pF | 1.6pF |
Parasitic capacitance, Cpp, Cps | 17.9pF | 6.0pF |
Load resistance, RLp, RLs | 4.9MΩ | 4.9MΩ |
Load parasitic capacitance, CLp, CLs | 4.2pF | 2.0pF |
Model parameters of the impact device: primary and secondary electrostatic transducers
Figure 11 shows the frequency response of the impact device compared with the reference device response for an RMS acceleration of 0.71g and a bias voltage Vb=7V in linear regime. At the small acceleration level, the primary mass motion is below limit and then there is no impact between the masses. The output power of the impact device is mainly from the ET1 transducers. The simulation results fit well to the measured results. The ET1 behaves similarly to the reference device. In design, the resonance frequency of the ET1 is the same as that of the reference device, but due to the over-etching effects, the ET1 resonance frequency is slightly smaller, about 1168Hz. The output power of the reference device is bigger than that of the ET1 at the same frequency. For example at the resonance frequency, the ET1 output power is 0.06nW, about three times less than the value of the reference device. The lower output power of the primary transducers originates from the smaller primary mass mp<m and the smaller transducer capacitance C0p<C0. This is due to the area sacrificed for the secondary transducers in the impact design.
Frequency response of the impact device compared to the reference device for bias voltage Vb=7V and RMS acceleration of 0.71g
Fig. 12 shows the output power for frequency up-sweeps for each transducer in the reference and impact devices at Arms=5.5g, which is sufficient to cause impacts between the primary and secondary masses. Compared with the reference output power, the primary output power is still smaller around the resonant frequency. The output power of the secondary transducer is significant in a frequency range from 1.15kHz to 1.30kHz. For example, at a frequency f=1.22kHz, the secondary output power is 108nW, but the output power of the reference and primary transducers is only 3.0nW and 1.4nW respectively. The energy from the impact is effectively utilized by the high transduction of the secondary transducers in the impact device. The nonlinear effect of the rigid end-stops is evident in the reference output response as saturation of the output power and occurrence of the jump phenomenon. The impacts have no performance benefit beyond up-sweep bandwidth enhancement in the reference device. Performance of the impact device is also modeled and simulated. The simulation results capture the main features of the measurements and thereby confirm that the essential mechanisms in the device have been identified.
A wider response bandwidth is obtained in simulation for the secondary transducer, while the primary transducer response behaves qualitatively like the measured result. The main differences between the measured and simulated results can be explained from the modeling of the impacts. We have seen in the simulations that when varying the loss in the impact model, the bandwidth is affected so inaccuracy in the loss representation can be at least partly responsible for the discrepancy. In addition, the design values for the device geometry have been used in the model and therefore small deviations in the distance of travel before impact could influence the impact events and thereby the bandwidth.
Output power frequency responses of the primary and secondary transducers in the impact device compared to the output from the reference device at RMS acceleration of 5.5g and bias voltage Vb=7V
Figure 13 compares the output powers of the reference and impact devices under bias voltage Vb=7V and at their resonance frequencies. For the impact device, the total output power is the sum of the primary and secondary output powers. For small accelerations, the primary mass does not impact on the secondary mass. The total output power is only contributed from the primary transducer. As a result, the total output power of the impact device is less than that of the reference device. For RMS accelerations larger than 3.5g, the primary mass begins to impact on the secondary structure. The secondary output voltage is dominant for RMS accelerations greater than 4.5g, giving a total output power significantly higher than that of the reference device. For example, the total output power of the impact device is approximately 200nW at an RMS acceleration of 5.5g, 33 times greater than RMS output power of the reference device which is 6.1nW. Further increase of the acceleration amplitude causes the secondary structure’s motion to be limited by its own rigid end-stops. In this case, the impact device saturates at much higher output power level than the reference device does. Since the proof-mass displacement constraint is the same for both devices and the masses are about the same, this means that the secondary transducers have provided a dramatic increase in the harvester effectiveness.
Figure 14 illustrates the frequency response of the total output power of the impact device and the reference device in frequency up- and down sweeps at an acceleration amplitude of 5.5g and bias voltage Vb=7V. The total output power of the impact device is considerably higher than that of the reference device in the frequency range of secondary-transducer activation. There is no jump phenomenon or hysteresis in the frequency response of the impact device at such accelerations. This differs from the reference device.
Comparison between measured output power of the reference and impact devices for bias voltage Vb=7V at resonance frequencies
One notable difference between this encapsulated low-Q device and the unpackaged high-Q device presented earlier [29] is the large difference in the RMS acceleration that is required for end-stop engagement. It is rather obvious that the difference in Q is responsible for this, but for both devices a subsequent additional increase in RMS acceleration is necessary before the output of the secondary transducers become appreciable, in the present device from 3.5g to 4.5g. For future designs, measures should be taken to ensure that the end-stop transducers are effective already when actuated a small distance so as to narrow down this range of RMS accelerations.
The merits of the impact-device concept can be quantified through the figure of merit energy harvester effectiveness as defined in [1]. Some example values are given for the present packaged device and a previous unpackaged device in Table 3. For conventional energy harvesters operating in the linear regime with displacement less than the limit Xmax, the effectiveness is proportional to the acceleration amplitude. Then, it degrades as the acceleration amplitude increases beyond the value needed to reach the maximum amplitude Xmax. This behaviour is displayed by the reference-device values in the table. With the active end-stops, the extra power improves the harvester effectiveness under displacement-limited operation. For the packaged devices presented in this book chapter, the effectiveness of the impact device is 4.25%, while this value is only of 0.11% for the reference device in the impact regime. The high mechanical quality factor Q in the previous unpackaged devices gives an even larger effectiveness up to 23.12%. Microscale energy harvesters have typical effectiveness in the range from 1% to 10% and it is lower for smaller displacement limits [1]. The impact devices therefore achieve effectiveness values under displacement-limited operation that are comparable, or even favourable, in comparison with other device prototypes in [9, 15, 17-18, 33-36] with the same scale of the displacement limit. Together these examples show that there is much to be gained from transducing end-stops.
Frequency response of the measured output power of the reference and impact devices for a RMS acceleration of 5.5g and bias voltage Vb=7V
Packaged prototype, Xmax=7µm | Unpackaged prototype, Xmax=10µm [29] | ||||||
RMS acceleration amplitude [g] | Effectiveness [%] | RMS acceleration amplitude [g] | Effectiveness [%] | ||||
Reference device | Impact device | Reference device | Impact device | ||||
Linear regime | 2.10 | 0.074 | 0.037 | Linear regime | 0.04 | 11.18 | 9.56 |
4.19 | 0.145 | 0.078 | 0.06 | 17.42 | 14.81 | ||
Impact regime | 5.15 | 0.139 | 3.017 | Impact regime | 1.76 | 5.40 | 14.44 |
5.50 | 0.106 | 4.249 | 1.87 | 5.09 | 23.12 |
Comparison of harvester effectiveness between the reference and impact devices
An electrostatic energy harvester with in-plane overlap-varying transducers on the primary mass and with secondary gap varying transducers as end-stops has been designed, modeled and characterized. The simulations are consistent with the measurement results. The performance was compared with that of a standard in-plane- overlap-varying type device. With the transducing end-stops we have seen that a considerable performance boost is obtained, with output power up to a factor 33 over the reference device, even though the reference device performed a factor of 3.4 better at low acceleration levels. None of the typical jump phenomena were observed in up and down frequency sweeps for this device. The frequency response of the impact device had approximately the same bandwidth as the reference device had on down sweeps.
This work was financially supported by the Research Council of Norway under grant 191282. We thank Prof. Eric Yeatman and Prof. Oddvar Søråsen for useful discussions and suggestions.
Falls from height cause significant death and disability worldwide, due to the severe traumatic load inflicted on their victims [1, 2, 3, 4]. According to the WHO, the yearly mortality due to suicide worldwide is approximately 800,000 people. What is more important is the fact that it affects mainly young people, suicide being the primary cause of death in the age group of 25–34 years [5]. The mean incidence of suicides across Europe in 2013 was of 11.7 deaths per 100,000 people. Low rates, under 8 deaths per 100,000 inhabitants were recorded in Italy, Malta, Cyprus and the United Kingdom. The lowest incidence was observed in Greece (4.8 cases per 100,000 people) [6]. There was a lag between the beginning of the economic crisis in Europe, and the manifestation of its effects on the Greek population. These became evident 3 or 4 years later, in the form of a reduction of household income and an increase in the rate of unemployment [7, 8, 9].
Causes for this mechanism of injury include both accidental falls and deliberate suicide attempts [10]. The latter constitutes a major social problem, with implications for the entire society, but particularly for the affected family. The psychological profile of people committing suicide is complex and unique for each case [11]. Thus, identifying contributing factors that may lead to suicide and establishing strategies for the safekeeping of mental health in communities are of paramount importance.
The type of injuries incurred after a fall constitute a unique pattern of blunt trauma, with a characteristic distribution of damage (multiple lesions in a variety of body areas) [1, 12, 13]. The most common form of trauma are fractures, followed by other areas, such as the head, the thorax, the abdomen as well as the retroperitoneum, being injured by varied degrees [14]. The quantity and the quality of traumatic load absorbed depend on factors like the height from which the fall occurred, the part of the patient’s body that had the first impact, the surface where the impact occurred and the victim’s age, taking into account the associated comorbidity, and reduced physiologic reserve that advanced age implies [15, 16, 17]. Anticipation and prediction of the exact areas being injured are not possible, because of the multitude of factors involved, and the exact unpredictability of the fall’s kinematic [18, 19].
As aforementioned, one can infer that the differential diagnosis of falls from height from other types of blunt trauma (for example, a road-traffic-collision with expulsion of the occupants from the vehicle) is difficult. Thus, a high index of suspicion must be maintained concerning the initial cause in cases of polytrauma in victims with an unknown history [20]. An array of papers have dealt with injury-related deaths in general, while others have differentiated between unintentional and intentional injury-related deaths [21, 22, 23, 24]. There are few studies though that have looked into patients with intentional or unintentional injuries, due to a fall from height, at a single centre [13, 25].
As noted by research in the past, self-harm due to a fall is a rare phenomenon, being responsible for 4–7% of deaths from suicide in the developed world [26, 27, 28, 29]. On the other hand, studies have shown that psychiatric disorders are a frequent finding in patients suffering trauma [30, 31, 32]. Nevertheless, the connection between mental disorders and specific injury patterns has not been adequately described. Furthermore, the elucidation of patterns of injury incurred after accidental falls and after intentional suicide jumps, might be of help to forensic pathologists while investigating the circumstances of a death after a fall from height.
From January 1990 to October 2012, 64 patients (15 males and 49 females) were studied as a result of falls from height. Fall from height ≥ 3 m is classified as high energy trauma in accordance to ATLS guidelines [33]. The mean patient age was 34 years (range 16–65 years). These 64 cases comprised our series and, for comparison, were divided into those without mental disorders (n = 32, group I) and those with mental disorders (n = 32, group II). Group II cases were further stratified according to their psychiatric diagnosis.
The principles of Advanced Trauma Life Support were followed in the management of all patients. Basic laboratory screening included haemoglobin level, prothrombin time, type and crossmatch and arterial blood gas analysis. Data collected included age, gender, associated trauma, injury severity score (ISS), Glasgow Coma Scale (GCS), haemodynamic status (systolic blood pressure less than 90 mm Hg on arrival), length of intensive care unit (ICU) and hospital stay.
Also, the following trauma variables were analysed: specific intracranial injuries (epidural, subdural and subarachnoid haemorrhage and brain contusion), spinal injuries (cervical, thoracic and lumbar spine), thoracic injuries, specific intra-abdominal injuries (liver, spleen, kidney, and hollow viscus) and specific fractures (pelvis, femur and tibia). The diagnosis of mental disorder was ascertained by psychiatric specialists using the criteria of the International Classification of Disease Ninth Version Clinical Modification (ICD-9CM).
The mean height of fall was 5.4 m (range, 3–25 m). The patients were separated in two groups: group I, without mental disorders (n = 32), and group II, with mental disorders (n = 32). The demographic data, including age, gender, height of fall, ISS, GCS, initial shock (SBP <90 mm Hg), hospital stay (days), ICU stay (days) and deaths are summarized in Table 1. The mean hospital stay was 29 days (range 19–45) and the mean ICU stay was 9 (range, 5–13) (Table 1).
Data | Patients |
---|---|
Age | 35 (18–65) |
Gender (M:F) | 15:49 |
ISS | 20 (12–58) |
GCS | 9 (6–13) |
Haemodynamic status-SBP <90 mmHg | 34 |
Hospital stay (days) | 29 (19–45) |
ICU stay (days) | 9 (5–13) |
Deaths | 13 |
Comparisons of demographic data of patients with suicide attempts from height.
Concerning their background psychiatric disorder in group II, the diagnosis was schizophrenia in 32 patients, depression in 12, drugs or alcohol abuse in 3, personality disorder in one, manic depression in one, another psychiatric condition in one and 14 cases without a specific diagnosis (generally marital or work related).
Patients due to suicide attempts from height comprised of 15 males and 49 females with a mean of age 35 years (range: 18–65 years). Of those, 16 were single, 14 were married and 2 were divorced. Thirty-three patients were employed, 6 were housewives, 7 were unemployed, 3 were students/pupils and 15 had various occupations. As far as religion was concerned, 48 were Christian Orthodox, one Roman Catholic, one Jewish, one Muslim and 13 of other religions.
Regarding their family status: 20 had children, 6 had only their parents, 3 had only their spouse, 2 had a step family, 2 had parents who were divorced, 6 had parents and/or siblings, one had both parents and children and 24 had no family at all.
The falls had occurred from a roof or balcony in 39 cases, from a window in 12, from a bridge in 7 and inside the house in 6. The mean injury severity score (ISS) was 20 (range 12–58) for all victims of fall. Sixteen patients arrived at the emergency department in shock. The most common body region having sustained severe trauma were the fractured extremities and/or spine, followed by the chest, the head and the abdomen for both groups (Table 2).
Fall from | Patients |
---|---|
Roof/balcony | 39 |
Window | 12 |
Bridge | 7 |
Inside the house | 6 |
Associated injuries | |
Abdominal trauma | 4 |
Thoracic trauma | 32 |
Head injuries | 16 |
Extremity fractures | 199 |
Spinal fractures | 32 |
Location where the fall occurred and associated injuries.
Head injuries were revealed by CT scan in 16 patients. The mean GCS was 9 (range 6–13) for both groups. The most common intracranial injury was brain contusion and subarachnoid haemorrhage, followed by subdural hematoma and epidural hematoma. The incidence of subarachnoid haemorrhage in the suicide group was significantly higher than in the accidental group.
Associated abdominal injuries were present in 4 patients. The most common injury was liver laceration, followed by kidney and spleen laceration. One died with an operative finding of a large central retroperitoneal haematoma due to a vena cava rupture. In the remaining 3 patients, ultrasonography showed minimal intraperitoneal blood and these patients were not operated on. Thoracic injuries were present in 32 patients. The most common of these were rib fractures—26 cases. Twelve of these patients had a haemopneumothorax and 6 had a sternum fracture. Conservative treatment with assisted ventilation was necessary in these cases (Table 3).
Patients | |
Skull, thorax and upper extremities | |
Skull | 16 (25%) |
Shoulder | 4 (6.2%) |
Scapula | 6 (9.3%) |
Sternum | 6 (9.3%) |
Ribs | 26 (40.6%) |
Humerus | 8 (12.5%) |
Elbow joint | 8 (12.5%) |
Distal radius | 7 (10.9%) |
Hand | 4 (6.2%) |
Spinal fractures | 32 (50%) |
Pelvis | 27 (42.1%) |
Lower extremities | |
Acetabulum | 9 (14%) |
Femoral neck | 38 (59.3%) |
Femur | 18 (28.1%) |
Knee joint | 17 (26.5%) |
Tibia | 19 (29.6%) |
Ankle joint | 36 (56.2%) |
Calcaneum | 34 (53.1%) |
The distribution of fractures in percentage across body region for the two groups of patients.
Upper extremity fractures were found in 37 patients, while pelvic and lower extremity fractures were found in 198 cases. Spinal fractures were noted in 32 patients. As far as the level of injury was concerned, in 16 cases, it was in the lumbar level, in 9 cases in the cervical, in 5 cases in thoracic and in 2 cases the sacral vertebrae were concerned. Regarding the neurologic deficit, in 23 cases, the injury was incomplete (14 with ASIA C and 9 with ASIA D), and in 9 cases, it was complete (4 with ASIA A and 5 with ASIA B). Further details with our data of 32 patients with spinal cord injury as a result of deliberate self-harm have been published previously [34]. It seems that the neurological complications of spinal injuries were correlated with the increase of the height from which the fall occurred.
Patients with psychiatric disorders were more frequently shocked on arrival at the emergency department than those in the accidental group, the most common reason for death being head injury. Fatalities were more common when patients fell from greater heights (over 4 m), or when their head hit a hard surface, such as concrete.
The final causes of inpatients’ death were: head injury in 8 cases, multiple organ failure in 3 cases, pneumonia in one case and cardiac complications in another one. The majority of patients who died of organ failure had sustained significant head injury. In one case, death occurred after a second suicide attempt 2 years later.
Each patient underwent a psychiatric evaluation by a consulting psychiatrist as soon as his condition and cooperation permitted. The assessment comprised of an interview. Regarding the type of treatment for the spinal fracture—dislocations, instrumentation devices included titanium rods, transpedicular screws, sacral bars and bone grafting in all patients. No new suicide attempt was recorded during the hospital stay.
All patients were discharged from hospital approximately 6–8 weeks after the operation with a custom-made thermoplastic thoracolumbar or lumbosacral orthosis for another 8 weeks and instructions for physical therapy and rehabilitation programs. The mean follow-up was 6 years (12 months to 10 years range). At follow-up, 27 patients were available for evaluation due to the death of 5 patients, 1–3 years post initial injury, because of suicide in one case (patient 7 of group II) and medical complications in 4 cases [renal failure in 3 cases (patients 8, 14 and 30 in group II) and pneumonia in one (patient 21)]. In the remaining patients, new unsuccessful attempts were recorded in 2 cases (7%) due to psychiatric disorders, 1–3 years after the first attempt (patients 10 and 24). All survivors received psychiatric follow-up. The overall mortality was significantly higher in those patients who fell from more than 10 m.
Suicides and suicide attempts constitute a major concern for public health services, with implications for both families and society [35]. Trauma incurred due to falls from height poses a great burden on health services due to its severity. This is particularly important if we take into account the fact that this is a largely preventable mechanism of injury. Prior knowledge of the possible traumatic patterns incurred after a fall from height can prove helpful in the initial evaluation of this group of patients. From an epidemiologic point of view, trauma due to falls may occur across all age groups, but it is the two extremes, the very young and elderly, which are particularly susceptible to it [36].
In this study, we have considered two groups of patients. Group I represented patients with no mental disorders and group II with mental disorders. It is quite difficult to identify someone who is prone to committing suicide. In addition, the observed number of suicides and suicide attempts being committed at a younger age (i.e. adolescence) has been a cause of concern worldwide and particularly in Europe [37]. The male-female ratio of suicide attempts varies across age groups. Thus, in the younger age group (15–24 years old), it is 1:1.9; and in the middle age group (45–54 years old) it is 1:1.7. This ratio further decreases for those older than 55 years to 1:1.4 [38]. In this study, the male-female ratio was 1:3. The female sex was associated with an increased likelihood of death due to a higher amount of energy involved in their attempted fall.
According to other studies [39, 40], young males tend to repeat suicide attempts more frequently than females and the methods used by them lead to an increased mortality. A suicide attempt in the past is a red flag for a possible attempt in the future; so, there is a strong correlation between suicide attempts and deaths from suicide both regionally and nationally, and particularly in young males [41]. Also, there is a strong correlation between repeated attempts and completed suicide, especially in the group of males who have used a violent method [42, 43].
The study by Dickson et al. had the aim of establishing a correlation between mortality and various factors, such as the patients’ injury severity score (ISS), the height from which the fall took place, the patient’s intention and the body regions that were injured. In addition, the height of the fall strongly correlated with the patient’s ISS and was an important predictor of mortality [44]. Head and/or chest injuries, if due to a fall from height, were strongly associated with an increased incidence of death. According to the authors, this mechanism of injury should be a triage priority when tasking ambulances. In addition, the best way of treating these injuries is their prevention. No other significant predictors of mortality were found in this study.
In the case series by Kent and Pearce, 282 suicide attempts were studied, 13 of which were completed. Of those, 8 happened at home, all patients were older than 49 years; and in 7 out of 8 deaths, ladders were implicated [45]. The retrospective study by Petratos et al. analysed in detail the musculoskeletal traumatic pattern resulting from falls from height, and focused particularly on the correlation between specific fracture patterns and the height from which the fall happened, as well as on the causation of the fall (suicide attempt vs. accident). According to their findings, with an increase in the height from which the fall occurred, the frequency of limb, thoracic and pelvic fractures also increased. Such a correlation was not evident for head injuries. Nevertheless, the anatomical regions having sustained fractures (including the cranium) varied in accordance with the height of the fall. Thus, we can infer a mechanism of injury that is varying proportionately to the height of the fall. There was no significant difference between the patients who attempted suicide and those who fell by accident as far as the number of fractures incurred or the regions having been injured were concerned. Nevertheless, with regard to our results that have been published previously, patients who attempted suicide had a significantly greater number of bilateral lower limb fractures than their accidental fall counterpart. In addition, logistic regression analysis shows a significant correlation between the cause of the fall and the presence of lower limb fractures. According to the authors, further research is necessary in order to establish a correlation between incurred traumatic pattern, the height of the fall and the patient’s intention [46].
Choi et al. in his recent study attempted to differentiate the characteristics of traumatic pattern between intentional and non-intentional falls [47]. In addition, he attempted to determine prognostic factors for suicide attempt-related injury and promote adequate measures for the prevention and management of such injuries. In this study, 8992 patients with an accidental fall (non-intentional group) and 144 patients who committed a suicide attempt (intentional group) were included. Falls from a height greater than 4 metres were more frequently encountered in the intentional group. Death prior to patient’s arrival in the accident and emergency department occurred in 54.9% of the cases of suicide attempt. Patients within the intentional group, having sustained increased traumatic load, had fallen from higher, were older and were more likely to be of lower educational level (high-school graduates, instead of college). Due to the fact that injuries sustained after an intentional fall were more likely to have a reserved outcome, the authors highlighted the importance of prevention. Such measures include telephone support and counselling lines, the installation of signs advising against suicide in high risk areas for an intentional fall, such as bridges, along with suggestions for government-coordinated programs aiming for the education of the public and the improvement of social conditions generally and the support of the community and family in particular.
The reasons behind a suicide attempt are multifactorial, hard to quantify and unique in every case. Nevertheless, the study of multiple suicide attempts puts into evidence some risk factors that would lead to such a decision. These are common across all age groups and include: the presence of mental illness, either currently or in the past, a history of alcohol or drug dependence, as well as the presence of depression [10]. Epidemiologically, one out of five persons who have attempted suicide will try once more within a year, and 10% of them will succeed in the end. Drug ingestion is the most common mechanism for a suicide attempt. Violent mechanisms such as hanging, falls from height and use of weapons are not common [48]. The persons who have attempted suicide by falling from height usually become polytrauma patients. The types of injuries incurred are two: deceleration injuries due to inertial phenomena, usually at viscera with vascular pedicles, and direct impact injuries [49].
The severity of fractures incurred will depend on factors like the area over which the impact is applied [50]. The smaller the area of spread of the impact, the greater the local load. Therefore, patients landing on their legs tend to suffer more severe injuries than those who have landed on their flanks, or prone, or supine [51]. Patients due to accidental falls mostly suffered spinal fractures and upper extremities fractures in an attempt to protect themselves. Patients due to suicidal high falls attempts suffered mostly of lower limb fractures, pelvis, spinal fractures and head injuries. Distal radius and hand was the most common affected region in upper extremities in patients with non-intentional falls, in an attempt to protect mainly the head and grab something stable to prevent further fall. In patients with intentional falls, kinetic energy is absorbed mainly by the lower limbs, pelvis, spine and head, leading to characteristic fracture patterns [52]. The most common cause for death is head injury [51, 53, 54] and this is accordance to our results. Turk and Tsokos reviewed 68 medicolegal autopsy cases (22 females, 46 males, age range 13–89 years) of fatal falls from height from 1997 to 2001 [55]. The cause of instant death was head trauma in 24 (35%), internal blood loss in 9 (13%) and polytrauma in 30 (44%) cases. Other causes of death, when the individuals survived the trauma for a longer period, included septic multiple organ dysfunction syndrome and pulmonary embolism. In general, suicides were from greater heights than accidents (mean height 22.7 m for suicides and 10.8 m for accidents, respectively). Strikingly, severe head injuries predominantly occurred in falls from heights below 10 m (84%) and above 25 m (90%). Head trauma was the cause of death in 11 of the 19 cases that were from 9 m or less (58%). Of all cases, 51 (75%) died within a few minutes. A survival time of several hours up to 1 day was observed in 8 cases. Nine patients survived for several days (up to 16 days). Five of them fell from heights below 10 m. Patients with intentional fall from height have a higher early mortality than patients due to accidental fall from height [56].
The easiest way to underline the suspicion that the mode is suicide is if a suicide note is found at the jumping site; this is, however, closer to being the exception than the rule. Analysing the distance of the body from the site of descent may sometimes also help us determine the manner of death. The distance of the body from the site of descent includes the falling height and the horizontal distance. The falling height in suicide was statistically higher than that in accident [57, 58]. For similar heights, Wischhusen et al. have demonstrated that in passive falls, the horizontal distance is usually farther than jumps [59]. From a mechanical point of view, during a fall from height, potential (dynamic) energy is converted into kinetic and this leads to fractures upon impact. Another important factor of the severity of injuries is the height of fall, as the kinetic energy is increasing due to acceleration during the fall and is maximum at the time of impact [60]. In suicide falls, kinetic energy is absorbed mainly by the lower limbs, pelvis and spine, leading to characteristic fracture patterns. In accidental falls, patients most probably extend their arms and flex their hips, which lead to a damping effect that protects the spine [61]. Hence, the most important determinant of survival after a free fall is the position of the body at the time of impact [49]. There were only 3 patients (cases 1, 22 and 31) in group II who have sustained solely upper extremity fractures. The most common body position at the time of impact is with the patient standing and landing with the lower extremities first. This usually leads to calcaneal or pilon fractures, as well as thoracolumbar fractures. If the impact takes place with the patient seated, then higher thoracic or cervical injuries are more likely to happen, which are associated with a higher rate of mortality. Finally, an unpredictable fracture pattern takes place when the victim suffers multiple secondary impacts, in various postures, after bouncing from the primary impact. The amount of injury incurred will depend on the rate of dissipation and absorption of energy, through the patient’s body.
According to the paper by Teh et al., there is a difference to the traumatic pattern incurred by jumpers compared to fallers [13]. Namely, the jumpers tend to impact their dominant lower limb first, as well as sustaining right sided thoracic injuries in the process. We did not confirm the above-mentioned findings in our study. The severity of spinal cord injuries was more important in the suicide than the accidental group [52]. This was in accordance with studies performed in the past, which also showed the early neurologic involvement in such cases. As far as prognosis of spinal cord injury is concerned, complete injuries will be unaltered both in level and extent in a year’s time. On the other hand, incomplete injuries may show signs of improvement for a period of 2 years after the impact [62]. Our results regarding prognosis for ambulation in ASIA A patients and for functionality in ASIA C patients are in accordance with current knowledge [63].
Anderson et al. performed a retrospective study, regarding the rehabilitation outcome of patients with spinal cord injury, as a result of deliberate self-harm (DSH) [29]. According to them, spinal fractures in the DSH group were mainly the result of falls from height. Underlying causes were revealed, such as psychiatric disorders and substance abuse, necessitating formal psychiatric review. There was no difference in short-term rehabilitation results between the DSH and accidental spinal cord injury group. In addition, DSH seemed to impact the length of stay only in patients with a spinal fracture, but without cord injury.
According to the literature, there are three studies on the subject of acute spinal cord injury following a suicide attempt that stand apart. The first is by Stanford et al. In his paper, 56 cases were followed over a period of 30 years (1970–2000). Fifty five cases were due to a fall from height and one open injury, through the use of a gun. Follow-up of 8 years on average was available for 47 cases (84%). The vertebral levels most frequently injured were C5 and L1. About 23 patients suffered from a complete spinal cord injury and 32 had a severe traumatic load (ISS > 15). The psychiatric background of these patients included personality disorder in 27, schizophrenia in 16, depression in 14 and substance abuse/dependence in 20. Of these patients, 4 were successful in subsequent suicide attempts [28].
The following two studies on this subject are from the UK [26] and Denmark [27]. Both of those are observational and retrospective, with a long follow-up. According to the latter, there is an increasing incidence of suicide attempts and associated spinal cord injury from 1965 to 1987. Approximately one third of the patients who attempted suicide suffered from schizophrenia. According to other papers [64, 65], schizophrenia is strongly correlated with falls from height (from bridges in particular). There were 7 patients in our study who have sustained a fall from a bridge. Damage control surgery principles are followed initially for the treatment of life-threatening injuries and for both limb and spinal trauma [66]. The primary goals of fracture fixation are timely mobilization and safe transfer to psychiatric services. Conservative treatment measures are not usually recommended for this group of patients.
Our findings are in accordance with relevant bibliography [67, 68], regarding the psychiatric background of patients who attempt suicide by falling from height. The spectrum of conditions encountered encompasses bipolar disorder, substance dependence and abuse, personality disorder and schizophrenia.
From an epidemiological point of view, schizophrenia is encountered in 5–10% of cases of suicide attempt. These patients may have well planned their suicide or even suffered from an active self-harm ideation. From the above-mentioned, we gather that management of these patients from a trauma point of view must take into consideration their psychiatric needs. The latter may cause significant disturbance in the delivery of medical care [69]. Most of the patients in this study had a positive response following adequate psychiatric intervention. Hence, we gather that prevention and early identification of persons at risk for a suicide attempt with the use of appropriate screening tools by health care professionals are invaluable.
Education of medical and nursing staff regarding the demands and particularities of care of this population, suffering from both spinal cord injury and psychiatric disorders, cannot be overemphasized. Regular follow-up with multidisciplinary team input and future research are necessary for the provision of high-quality care to this population.
According to the literature, it has been difficult to obtain comparable international data on suicide attempts, owing to disparities in definitions, survey designs and study methods, because the combination of free falls and mental disorders produces a unique group of patients. It has been our experience that psychiatric conditions, and especially the suicidal risk, should be evaluated and treated as early as possible during the orthopaedic or surgical hospitalization. Management requires both psychopharmacological therapy and psychotherapy. It has to be directed towards the achievement of symptomatic relief and, if possible, towards the remission of the primary psychiatric disorder.
The management of these patients in the orthopaedic or surgical ward is difficult, because of restlessness, non-cooperation of the patient and the problem of staff inexperienced in handling the psychiatric patient. When prolonged orthopaedic and rehabilitation management are necessary, it is suggested that the patient be transferred to a psychiatric hospital while continuing the necessary orthopaedic treatment. The outcome data provide critical information concerning those individuals who have attempted suicide and suggests future methods for the identification of suicidal factors.
The authors declare that they have no conflicts of interest.
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