Stretchable Thermoelectric Generators Based on Bulk Bi2Te3 and Liquid Metals

Flexible energy‐generating electronics have attracted great interest. Generally, such devices can be classified into three categories: energy storage, energy harvesting, and wireless charging. In this work, a flexible thermoelectric generator (TEG) for energy harvesting is fabricated. Bi2Te3 ingots are used as thermoelectric legs. To introduce flexibility, liquid metals and elastomers are utilized as the interconnects and encapsulation, respectively. The as‐fabricated device exhibits an output density of 34.5 µW cm−2 over a temperature difference of 25 °C. The device exhibits promising robustness under mechanical stretching, as the fracture limits reach 100%. The device is tested on the human body, and it delivers an instantaneous output of 75.2 mV. Therefore, the as‐fabricated TEG is suitable for human wearable electronic applications.


Introduction
Nowadays, to fulfill the multiple and functional wearable electronic market, such devices are endowed with diversified features, as wearability, biological comfortability, high efficiency, self-powering, self-cleaning, self-healing, and so on. [1,2]With these features, wearable electronics are applied into human health care, motion monitoring, bio-/chemical-signal detection, and many other fields.The self-powering, one of the criteria for a promising electronic, is of great significance as it provides an essential precondition to a long lifetime and high device efficiency.Energy storage devices like batteries or supercapacitors can store DOI: 10.1002/aelm.202300300[5] These devices exhibit high capacity, small volume, lightweight, and recyclability.The power density can reach a few hundreds of watt-hours per kilogram. [6]However, these devices show relatively low thermal stability and potential toxicity to the environment.Besides, the power supply is not permanent as a charging/recharging cycle must be involved.On the contrary, energy harvesters can produce energy with no concerns of finite lifetime.
In this field, thermoelectric (TE) materials are promising candidates as they can convert thermal energy into electrical energy based on a temperature difference.To date, high-efficient TE materials contain Bi 2 Te 3 , PbTe, SnSe, SiGe, etc. Especially, Bi 2 Te 3 has become the most popular commercial TE material as it is non-toxic, high-efficient, easy to produce, and abundant on Earth.It can bring a maximum conversion efficiency of 8% to 12% at 100 to 200 °C. [7]However, most TE materials are rigid alloys, which leads to difficulties to introduce flexibility.One solution is to employ the "island and bridge" architecture to disperse the strain.10] Another solution is to use deposit TE nanoparticles on fiber/yarn substrates. [11]This approach is easy to bring flexibility and scale-up.TE fiber devices still face challenges of the low fill factor from the limited thickness of the TE layer, poor washability due to the weak bonding between TE materials and substrates, and relatively low use of the skin.As a result, TE fibers can only reach power density in the magnitude of nanowatt per centimeter squared. [12,13]iquid metals have been used as flexible interconnects to enhance the device ductility.Early in 2017, a flexible TEG was made of Bi 2 Te 3 , EGaIn, and PDMS. [14]The contact resistance at EGaIn/TE leg is only 1 mΩ.Moreover, EGaIn has the feature of self-healing and large tolerance to tensile stress.The as-fabricated TEG can provide a power of 11.5 μW when worn on the human wrist.Jeong et al [15] further investigated the surface condition of EGaIn/TE.When exposed to air, an oxide layer can be formed spontaneously at the EGaIn surface.This causes a reduction of electrical conductivity of the TEG.Another issue is the encapsulation of the liquid metal.Though flexible substrate can seal the liquid metal and prevent leakage.However, the encapsulation layer has a relatively higher thermal conductivity than air.This causes a slow thermal transmission from one end of the TEG to another end, leading to the decrease in the temperature difference.To overcome the issue, Lee et al [16] developed a TE cooler with half connected by liquid metals and encapsulated, and the another half was connected by solid interconnects.The half-encapsulated design brings two advantages: a reduced device thickness and an annihilated thermal bypass from the air gap.The device has a EGaIn/TE contact resistivity of 7.92 μΩ cm 2 , and can perform a temperature drop of 3.8 K at the power density of 30 mW cm −2 .To enhance the thermal dissipation at the device surface, Öztür's group [17] employed high thermal conductivity (HTC) elastomer as the encapsulation layer.Besides, a copper sink was attached on the device surface.At last, the device can stand a bending radius of 17 mm and has a power density of 5 μW cm −2 .
Though numerous works about liquid metal-based TEG have been reported, only few works focused on the stretchability, which limited the application of the flexible TEGs.Besides, most of the works have utilized complicated synthetic routes, which are not feasible for large-scale manufacture.Therefore, in this work, we developed a convenient approach to highly stretchable TEGs.Bi 2 Te 3 ingots were chosen as the TE legs due to their promising TE efficiency and commercial matureness.Liquid metal interconnects were used to minimize the device resistance due to their excellent conductivity and ductility.To achieve good flexibility and stretchability, the Ecoflex 00-50 (Ecoflex) was employed as the flexible substrate.The as-fabricated device is in the demission of 13 mm × 10 mm × 1.7 mm.The presence of Ecoflex substrates and gallium indium eutectic (EGaIn) interconnects leads to good device flexibility.The thermoelectric performance of the TEG film is characterized, as it delivers a maximum power density of 34.5 μW cm −2 at a temperature difference of 25 °C.The results indicate that the EGaIn-based TEG is a promising wearable power supply device that can be used in various applications.

Device Resistance
A common TE module has an "island and bridge" configuration as shown in Figure 1A.The TE ingots are connected electrically in series and thermally in parallel, with the temperature direction vertical to the device plane.Liquid metals have been employed as the interconnects.This takes the advantage of the fluidity of the interconnects, which can maintain the electrical connection of the circuits in large deformation conditions.Computation works have been carried out to estimate the capability of such a design when the device is subjected to the tension.As shown in Figure 1B, under the tension stress, the soft interconnects are stretched to share most of the deformation.The rigid TE legs are steady and only undertake minor deformation.This result indicates such a design is feasible for a wearable TEG.
To choose a proper encapsulation material, the mechanical behaviors of Sylgard 184 silicone elastomer (PDMS) and Ecoflex elastomer are compared.[20] The results show that PDMS has a better fit with the Yeoh model, which indicates strain hardening may occur upon large deformation (Figure S1, Supporting Information).This deduction was further proved by our experimental characterizations.Based on the uniaxial and biaxial tensile test, PDMS has a Young's modulus of 2.39 MPa and a Poisson's ratio of 0.23.The material yields maximum elastic strength of 80%.When the strain is over 80%, plastic deformation and hardening effects start to emerge (Figure S2A, Supporting Information).This brings a risk of mechanical failure of the encapsulation when the device is under large stress.Comparatively, Ecoflex elastomer shows a relatively low Young's modulus of 0.056 MPa, indicating its better softness.From a further cycling test, it is shown that the material yields a maximum elastic strength of 200%, which is twice larger than that of PDMS.Within this range, Ecoflex is capable of self-reversing after unloading.When the deformation increases to over 200%, the material starts exhibiting a permanent distortion, as known as the plastic deformation.This is possibly due to the irreversible movement of the polymer chains. [21,22]The elastomer finally breaks at the maximum strain of 400% (Figure S2B, Supporting Information).This fundamental study suggests that Ecoflex elastomer is more suitable for stretchable applications due to its softness and low Young's modulus.Hence, Ecoflex was chosen as the encapsulation material in this work.
In this work, 20% Sb-doped and 5% Se-doped Bi 2 Te 3 have been used as the p-type and n-type TE leg materials, respectively.Compared to the reported synthetic procedure, [14,15] our approach takes advantage of no adhesives were used to fix the TE legs to the stencil.As the presence of adhesives can trap pores during the curing stage.Another merit is that the height of the elastomer was controlled by a template, which is accurate and convenient.Hence, our approach is feasible for large-scale manufacture.
The deposit of the liquid metal faces the challenge of its poor wettability on Bi 2 Te 3 alloys.The reason is explained by the instantaneously formed oxide layer when EGaIn exposed to oxygen.The oxide layer not only prevents Bi 2 Te 3 alloys from touching the EGaIn directly but also constrains the spreading of EGaIn on the Bi 2 Te 3 surface.As shown in Figure 2A, the EGaIn drop forms a sphere instantaneously on the Bi 2 Te 3 surface, which indicates the material is unwetted.The contact angle measurements showed the contact angle is >90°(Figure 2B).Strategies to overcome the issue include applying the Au coating, using HCl to corrode the oxides or spray coating the EGaIn.In our work, the Ecolflex /TE layer was undergone an oxygen plasma treatment to improve the surface energy and provide more free covalent bonds on the alloy surface. [14]After plasma treatment, the alloy was immediately transferred into a glovebox with the argon atmosphere for the next stage of processing.The treated contact surface provides a good wetting condition, and the argon atmosphere restricts the formation of EGaIn oxide.Hence, the Bi 2 Te 3 alloy can be completely wetted.Furthermore, when the wetted alloy was taken out from the glovebox, the oxide layer was formed again, stabilizing the wetting condition and preventing detaching of EGaIn from the alloy surface.This was proved by the contact angle measurements.From in Figure 2C, the plasma-treated Bi 2 Te 3 alloy presented a well-wetting condition as the angle at the alloy/EGaIn interface is only ≈36 o .The wetting is stable and can be maintained over 24 h without shrinking of EGaIn.It should be aware of that the angle measured here can prove a promoted wetting by plasma treatment, but it cannot represent the real contact angle of Bi 2 Te 2 /EGaIn.As the circumstance here is a ternary system with the Bi 2 Te 3 alloy, the EGaIn, and the EGaIn oxide.The measurement of the real contact angle should be taken in a nonoxygen atmosphere to eliminate the presence of EGaIn oxides.To complete the device fabrication, the wetted Ecolflex /TE layer was then encapsulated with another Ecoflex layer.Copper wires were welded to the device to access the external circuits.The synthetic route is illustrated in Figure 1B.
At the early stage of our experiments, liquid metals had been attempted to be deposited by contact printing on the Bi 2 Te 3 ingots.The Ecoflex/TE integration was attached to a circuit mask and dipped into the liquid metal eutectic.The exposed surfaces were pre-coated by a thin layer of gold particles to improve the wettability of EGaIn.Though the process was undertaken under a reducing atmosphere as well, the wetting did not work out smoothly, as the liquid metal preferred sticking to the Ecoflex surface.A long-term immerging allowed full coverage of EGaIn on both exposed Ecoflex and Bi 2 Te 3 surfaces.However, the weak bonding between the EGaIn and Bi 2 Te 3 can be broken when removing the circuit mask.The EGaIn tended to immigrate and merge into larger drops on the Ecoflex surfaces, leaving the Bi 2 Te 3 surfaces uncovered.It is indicated more energy should be endowed to enhance the wettability.Therefore, a rolling process was employed before the removal of the mask to strengthen the EGaIn/Bi 2 Te 3 bonding.As a result, a well-defined liquid metal interconnect pattern can be achieved on the Ecoflex/Bi 2 Te 3 surface.
The TE performance of the as-fabricated TEG strongly depends on the electrical resistance of the device.According to the module shown in Figure 1A, the total resistance of the TEG contains several components.It consists of the intrinsic resistance of each material, including the Bi 2 Te 3 ingots, the liquid metal, and the Cu wires.Besides, it involves the contact resistances between the EGaIn/Bi 2 Te 3 and the Cu/Bi 2 Te 3 .Therefore, the total device electrical resistance (R T ) can be expressed by the equation as below: where R TE refers to the intrinsic resistances of both p-type and ntype Bi 2 Te 3 , R LM refers to the total resistance of the liquid metal interconnects, R Cu refers to the resistance of the solid wires, n refers to the number of EGaIn/Bi 2 Te 3 contacts, which mathematically equals to (the number of TE legs × 2 -2), R C(LM/TE) refers to the contact resistance of EGaIn/Bi 2 Te 3 , and R C(Cu/TE) refers to the contact resistance of Cu/Bi 2 Te 3 .The electrical resistivities of the TE legs were measured via the four-point-probes method at room temperature, with the values of 3.8 × 10 −4 Ω cm for the n-type Bi 2 Te 3 , and 5.1 × 10 −4 Ω cm for the p-type Bi 2 Te 3 , respectively.Given the dimension of the device, the resistances of the two TE legs were calculated using the equation of R = L A −1 , which are obtained as 53.4 and 71.7 mΩ.Hence, R TE was calculated to be 39.1 mΩ.From the literature, it is obtained that the resistivity of the liquid metal at room temperature is 2.94 × 10 −5 Ω cm, [18] which leads to an approximate resistance of 33.5 mΩ for the interconnects with the given demission.R T , R Cu , and R C(Cu/TE) were directly measured from the multimeter at room temperature, as the values were obtained as 2.8 Ω, 12 mΩ, and 18 mΩ, respectively.Therefore, from (1), the contact resistance at the liquid metal/Bi 2 Te 3 interface was calculated to be 20 mΩ, with the corresponding specific contact resistance of 0.36 mΩ cm 2 .All the data have been listed in Table 1.The low specific contact resistance can be attributed to several factors.The plasma surface treatment is suggested to remove the contaminations and provides an energetic Bi 2 Te 3 surface for the wetting of EGaIn. [19]The coating of Au and the rolling process have a similar effect on the enhancement of electrical contact at EGaIn/Bi 2 Te 3 .
Besides, the absence of oxygen during the liquid metal deposit process minimizes the formation of passivating oxide skin and decreases resistance.It should be noticed that the as-obtained Bi 2 Te 3 blocks own a relatively rough texture, which stands as a detrimental factor toward the contact as it increases the separation between the interconnects. [17]In this work, the influence of the Bi 2 Te 3 roughness has been compensated by the advantageous approaches above, however, it is believed the contact resistance can be further reduced with a smoother Bi 2 Te 3 surface.

Device TE Performances
The thermoelectric performances of the TEG are illustrated in Figure 3.To proceed with the measurements, a heating table was employed to maintain a hot temperature (T H ). When the temperature reading is stable, the TEG was placed on the heating table, and the maximum voltage reading was recorded immediately.As the TEG is thin in thickness, and the time for heat transferring is short.Therefore, the TEG was removed from the heating table for 10 min of by the end of each measurement.Assuming the TEG cold end has the same ambient temperature, T C was recorded as 25 °C, and the temperature difference equals to T H − T C .By altering the temperature of the thermal couples, a temperature difference ranging from 5 to 25 °C has been generated.As shown in Figure 3A, within the measurement range, V OC increases as the temperature difference, reaching the maximum value of 47 mV when ΔT = 25 °C.The device Seebeck coefficient (S) can be calculated to be ≈1.9 mV K −1 .As a comparison, the ideal device Seebeck coefficient, S' = Σ (S p -S n ), where S p = 172 μV K −1 and S n = −168 μV K −1 were obtained from the initial TE material characterizations by a ZEM-3 instrument.Hence, S' is obtained as 10.2 mV K −1 for 32 pairs of Bi 2 Te 3 TE legs.It is suggested that the real device Seebeck coefficient is no more than 8% of the ideal value.The reason is explained by the fact that heat loss occurs during the measurements.The main contribution is the low thermal conductivity of the encapsulation Ecoflex layer (≈0.1 W (m•K) −1 ), [25] which acts as a thermal insulating layer that causes a temperature drop between the Bi 2 Te 3 legs and the thermal couple.There are several reported works focusing on improving the thermal conductivity of the encapsulation layer. [26,27]Such approaches include using use a copper sheet as the heat sink or using liquid metal elastomer composites (LMEC) for a fast heat dissipation.However, the rigidity of copper may sacrifice the device stretchability.Nevertheless, even LMEC may not stand a tensile strain over 70% before mechanical failure occurs. [27]Another reason for the Seebeck coefficient drop is the effect of the fill factor (FF), which is defined as the cross-section area ratio of TE legs over the entire device.The device efficiency is proportional to the Figure of merit (ZT), which can be written as: where T is the temperature, R is the resistivity, and K is the total device thermal conductivity.Considering the existence of Ecoflex, the real thermal conductivity can be rewritten as: where K TE and K Ecoflex represent the thermal conductivity of TE legs and flexible substrate, respectively.According to the (3), the theoretical maximum ZT is ≈0.8, with the corresponding conversion efficiency of 4%.In an ideal case where the device is filled with air, which shows superior low thermal conductivity, the K Ecoflex (1-FF)/FF term in (3) can be ignored, and K' is independent of the FF.While in the real circumstance, the K Ecoflex (1-FF)/FF term cannot be ignored with Ecoflex as a filler, and the device efficiency decreases rapidly with the decreasing FF (Figure 3).To achieve a desirable ZT drop (<5%), the FF should be >50%.This means the interval between TE legs is only 41% of the TE leg length.However, in this case, the flexibility of the device is strongly weakened.When FF = 0.25, where the TE leg interval is equilibrium to the TE leg length, the device can exhibit remarkable flexibility, but the ZT drop is ≈19%.Therefore, FF is a key factor to balance the device flexibility and efficiency.In this work, the FF is chosen as 0.25.
The Output power (P) is a function of the load resistance (R L ), which can be expressed by the equation as follows: where I refers to the circuit current.From (4), it is indicated the maximum value of P can be obtained when R L = R T , and ( 4) can be rewritten to the equation as follows: B) The variation of output power with the current at different temperature differences.
Hence, the device generation characteristics as a function of the current are presented in Figure 4B.When ΔT = 25 °C, the maximum device output power of 200 μW with the corresponding current of 8.4 mA is achieved.Considering the device demission, the maximum output power density is calculated to be 34.5 μW cm −2 with a matched load.

Device Stability
The as-fabricated wearable TEG is demonstrated in Figure 5A.Due to the softness of the Ecoflex encapsulation and liquid metal interconnects, the device can stand mechanical bending along arbitrary directions.An experimental setup was established to measure the device resistance upon stretching, which is shown in Figure 5B.The device was placed in the tensile test machine (Lloyds-LS1E), and the external wires were connected to a resistance meter.The displacement of the crosshead was recorded as the measured displacement.However, during the stretching test, the edges take more stretching deformation.Hence, the actual displacement of the device body was only ≈75% of the measured data.Hence, a correction factor of 0.75 was used to convert the actual displacement.First, the device was undertaken a cycling test with the maximum strain of 75%.The resistance  curve was recorded at the 1st, 10th, 20th, 40th, and 50th cycle.The results are plotted in Figure 5C.At the early stage of the initial stretching (0-30%, 1st), the resistance decreased from 2.8 to 2.6 Ω.This is possibly because the tension along x-axis gave rise to a comparison along the y-axis.This led to a more closely contact of the TE legs and EGaIn, which accelerated the electron transport and reduced the resistivity [28] (Figure 5E).With the continuous stretching (30-75%), the EGaIn contacts were prolonged, and the electron transport path was lengthened.Hence, the device resistance increased from 2.6 to 3.0 Ω.As a result, the device resistance was 2.8 ± 0.2 Ω, with a variation <7%.After 50 cycles, the device only exhibited a slight resistance growth of 0.04 Ω (+1.4%), indicating the as-fabricated wearable TEG shows great reliability upon repeating stretching.To further study the tensile strength of the device, the wearable TEG was undertaken a continuous tension.As shown in Figure 5D, when the strain increased from 75% to 200%, the resistance gently increased from 3.0 to 10.3 Ω, and suddenly jumped to an extremely high value of 20 Ω.This represented the mechanical failure of the device.The broken device was reviewed under an optical microscope, and a tiny fracture was found at the Ecoflex encapsulation closed to the TE leg edges.It is implied that during stretching, the TE leg edges provided the maximum pressure to the encapsulation.When the Ecoflex mechanical strength was reached, the encapsulation was punctured, and an irreversible leakage of EGaIn occurred.This resulted in the device failure.

Applications
The liquid metal-based wearable TEG possesses favourable TE performance and softness.This allows electricity generation from a curved platform, such as human skin.A human test has been carried out as demonstrated in Figure 6A,B.The 128-pair TEG was attached to the human arm.The device gives an instantaneous voltage output of 75.2 mV, but slowly dropped to 14.0 mV after a few minutes.As heat was conducted from the inner side to the outer side until the thermal equilibrium was reached.At the equilibrium condition, the temperature difference was much lower than the initial condition.Hence, the stable voltage output was quite lower than the instantaneous value.To address the issue, an in-plane TEG was designed and fabricated (Figure S4, Supporting Information).In this design, the voltage output is independent of the temperature difference vertical to the plane, but to the one parallel to the plane. [29]One in-plane TEG and one cross-plane TEG with the same demission were attached to the thermal couple to create temperature difference.The real-time voltage (V') was recorded and shown in Figure S5 (Supporting Information), and the ratio of V'/V0 as a function of the maintaining lifetime is plotted in Figure 6C, where V0 is the maximum voltage that is obtained at t = 0 s.The results can be divided into three regions.Due to the unavoidable heat transfer from the hot end to the cold end of TE legs, both TEGs exhibit decreasing V' throughout all three regions.In region I (t < 15 s), both devices show a rapid decrease in V' with the rate of ≈−3.3% s −1 .This is related to the high thermal conductivity of TE legs that the temperature difference suddenly shrinks.As a result, only ≈50% voltage is maintained after 15 s.In region II (t < 190 s), the inplane TEG starts performing a slower decreasing rate than that of the cross-plane TEG, leading to a higher maintaining voltage.This can be explained by the different device configure rations where the in-plane TEG is capable of faster heat dissipation, leading to a slowdown of the temperature difference drop.Besides, the Ecoflex filler can further retards the heat diffusion along the in-plane direction, and it results in 26% of voltage maintained for the in-plane TEG after 190 s.While the value for the cross-plane TEG is <5%.When the system reaches thermal equilibrium, it enters the region III (t > 190 s) where a stable output voltage can be obtained.The in-plane TEG maintains 22% of the voltage, while the cross-plane only exhibits a retention voltage of <2%.Hence, the in-plane TEG is more favorable to maintain the temperature difference and leads to a higher final voltage output.

Conclusion
To summarize, an in-plane TEG based on Bi 2 Te 3 TE legs, liquid metal interconnects and Ecoflex encapsulation has been realized via an easy fabrication approach.Strategies have been used to enhance the contact between liquid metals and TE legs, and low contact resistance of 0.36 mΩ cm 2 is finally achieved.The TEG can generate power of 34.5 μW cm −2 under a temperature difference of 25 °C at room temperature.The device also shows long-term thermal stability and repeatable mechanical robustness.The device exhibits the maximum mechanical strength of 200%.When attached to the human skin, 128-pair TEG can give an instantaneous output of 75.2 mV.Hence, the device is capable to harvest energy from the waste heat when worn on human bodies, and it potentially encourages new approaches to flexible energy harvester in the future.

Experimental Section
A demonstration revealing the device fabrication route is presented in Figure 7.The Bi 2 Te 3 ingots were in the dimension of 1.35 mm × 1.35 mm × 0.8 mm.Thirty-two pairs of TE leg (purchased from Alpha Material Company, Chengdu).1) A template was designed with an 8 × 8 array of cubic humps on one side.Each hump had the same dimension as the TE leg ingot.2) The prepolymer of Ecoflex was mixed at the weight ratio of polymer A:polymer B = 1:1.After sufficient stirring on a magnetic stirrer, the mixture was poured on the designed template.Then the tray was vacuumed to remove the residual air in the Ecoflex mixture.To achieve the planarization, a piece of flat glass was attached to the top surface of the template.After curing at room temperature for 1 h, the planarizing glass was removed from the surface of the template.3) Then the Ecoflex elastomer was carefully peeled off from the template, and the rough edge was picked off with a tweezer under the microscopy until a complete holey flexible substrate was obtained.4) The TE leg ingots were then inserted into the vacancies of the Ecoflex substrate in order.5) The liquid metal of EGaIn was utilized as the interconnects due to its extremely high electrical conductivity and ductility.The interconnects were printed with a stencil on the top side of the substrate, and 6) followed by the Ecoflex encapsulation using the same formula as step (2).7,8) At last, the bottom side of the substrate was printed with interconnects and encapsulated, and the final product was obtained.
The as-fabricated wearable TEG was characterized using a multifunctional electric meter (2450-Probe, Tektronix) for the electrical resistance measurements.The internal resistance of the multi-meter was measured first, and the value was subtracted from the TEG measurements.
In terms of open circuit voltage (V OC ) measurements.A temperature difference was created with a pair of thermal couples monitored by two digital temperature controllers (ED330L, Zhenglong).The thermal couples were attached to the two different faces of the TEG, which were controlled by two separate temperature controllers, respectively.One thermal couple was fixed at the temperature of 25 °C, while the other one was controlled to deliver a gradually increased temperature range from 25 to 50 °C.The temperature variation was within ± 1 °C.The V OC measurements were taken after 5 min since the placement of the thermal couples.All measurements were repeated three times, and the averages were taken as the results.All necessary approvals were obtained, that all participants consented to this work agreed to this publication.

Figure 1 .
Figure 1.A) The schematic illustrations of TEG designs.The temperature direction is vertical to the paper.B) Computation works show that the "island and bridge" design exhibit segmented stretchability.

Figure 2 .
Figure 2. A) The photographs of pure, unwetted, and completely wetted TE ingot.B) The raw Bi 2 Te 3 surface that provides a poor wetting condition with EGaIn.C) Plasma-treated Bi 2 Te 3 exhibits a reduced contact angle with EGaIn, indicating an improved and stable wetting condition.

Figure 3 .
Figure 3.The theoretical ZT as a function of FF.

Figure 4 .
Figure 4.The TE performance of the as-made TEG.A) The open circuit voltage and the maximum output power as a function of temperature difference.B) The variation of output power with the current at different temperature differences.

Figure 5 .
Figure 5. A) The as-fabricated wearable TEG shows good stretchability and bendability.B) The experimental setups for the measurement of device resistance under stretching.C) The cycling tests of device resistance as a function of strain.D) The dependence of device resistance on the increasing strain until failure.E) The resistance change of the TEG upon stretching.

Figure 6 .
Figure 6.The wearable TEG used on the human arm with A) the instantaneous output and B) the stable output.C) The retention voltage with the time for an in-plane and a cross-plane TEG.

Figure 7 .
Figure 7.The schematic demonstration of the fabrication route of the wearable TEG.

Table 1 .
The resistivity of the raw materials and the measured resistance data of the device.