Liquid metal architectures for soft and wearable energy harvesting devices

Thermoelectric energy conversion in which heat is directly converted to electricity has been considered a power source for wearable electronics [6, 8, 95]. Wearable TEGs recover the wasted heat energy from the human body by using the Seebeck effect in which temperature gradients ($\nabla {\text{T}}$) between the skin (heat source) and ambient air (heat sink) applied across parallel p (electron donating) and n (electron receiving) type semiconductors induce current flow [3]. The effectiveness of these semiconductors is captured by the semiconductors figure of merit (ZT), which is described as

$$\begin{equation}{\text{ZT}} = \frac{{{\alpha ^2}\sigma }}{\lambda }T,\end{equation} \tag{ 1 }$$

where $\alpha$ is the Seebeck coefficient, $\sigma$ is the electrical conductivity, $\lambda$ is the thermal conductivity, and $T$ is the temperature [96]. The TEG output power (P_TEG) depends on semiconductor Seebeck coefficient $\alpha$, local thermal gradient ($\nabla T$) and the electrical conductivity (σ) [96]. Researchers have used LM microfluidics as flexible electrical electrodes [97] and LMEE as thermal interfacing between skin and the TEG [27]. These approaches ensure that TEGs have high internal electrical conductivity (σ) and large thermal gradient ($\nabla T$) across the TEG, which is achieved by maintaining skin to TEG contact even under large deformations. Optimizing these factors maximizes the power output of the TEG where ${P_{{\text{TEG}}}} = {\left( {\alpha \nabla T} \right)^2}\sigma$.

One of the major challenges is to provide sufficient mechanical compliance to suit such applications. Flexible thermoelectric modules have been developed through different fabrication techniques including the use of solution processable organic semiconductors [98, 99], fiber-based devices and textiles [100–105], printable nanosheet and thin film based devices [106–108], a screen printed and polymer hybrid device [109], and encapsulation of rigid inorganic semiconductors in soft elastomers [4, 110]. Different material architectures across multiple length scales have been used for fabrication of flexible TEGs. For instance, pellet semiconductors have incorporated soft packaging materials such as PDMS in an effort to replace traditional materials such as copper and ceramics to better harvest energy. This is due to an increased contact area with skin and efficient thermal transport at the material interfaces, enabling interfacing with other wearable devices while maintain high thermal absorption from the skin [111–114]. Moreover, these devices can be utilized to power other wearable electronics like an ECG device [4, 110]. Other wearable TEGs have incorporated nanosheets of printable semiconductors on polyimide and flex PCB to fix rigidity and thickness issues raised by pellet based semiconductors for wearable applications [106, 115–118]. While these nanostructured TEGs are extremely thin and bendable, their voltage and power generation is quite low with 16.9 nW and 4.8 mV, respectively, at a temperature gradient of 47.2 °C [106]. Moreover, some implementations require silver wires and silver paste to wire the semiconductors together [115].

More recent work has begun to incorporate LM into nanowire-based printed semiconductor TEGs in order to reduce the contact resistance between different components. Using LM screen printed interconnects, this device recorded the highest power values at the time of publication for this class of thin-film device (table 1) [119]. Using low-temperature solution-phase synthesis methods [120], bismuth telluride (Bi₂Te₃) and bismuth antimony telluride semiconductors were integrated using a nanowire-based printable ink. Once formulated, the semiconductor ink was inkjet printed onto a polyimide base layer. Using stencil lithography, EGaIn was selectively deposited with a nitrogen assisted spray coating procedure to create flexible interconnects between the p and n type semiconductors. The device was then sealed with a silicone elastomer. The authors reported that when compared to using silver paste for the interconnects, EGaIn interconnects increased flexibility of the device [119]. The device tested had five thermoelectric modules and recorded a maximum power output of 127 nW at ΔT = 32.5 °C. At temperatures consistent with the ambient environment and human body, the TEG generated 14.1 nW and 10.5 mV at ΔT = 7 °C. While these devices are extremely thin and can deform well to the human body, even with LM's incorporation, their low power limits their applicability for wearable devices.

Comparatively, pellet-based semiconductors (which are used in commercially available TEGs) embedded in elastomer composites, pose a solution to these low energy harvesting issues. Pellet based TEGs represent a more traditional design, with semiconductor pellets/cubes connected in series to create a Seebeck effect. The electrical and mechanical performance of pellet based TEGs has also been improved with the development of LM material architectures. For pellet based TEGs, LM was first proposed as an approach to mitigate thermal stress between semiconductors and their connectors [122]. The first device to begin to realize this potential was published over 10 years later in 2017 [97]. It was based around a 16-semiconductor array of Bi₂Te₃ pellet semiconductors that were suspended in an Ecoflex^™ elastomer matrix (figure 2(a)). This device was fabricated in layers with stencil lithography again being used to create LM interconnects [97]. The semiconductors, which were prewetted with LM, were then placed in series with Ecoflex filling in the open spaces. The same process was completed on the top side with Ga deposited again as interconnects and sealed with another layer of Ecoflex. The fabricated devices dimensions were 1.5 × 2.0 cm. At ΔT = 20 °C with resistance load matching, the device's recorded power density was 19.8 μW cm⁻² with one thermoelectric module generating ∼3 mV. The major improvement of this system to previous work is that LM was used as the interconnect material instead of copper or, in some cases, flex PCB [123]. The use of LM allowed for the device to stretch while guaranteeing solid semiconductor to interconnect contacts. Compared to previous semiconductor pellet based TEGs, this device does not fail mechanically or electrically under linear cyclical loading. After 1000 cycles of 20% strain the internal resistance of the device actually decreased by 10% as the loading improved interconnect semiconductor contact [97]. Rigid electrodes often used for interconnects have been identified as a major limiting factor in stretchable electronics in a past review [64].

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This fabrication approach and architecture continues to be widely adopted, with another paper in 2017 implementing 32 Bi₂Te₃ thermoelectric modules suspended in a PDMS matrix with LM interconnects [121]. The semiconductors were pre-wet with EGaIn, followed by EGaIn traces deposited onto the surface connecting the legs together. The completed device had a TEG area of 4 cm² (figure 2(b)). To better emulate real world applications, this TEG was tested on skin with the temperature differential calculated in different air velocity environments and ambient temperatures. The performance of this device was limited with the maximum voltage and power being 0.69 mV and 0.33 μW (0.0825 μW cm⁻²) respectively at ΔT = 0.09 °C for an air velocity of ∼0.9 m s⁻¹. At ΔT = 0.4 °C with no air velocity the device recorded results of 1.47 mV and 1.48 μW (0.37 μW cm⁻²). The highest recorded values were 8.22 mV and 46.28 μW at ΔT = 1.1 °C. The lack of proper thermal and insulating materials leads to a degradation in the thermal gradient over time. This leads to a loss of power and voltage when applied to the body for long periods of time when applied to the body. One test saw the voltage drop by 17% and power by 30% after ∼40 min on skin. This device was cyclically bent in order to test its ability to move and deform on the human body. Over 1000 cycles with a 5 mm radius bend the TEG showed no degradation of resistance and even self-healed during three separate occasions during loading highlighting the robustness of this material architecture for wearable energy harvesting compared to non-LM work. A later study used a similar approach but implemented flex PCB boards instead of LM on the underside of the device to act as interconnects and to help ensure good contact with the body [124]. PDMS was again used to hold the device together with LM deposited in grooves on a sealed top layer as interconnects. Although the flex PCB has a low thickness and can conform to the skin, it increases the complexity of the device requiring extensive amounts of soldering and limiting the device from stretching. This design choice of both LM and flex PCB interconnects also led to visible structural integrity issues.

These previous devices address issues related to flexibility and electrical connectivity that stem from poor contacts and an inability of the rigid interconnects to deform under tensile strain. However, they still lack proper thermal management and there still remains to be significant room for improvements in energy harvesting performance. One issue is that PDMS elastomer has a low thermal conductivity of 0.18 ${\text{W}}{} {{\text{m}}^{ - 1}}\ {{\text{K}}^{ - 1}}$, making it an insulator [125]. It is not a proper material for holding temperature gradients between the sides of a TEG, which is key for proper power generation. PDMS is not ideal for two reasons. As an insulator it cannot absorb heat from the body well nor release the heat on the cold side of a TEG. Separately, the use of a single layer of elastomer allows for heat to transfer from one side to the other decaying the temperature difference between the two sides of the pellet semiconductors. To fix this problem LM elastomer composites were introduced as thermal interfaces into TEGs.

LMEE has already been introduced as a thermal absorber and heat sink for TEGs, enabling fabrication of a cold-resistant self-powered 'electronic sleeve' for monitoring an individual's heart rate at low temperatures (0 °C or lower) [36]. Compared to unfilled PDMS, LMEE thermal interfaces have shown significantly higher efficiency when used with conventional rigid TEGs. Since the EGaIn–PDMS composites are extremely compliant, they provide a better conformal contact over a large area, which results in a comparable performance when compared to unpackaged rigid thermoelectric modules. These thermally conductive elastomers have become a popular choice for flexible thermoelectric energy harvesters. A recent study has incorporated EGaIn and graphite nanoplatelets into an elastomer matrix to increase thermal conductivity at the interfaces (figure 2(c)). Similar to the previous study, the high thermal conductivity of the elastomeric nanocomposite greatly outperformed unfilled PDMS. LM traces were again used for the interconnects between the semiconductors in this study [29].

The most recent work in this field builds on these previous approaches and uses LMEE to implement LM as both a thermal material and, through mechanical activation of its surface, as electrical interconnects for the semiconductors (figure 2(d)) [27]. This solves the problems of thermal management, rigid interconnects, and complex design processes requiring LM to be deposited separately onto the surface and then sealed into the device. The device is fabricated in three layers with LMEE on the top and bottom and a film of PDMS in between that contains the Bi₂Te₃ semiconductors. The undersides of the LMEE was patterned creating traces to act as the interconnects for the thermoelectric modules. The PDMS center sheet was cured and then laser cut to hold 100 rigid semiconductors (i.e. 50 pairs of n-type and p-type thermoelectric modules). To bond these pieces together, oxygen plasma treatment was used [126, 127]. The components were aligned and then cured. This work generates excellent power and voltage with its new thermal management advantages exceeding all previous LM TEGs. At ΔT = 10 °C, 60 mV was recorded with differences of 80 °C generating more than 400 mV. Resistance matching the internal and external loads at ΔT = 60 °C gave a max power of 1.68 mW and power density of 86.6 $\mu {\text{W c}}{{\text{m}}^{ - 2}}$. The internal resistance of ∼13.5 Ω was measured at room temperature. Other unique features of these wearable TEGs are high stretchability and durability due to the elimination of rigid interconnects. During cyclical and tensile loading, the device did not electrically or mechanically fail indicating the excellent structural integrity and functional stability of the device. Using an 18-semiconductor version of the TEG as a tensile specimen for electromechanical testing, failure occurred between 50% and 70% strain with the internal resistance increase below 20%. The tensile specimen was also cyclically loaded at a 30% strain for 1000 cycles with no mechanical or electrical failure with a 21% increase in resistance. These excellent strain results are attributed to the simple design process of incorporating LMEE as a deformable thermal and electrical interface.

While these devices have been improving in thermoelectric performance over time, they still lack the performance of traditional rigid commercial TEGs. For example, by comparing the performance of a 40 × 40 mm and 256 semiconductor commercial TEG (TEG2-126LDT Scavenger TEG module) to the TEGs in table 1, a significant difference in performance is found. At ΔT = 15 °C, V_oc = ∼800 mV and the matched load output power is ∼30 mW for this commercial device, highlighting the improvements that will need to be made for deformable TEGs to eventually enter the commercial space.

TENGs have attracted considerable attention as a low frequency energy harvester for wearable technologies. TENGs excel in their ability to convert ambient mechanical energy into electrical energy while being lightweight, having a simplistic design, and allowing the use of inexpensive materials for fabrication. Mechanical motion can drive the contact separation modes (i.e. vertical, sliding or both) of a pair of distinct dielectric layers in contact, each with inherently different tendencies to release or receive electrons [11, 128]. This tendency is quantified by their rank within the triboelectric series. This contact separation mode causes coupling of triboelectric and electrostatic induction to generate current and power.

The triboelectric effect is comparable to the Van der Graaf generator [129], which consists of a metal dome that becomes highly positively charged through internal friction of a rotating rubber belt. This charging occurs through continuous electron removal by a dielectric material before eventually being neutralized by abrupt electron donations from the ground. This can be observed as a sudden corona spark discharge. The positive charging of the metal dome and the charge leakage phenomena are analogous to a user's skin rubbing or pressing onto TENG's surface followed by the induced current, respectively.

Natural biomechanical motion such as arm flexing, gaiting, hip motions, and tactile actions can deform wearable TENG devices which in return generate large output electrical power. The first TENG was made of paired unbonded polymer films sandwiched by gold films with a power density of 10.4 mW cm⁻³ [130]. Bending of the film laminates causes frictional agitation between the polymer film, inducing electrostatic effects and current flows. Since then, TENG designs have been developed by harnessing a wider range of mechanical motion such as press and release and rotational motion with improved charge trapping abilities. This improvement was fabricated by micro-texturing the contact surface to increase surface area for more electron exchanges to occur [131, 132]. Despite its decent power density, the Young's modulus mismatch of gold electrodes and polymer laminates can reduce the range of TENG deformation. This degrades its maximum power efficiency potential, restricts bending deformability, and creates lower durability or fatigue life. These drawbacks have prompted researchers to pursue more advanced electrode materials for wearable TENG that have enhanced dual mechanical durability when strained and with excellent charge-trapping properties.

One of the earliest choices of electrodes to solve this problem is to utilize liquids or liquid-infused materials such as microfluidic architectures with ionic solutions [133, 134] or ionic hydrogels [135, 136]. Alternatively, LMs have garnered recent traction as a solution to these issues because of its better mechanical and chemical stability compared to ionic liquids. This is likely due to the native oxide skin of Ga-based LM alloys like EGaIn [52, 54, 78, 137, 138]. TENGs based on a single LM microfluidic electrode are generally fabricated by transferring LM into micro patterned channels of partially cured elastomer before being sealed with another uncured layer prior to additional complete curing. This embedded electrode is capable of stretching up to 300% strain while remaining electrically conductive since conductive bulk LM will continuously fill the elongated fluidic channel during deformation [52]. These electrode features are highly beneficial for wearable TENG because it ensures maximum extraction of charge flows/current (i.e. unchanged resistance) even during large deformation (i.e. knee or arm flexion movements) allowing possible fabrication of a functional 3D TENG. This type of electrodes allows control over the geometry of the electrical pathways in TENGs as a design parameter. For instance, LM interdigitated [49] and spiral [50] electrodes have been used to harvest energy in either vertical separation or lateral sliding modes (hand patting and rubbing). Both configurations allow the TENGs to function as stretchable self-powered capacitive or proximity sensors, but the latter design can also function as a force sensor. However, such LM microfluidic based electrodes can leak by impulsive or dynamic mechanical damage to the TENG that may rupture the encapsulating elastomer layer. This can be mitigated by using a mixture of shear stiffening gel (SSG) and an elastomer as the encapsulating matrix material for the LM microfluidics [51]. A SSG is viscous at rest (or at low frequency deformation) and becomes rigid upon impact (high frequency deformation) due to having a larger storage modulus (i.e. stiff) at high frequencies. Introduction of SSG into elastomers protects LM content of electrode when stretchable TENG is inflicted by sudden extreme mechanical impact or damage.

In the pursuit of lightweight and thinner wearable TENGs, electrodes are required to be ultrathin while not sacrificing conductive and deformable performance, prompting researchers to use LM NP thin films as an alternative to microfluidic type electrodes (table 2) [139]. This thin-film device consists of clusters LM NPs (<1 μm diameter) that have large effective surface areas, each with electrically insulating oxide shells (∼3 nm thick). The sintering process coalesces the LM NPs to form wide electrically conductive pathways during stretchable TENG fabrication. Thinner electrodes of TENGs can enhance its sensitivity to tactile stimuli but will have limited stretchability up to 100% before conductance drastically drops (figure 3(a)) [139]. Since, LM NPs electrode will have smaller conductive LM content than microfluidics, upon large applied strain (1.0–2.5 stretch ratio), there will be increased resistance up to 2× because of the increased detachment of coalesced networks of conductive LM NPs (i.e. reduced percolation effect).

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It is mentioned that LM elastomer composites possess dual dielectric and human skin-like compliant properties that have encouraged researchers to explore LM polymer composites for improving TENG performance as both an electrode and charge trapping material. Based on recent works, LMEE has been used as either the triboelectric dielectric component with high dielectric properties (figure 3(b)) [138] or as both electrode and triboelectric component (figure 3(c)) [30]. Recently, it has been shown that LMEE can increase the surface charge density of triboelectric components and reduce surface leakage charge at contact layers by using LMEE with higher dielectric constants [141]. The high dielectric constant of LMEE (${\varepsilon _{LMEE}}$) allows for increased surface charge density (σ). Using the expression for TENG output voltage (V) [142]:

$$\begin{equation}V = \frac{{\sigma x\left( t \right)}}{{{\varepsilon _0}}} - \frac{Q}{{S{\varepsilon _0}}}\left( {x\left( t \right) + \frac{d}{{{\varepsilon _{{\text{LMEE}}}}}}} \right)\end{equation} \tag{ 2 }$$

where _o, $x\left( t \right)$, Q, S, and d are the electrical permittivity in vacuum, time-dependent gap difference of the electrodes, number of transferred charges between the electrodes, contact area, and thickness of the dielectric material, respectively. High dielectric constant LMEE composites will increase the values of σ and ${\varepsilon _{{\text{LMEE}}}}$, thereby maximizing output voltage as well as minimizing surface leakage effects (2nd term in equation (2)).

Another TENG design incorporates dual electrode and triboelectric layers with a porous composite of LMEE, which is termed a liquid-metal-elastomer foam (figure 3(d)) [28]. To increase surface charge density within this composite and induce electrostatic current during mechanical deformation, the surface of the interior inclusions of this foam become contact layers with each inclusion acting as miniature triboelectric generator as the foam compresses and releases. Most of these studies integrating compliant LMEE as triboelectric layers for TENGs still use stiff metal electrodes (gold and copper), which negates the overall deformability of the TENG. As discussed in the TEG section, LM electrodes help solve the rigidity issues that copper and rigid electrodes have in soft devices. More recent work has solved this problem by incorporating LMEE as a functional electrode and triboelectric component of TENG [30]. In this study, the LM droplets are non-uniformly distributed through the thickness of LMEE in order to create a functionally graded LMEE composite for wearable TENGs (figure 3(c)). This is achieved by suspension of large EGaIn inclusions in uncured elastomer, which settle to the bottom prior to the curing process. Two regions, an elastomer-rich and an LM-rich region, form in this material. This is referred to as 'sedimented liquid metal elastomer'. The elastomer-rich side serves as the triboelectric layer, while the LM-rich side performs as an electrode. Unlike sandwich structured TENGs, this TENG configuration creates a gradual increment of Young's modulus over the thickness of LMEE, reducing the delamination effect from bending, while increasing fatigue life. The result is a LMEE based stretchable TENG with high stretchability (>500%), skin-like softness (<60 kPa), and high electrical/mechanical durability (>10 000 cycles). With these mechanical improvements, this technology has been integrated into highly stretchable fabrics for exercise clothing, while exhibiting a peak power area density of 1 mW cm⁻² [30].

A recent study uses LMs welding of low dielectric polymer fibers (i.e. polyacrylonitrile) in elastomer composites as the TENG active layer and requires smaller amounts of expensive LM (<1.5 weight %) during fabrication. This device achieves a max power area density of 4 W m⁻² even after 800 contact cycles on the device. Similarly, this modified LM architecture, resembling LMEE, improves the charge-trapping capability of the triboelectric layer [143]. Another study uses a low weight percentage of LM fillers with rigid silver flakes embedded in a self-healing polymer (polyurethane acrylate (PUA)). This material architecture creates a self-healing stretchable TENG as shown in (figure 3(e)) [140]. This self-healing TENG can be stretched to an impressive 2500% strain and regain its energy harvesting performance even after extreme mechanical damage despite its electrodes being made of rigid conductive fillers (i.e. silver flakes) [140]. In this study, ruptured LM NPs act as percolative nodes or electrical anchors for silver flakes when hyper-stretched (>2000%) to reduce resistance increment. Upon mechanical damage, this electrode composite can be heated for self-healing of the PUA matrix, regaining its stress–strain behavior close to its pre-damaged state. At the optimum load resistance of 1 MΩ, the output power density was 40 μW cm⁻².

5.1.1. Physics of DEGs

5.1.2. Early elastomer DEGs

5.1.3. LMEE based DEGs and their advantages

While unfilled elastomers have been popular for DEGs, researchers have also explored the use of polymer composites with enhanced electrical permittivity. These are composed of elastomer that is embedded with a suspension of filler particles. Such particles can be dielectric or conductive and have included non-ferroelectric fillers such as titanium dioxide, ferroelectric NPs such as barium titanate (BaTiO₃), Ag flakes, Ni and Ag NPs, and carbon nanotubes [149–152]. While these composites have increased dielectric properties, the rigid inclusions can lead to premature mechanical failure [61, 153, 154]. One paper highlighted that the addition of a 30% fill of ferroelectric powder to the elastomer decreased the strain limit from 500% to 200% [155].

LM droplets represent a promising alternative to rigid filler particles since they do not introduce significant stress concentrations or mechanical resistance. As with TEG and TENG devices, LMEE can again be used—in this case as a 'high-κ' dielectric material that generates power through its strain-based electromechanical coupling abilities [61]. This particular LMEE formulation uses EGaIn microdroplets on the order of ∼4–15 μm diameter. Compared to unfilled control samples, a 50% volume fraction filled LM composite increased the relative permittivity by 400%. A separate study has confirmed these permittivity results up to a 50% volumetric fill fraction [156]. The dissipation factor (D) of LMEE was found to be similar or less then unfilled elastomer (D < 0.1) [61]. Conversely a large dissipation factor for rigid high-κ composites materials can occur at fill fractions as low as 30 vol% [150]. Lastly, the electromechanical coupling of this material is excellent, with the ability to support up to 700% strain while having a relative capacitance increase of ∼4.5 at this extreme strain.

While LMEE composites with micron-sized inclusions have many excellent material attributes for DEGs, the material suffers from a low dielectric breakdown strength due to its heterogeneous microstructures. A previously fabricated microcomposite LMEE made with silicone elastomer (Ecoflex 30) has heterogeneous droplet sizes from as low as 4 μm to 15 μm with a breakdown field of 5.0 kV mm⁻¹ [61]. A separate work on heterogeneous LMEE with average inclusion size of 47 μm and a matrix material of PDMS (Sylgard-184) produced a breakdown field of 2.1 kV mm⁻¹ [85]. Comparatively, by controlling the droplet distributions to become homogenous with an average droplet size of ∼100 nm, recent work was able to sustain a breakdown field of 94.1 kV mm⁻¹ [37]. In contrast, heterogeneous microcomposites have a lower breakdown strength due to large internal field concentrations that arise from the greater LM inclusion sizes. LMEE nanocomposites with homogenous inclusions of ∼0.1–1 μm dimension were instead selected as the dielectric DEG since they enabled greater internal electric field and energy harvesting voltage output. Using the cone shaped design, the LMEE dielectric material was coated on both sides with a thin-film of EGaIn and cyclically displaced with an amplitude of 80 mm [37] (figure 4(a)). The LMEE-based DEG had a measured capacitance of 8.08 nF measured under 1 kHz at 1 V compared to 5.54 nF for the unfilled elastomer. This increased capacitance led to an estimated electrical energy that was 53% greater than unfilled elastomer. Compared to unfilled elastomers, the electrically efficiency of this device was increased from 12.3% to 17.7% with the incorporation of LM [37]. A separate study has confirmed 1 μm diameter droplets as the ideal droplet size for relative capacitance as a function of strain compared with 80 μm and 20 μm diameter droplets. This study used a 60 vol% fill fraction and recorded a relative capacitance increase of ∼4× at 270% strain [56].

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Recent advances in synthesis and integration of LM nanodroplets have significantly increased the number of promising matrix materials for DEG devices. For instance, surface-initiated atom transfer radical polymerization (SI-ATRP) has been recently introduced as a versatile synthesis method for LM-polymer nanocomposites with both thermoset and thermoplastic matrices [159, 160]. These advances have led to the most recent work on LM-based dielectric composites where EGaIn droplets with a poly (n-butyl methacrylate) coated shell are synthesized using ATRP [161]. At a 40 vol% LM, the relative permittivity was 22.5, much higher than previous work, and with a max strain above 400%. Compared to this latest work, the previous DEG work [37] recorded an ${\varepsilon _{\text{r}}}$ of ∼11.5 at a 40 vol% LM for 10 μm dispersions and 8.8 at a 30 vol% for 1 μm dispersions. This study shows that future LM-DEG devices can show significantly higher performance if the composite parameters such as filler size, volume fraction, and polymer matrix are carefully selected.

5.2.1. Background on flexible piezoelectric materials

5.2.2. LM for piezoelectric energy harvesters