Soft Electronic Platforms Combining Elastomeric Stretchability and Biodegradability

Soft and stretchable electronic devices are expected to offer technological advances in the field of robotics, human–machine interfacing, and healthcare. Employing biodegradable elastomers, hydrogels, and nontoxic conductors would add significant value to and minimize the ecological impact of such disposable and transient electronic applications. Here, the biodegradable and photo‐crosslinkable elastomer poly(glycerol sebacic) acrylate (PGSA) is characterized for its use in soft and stretchable electronics. Its mechanical properties are investigated in terms of their chemical composition and compared to commonly used gelatin hydrogels. Furthermore, these materials are combined with interconnects made of liquid Galinstan in order to create functional substrates with certified biodegradability under ISO standards. The combination of these materials produces elastic circuit boards that act as soft platforms for body‐mounted sensors or biodegradable stretchable light‐emitting devices. These soft platforms reveal linear elongations at a break of 130% to 350% and similar moduli to nondegradable elastomers and human tissue, without any decrease in conductivity. Advanced applications in biofriendly packaging, soft robotics, and healthcare will greatly benefit from these biodegradable devices.


Introduction
Stretchable electronic devices are currently being considered for numerous applications in bioelectronics as they eliminate the mismatch between mechanically rigid electronic devices and soft biological tissue. [1,2] These new technologies offer exciting possibilities in the field of bionics and soft robotics, [3] for agricultural sensors, [4] smart packaging, [5] or wearable electronic devices in clothing and accessories. [6] Most of the existing literature reports on this topic present devices supported by silicone-or polyurethane-based substrates, while disregarding their biodegradability. [7] An innocuous degradability fumarate moieties [37] or using maleic acid as a monomer. [38][39][40][41][42] The photocurability of PGSA has been exploited as a glue for heart surgery [43,44] and as bioinspired structural adhesives. [45] Compared to natural biopolymers, their synthetic counterparts are, in general, immunogenically inert, provide a batch-to-batch uniformity and usually undergo hydrolytic degradation, so they show only a minimal site-to-site and patient-to-patient variation. [46] A biodegradable elastic substrate requires equally stretchable interconnect electrodes. While gold is intrinsically biocompatible, its evaporated thin films or foils are merely bendable and require prestretched substrates. [47,48] However, prestretching and buckling decreased the transmittance of a stretchable LEC dramatically. [47] As an alternative, gold nanoparticles and nanowires require annealing and percolation, [49][50][51][52] usually achieving around half of the conductivity of the evaporated film (1 × 10 5 S cm -1 ). [43] Additionally, they require strong adhesion to the elastomer matrix. [53] Other conducting materials like carbon black, graphene, and PEDOT:PSS exhibit lower conductivity (Graphene: 550 S cm -1 , [54] PEDOT:PSS: 0.1-1 S cm -1 [50,55,56] ) and provide elasticity only for strains of up to a few percent.
In contrast, liquid metals such as Galinstan (eutectic alloy of 68 wt% Ga, 22 wt% In, 10 wt% Sn) combine the high conductivity of metals (3.5 × 10 4 S cm -1 ) [57] with an intrinsic ductility due to the liquid state and biochemical inertness. [58][59][60][61] In this work, we synthesize and characterize the biodegradable and photo-crosslinkable elastomer PSGA and demonstrate its applicability in soft and stretchable electronics. We investigate its mechanical properties in terms of chemical composition, and compare it to commonly used gelatin hydrogels. Furthermore, we combine these materials with interconnects made of liquid Galinstan in order to create functional substrates. The biodegradation of PGSA is demonstrated via an internationally standardized test. Finally, a biodegradable light-emitting electrochemical cell pixel is mounted on top of these elastic circuit boards in order to attest their functionality as a carrier substrate for electronics. The demonstrated biodegradability and flexibility of our soft electronic platforms open up opportunities to address mechanically challenging applications in the field of smart packaging, agricultural sensors, soft-robotics, or healthcare.

Synthesis and Fabrication of Elastic Films
The synthesis of poly(glycerol sebacate) (PGS) consists in a solvent-free polycondensation, in which the natural educts glycerol and sebacic acid react in an environmentally friendly process (Figure 1a, details in Section S1.1, Supporting Information). [3,24,29,30,62] The polycondensate PGS has a linear structure due to the inert reaction conditions and shows thermoplastic behavior. [63] By thermal curing, its secondary hydroxyl groups may react with an excess of sebacic acid or terminal carboxylic groups to produce crosslinks and restrain its thermoplastic properties. For this, PGS was kept at 120 °C under pressure for at least 3 d, forming an 800 µm thin elastomer film (details in Section S2.1, Supporting Information). Clearly, the large timescale and harsh conditions of this thermal crosslinking process represent a major drawback. [24,31] Note that the crosslinked PGS has been previously applied as a tissue implant and can therefore be regarded as biodegradable. [29,30,32] As an alternative to thermal crosslinking, chemical crosslinking of PGS by toluene diisocyanate would limit the pot life of the solution and create the risk of a residual toxic chemical in the elastomer. Conversely, by introducing acrylic groups through the free hydroxy groups of PGS, the resulting poly(glycerol sebacate) acrylate (PGSA) would offer the possibility to be photo-crosslinked faster and at room temperature through the double bonds of the acrylic moieties, creating an elastomeric network. [33][34][35]65] Thus, we conducted an acrylation reaction in solution, in which the amount of added acryloyl chloride determined the final degree of acrylation (DA) of PGSA (Figure 1b, details in Section S1.2, Supporting Information). The removal of triethylamine chloride, which otherwise formed crystals in the final elastomer film, presented an essential step in the synthesis procedure of elastic PGSA. This step was not elaborately discussed in previous synthesis protocols [33][34][35] and is therefore detailed in Section S1.3 (Supporting Information).
The solution-processability of PGSA opens up various film fabrication routes, e.g., spin-coating, blade-coating, microdispensing, making it attractive for industrially relevant processes. Thus, we fabricated films with a concentrated and viscous solution of PGSA by drop-casting onto a carrier substrate or into a casting mold (details in Sections S2.2 and S2.3, Supporting Information). A UV photo-crosslinking step with the aid of the photoinitiator 2,2-dimethoxy-2-phenylacetophenone for the radical polymerization (details in Section S1.4, Supporting Information) successfully produced either thin elastomer films or thicker elastomer films with embedded channels. The lower temperature and shorter timescale of this process are highly advantageous compared to the thermal crosslinking of PGS. Multiple layers with intermediate crosslinking ensure a firm structure within the bulk of the film.
Hydrogels can be used as an alternative concept to elastomers for elastic biocompatible substrates, [66,67] despite exhibiting a drastically different molecular structure and properties compared to elastomers. Gelatin forms elastic hydrogel films when processed from aqueous solution, to which we added glycerol as an environmentally friendly plasticizer (details in Section S2.4, Supporting Information). Note that the heterogeneous polypeptide mixture of gelatin exhibits variable molecular weight and composition that heavily depends on its extraction procedure from collagen. The acidic pretreatment route forms gelatin type A that includes roughly 33% glycine, 11% alanine, 11% proline, 10% hydroxyproline, 5% arginine, 5% glutamic acid, and 25% remaining amino acids. [68][69][70] On average, the order of amino acids follows the pattern Gly-X-Y (Figure 1c). [64,71] On a molecular level, the intermixed structural polypeptides of the hydrolyzed collagen provide a high elasticity without chemical crosslinks.

Biodegradation of PGSA
As gelatin originates from a natural material and is composed of oligopeptides, it is obviously biodegradable. [66] In contrast to gelatin, synthetic elastomers that are suitable for tissue engineering, such as PGSA, [33][34][35] have not been investigated with regard to their biodegradability, yet. Thus, the biodegradability of PGSA with a 19% DA (PGSA-19) was investigated with a standardized biodegradation test in aqueous media according to DIN EN ISO 14851 (2004-10), [72] using microcrystalline cellulose as a reference. Both test volumes of the reference and PGSA-19 revealed a decrease of pH value from 7.0 (blind control sample) to 6.9 and small white flakes of biomass, indicating biodegradation.
Recording the oxygen consumption of the degrading sample with a respirometer enabled the comparison to the theoretical oxygen demand, which serves as the figure of merit for biodegradability. A 100% of the theoretical oxygen demand represents full oxidation of the sample's carbon. This value may be exceeded by the development of additional CO 2 from the primer, when the addition of the sample triggers the biodegradation of the seeding material (activated sludge), causing a higher oxygen consumption than the theoretical demand. PGSA-19 and the reference showed a typical exponential decay behavior (Figure 1d). The cellulose reference reached 133 ± 1% of the theoretical oxygen demand within 68 d. The second rise in degradation beyond 21 d and exceeding 100% resulted from the afore-mentioned priming effect. The biodegradation of PGSA-19 reached 89 ± 10% of the theoretical oxygen demand in the same period. The plateau beyond 60 d indicates that the sample reached its maximum degradation. Consequently, PGSA-19 is certified biodegradable in aqueous media by reaching 90% biodegradation in the plateau phase, satisfying the definition of biodegradability of DIN EN 14995 and DIN EN 13432. [73,74] These results add to previous data that proved PGSA to be biocompatible and to encourage cellular adhesion and growth in vivo. [33] Likely, PGSA will exhibit the same degradation mechanism as PGS, i.e., via a surface erosion mechanism that maintains the mechanical properties and the integrity of the bulk material. [23] In contrast, the bulk hydrolytic degradation of traditional polyesters like poly(glycolic acid) is based on a random chain scission, thus altering the molecular weight and mechanical properties of the bulk. [23,24] The certified biodegradability of PGSA allows a plethora of applications on living subjects (Figure 1d inset).

Mechanical Properties of Biodegradable Elastic Films
An application of elastic materials as a substrate for biodegradable electronics will encompass compressive and tensile strain, where the latter truly exposes their advantage over thermoplastic foil substrates. Linear tensile tests (Figure 2a,b) revealed the mechanical key properties of candidate elastic materials (Table S2, Supporting Information). When applied to the body, the Young's modulus E of the elastic substrate should match the

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© 2021 The Authors. Advanced Sustainable Systems published by Wiley-VCH GmbH E of the respective body tissue in order to minimize the need for geometric compensation in case of a difference in stiffness.
As gelatin originates in the collagen of connective tissue, it offers very similar mechanical properties to the tissues of the human body. The low Young's modulus E = 48 kPa of this gelatin-plasticizer hydrogel matches the modulus range of muscle tissue (10-500 kPa). [24,[75][76][77] Additionally, gelatin exhibited a high elongation at break ε B = 155% (Figure 2c). However, as gelatin is a natural product with nonuniform chemical composition, its mechanical properties are subject to batch-to-batch variations. Thus, ε B may be as low as 120% ( Figure S5b, Supporting Information). The mechanical properties of gelatin could be further tuned by mixing gelatin products of different gel strengths.
Unlike gelatin, PGSA offers the possibility to tailor its mechanical properties by adjusting the DA. Therefore, the properties of the target tissue can be imitated with PGSA by controlling an additional parameter besides changing the molecular weight or adding additives. [33][34][35] Our experiments revealed that PGSA with a 28% DA (PGSA-28) exhibited the expected rubber-elasticity of an elastomer but with a much higher E = 592 kPa than gelatin. With a similar ultimate tensile strength σ UTS , this led to a low ε B of only 20%. By reducing the DA to 19%, the reduced number of crosslinks decreased the E to 143 kPa, still in the range of muscle tissue, and increased ε B significantly to 130%. PGSA-19 exhibited more rubberelastic behavior than PGSA-28. Reducing the DA further to 0% (PGS) retained E, but decreased the ε B to 103% due to a lack of crosslinks. Thus for an optimized ε B with ductile deformation, one must find an optimum DA between 0% and 28%.
However, a sheet of prematurely crosslinked PGSA (PGSA-19*) emphasized the full potential of PGSA's mechanical properties. The obtained Young's modulus of 80 kPa, high ultimate tensile strength of 265 kPa and a high elongation at break of 350% ( Figure S5c, Supporting Information) are competitive to soft silicone rubbers, such as Ecoflex with a low E = 34 kPa and very high ε B = 522%. Increasing the molecular weight of PGSA would certainly increase the ultimate tensile strength further.
Silicones, such as Ecoflex or poly(dimethylsiloxane) (PDMS), are commonly used as stretchable substrates due to their ability of liquid processing. However, they are not biodegradable and thus inferior to PGSA and gelatin for ecofriendly or healthcare applications. The stiffness of PDMS (E = 1124 kPa) is in the range of skin (0.7-16 MPa) [24] and significantly larger than that of Ecoflex, PGSA, or gelatin. The same disadvantages apply to vulcanized natural rubber, which also cannot be cast after vulcanization.
Biodegradable alternatives to gelatin and PGSA are commercially available latex milk and "Terratek Flex", which is a thermoplastic elastomer with a 35% biobased content and certified compostability. Both exhibit a drastically higher Young's modulus in the MPa range, much higher than that of typical muscle tissue. Conclusively, these two materials are too stiff for skin-mounted devices. Terratek Flex even revealed a plastic deformation above 26% tensile strain, despite being marketed as a biodegradable rubber replacement. Likewise, poly(ethyleneoxide) with a trimethylolpropane ethoxylate plasticizer did not show elastic properties (details in Section S2.5, Supporting Information).
Cyclic linear tensile tests confirmed the results of the destructive tensile tests (Figure 2d). The small hysteresis emphasized the reproducibility of the discussed mechanical properties under periodic load and the absence of a dominant viscoelastic component in the Young's moduli. In order to add a visual perspective to the stress-strain data, Figure S6 (Supporting Information) shows photos of stretching each of the elastic materials by hand. Combinations of two materials could further enhance the mechanical properties, [78] as the bonding strength of PGSA-gelatin exceeded the one of gelatin-gelatin. [79] As a result of this extensive comparison of biodegradable and nondegradable elastic material candidates, the mechanical properties of gelatin and PGSA-19 in combination with the ability of solution processing qualify them to be further investigated for biodegradable stretchable platforms.

Biocompatible Stretchable Interconnects
In order to equip the PGSA-19 and gelatin substrates with biodegradable electronic components in a printed-circuit-boardlike manner, [80][81][82] a biocompatible stretchable conducting material is required for the interconnects. Galinstan was chosen because of its biocompatible constituent elements and constant conductivity under deformation, resulting in infinite mechanical compliance. [83,84] Several patterning concepts for liquid metals in elastomers have been reported previously, including lithography processes, injection into predefined features (e.g., microchannels), subtractive methods, i.e., selective removal of the metal, and additive methods where the metal is deposed only in desired regions, e.g., via inkjet printing. [60,61,85] However, none of these turned out to be compatible with PGSA-19 and gelatin. Depositing gelatin or PGSA-19 solution into a laser-cut cast mold enabled drop-casting the liquid metal into these predefined channels (details in Section S2, Supporting Information). This step could be readily upscaled with a microdispenser or a modified inkjet printhead. [60,61,86,87] Galinstan dewets from the surface of gelatin and PGSA-19, which predicts a stable laminar flow in the channel. Subsequently, the Galinstan trace was covered with a second layer of elastomer (PGSA solution or a solid gelatin layer), embedding the Galinstan interconnects in the substrate material. This fabrication process enables the patterning of biodegradable soft platforms, i.e., elastic circuit boards, on an industrial scale with arbitrary complexity.

Soft Platforms under Strain
Four different patterns of interconnect traces were tested in a linear tensile test: A straight line and three serpentines with a sinusoidal, semicircle, and horseshoe shape (Figure 3a,b). Even though serpentine geometries are well known to be beneficial for solid and especially printed interconnect traces, [88][89][90][91][92] the liquid metal equivalent has not yet been compared to linear interconnects.
Samples of PGSA-19 and gelatin, each with embedded Galinstan interconnects, were stretched repeatedly with an increasing elongation while the resistance of a serpentine geometry and the stress were simultaneously measured (Figure 3c). The initial absolute resistance ranged around 2 Ω for each trace and remained constant for every cycle. Hence, the strain-dependent resistance was normalized by the initial resistance for every cycle to account for the different lengths and thicknesses of the serpentines. For PGSA-19, none of the geometries exhibited a rise in resistance neither within a strain cycle nor after various strain cycles, highlighting the high strain compatibility of the liquid metal electrode material (Figure 3d). A serpentine geometry of the interconnect did not result in a lower resistance or more strain-resistant behavior compared to the straight trace. Similar results were obtained for the gelatin sample (Figure 3e). A slightly decreasing resistance at higher strain can be attributed to a better interfacial contact of the Galinstan with the measurement clamps due to the increased pressure in the embedded channel because of the Poisson effect. Correspondingly, a fatigue test, i.e., repetitive but constant strain, of surface-mounted Galinstan serpentines on PGSA-19* revealed a reduction of the resistance by around 6% within 500 strain cycles ( Figure S7, Supporting Information).
Overall, both biodegradable elastic materials were successfully equipped with stretchable and biocompatible interconnects of arbitrary shape. Each of the four trace geometries exhibited full compliance for a cyclic tensile strain of up to 100%. Thus, these ecofriendly stretchable platforms are well suited for reliable biodevices that experience high strain.
In order to demonstrate the application of the embedded Galinstan interconnects in a biodegradable matrix, we embedded a straight trace in a gelatin substrate and attached it to a human elbow (Figure 4a). The elbow constitutes a part of the human body where the skin gets stretched with a very high magnitude and frequency. As the ends of the specimen are fixed, it experiences both bending and tensile stress, where the Galinstan trace is roughly located in the neutral axis. The resistance was measured by piercing wires into the terminal reservoirs of the interconnect, exhibiting an initial value of 0.6 Ω. When bending the elbow, the recorded resistance plateaued at 0.7 Ω at a 90° bending angle, corresponding to a strain of approximately 30% (Figure 4b). When straightening the elbow, the resistance returned to its initial value. This reversible change of R/R 0 = 16% exceeded the relative resistance variation of the straight trace in the linear tensile test (R/R 0 = 8% for 30% strain in Figure 3e) due to a combination of the tensile stress induced Poisson effect and the bending stress induced compression of the channel. Still, this absolute change in resistance of 0.1 Ω is very small and constant over seven bending cycles (Figure 4c, videos in the Supporting Information). The very short resistance spikes are attributed to the volatile interfacial resistance of the moving wire in the Galinstan reservoir, which is not considered a part of the device but was necessary for the setup of the experiment. Interfacing with hard wire electronics could be more reliable when employing nanowire-elastomer blends, [53] nanoparticle-elastomer blends [86,93,94] or HCl vapor treatment. [95] The overall reproducible and constant change in resistance of these stretchable and biodegradable interconnects renders them suitable as a soft platform for applications on human patients or farm animals, exploiting the biodegradability in case of accidental detachment.

Soft and Biodegradable Circuit Boards
A further step toward biodegradable circuitry [81] is mounting electronic devices onto the soft platforms that are made of embedded Galinstan in biodegradable elastomer matrices. First, two embedded interconnects in the PGSA-19 or gelatin substrate linked a light-emitting diode (LED) to a power supply (Figure 5a).
While many previous reports focus on testing such an arrangement merely under linear strain, [94,96] stretching the elastomer film over an object emulates the isotropic bending stress of a body-mounted device, as it was produced on the   (Figure 5b). Increasingly larger half-spheres in the setup produced an increasing strain of up to 21% (Figure 5c). Indeed, this value falls much below the maximum linear strain of the tensile tests, but the isotropic nature of the puncturing load did not permit a Poisson-effect-like compensation of the stretched material as it would be the case at linear stress.
Under mechanical load, the embedded Galinstan interconnects remained visually undistorted, even at a high strain (Figure 5d), confirming the durability of the PGSA-19 elastomer and gelatin hydrogel as presented in the linear tensile and elbow stretching tests. Thus, the Galinstan interconnects in both elastomer substrate materials supplied the LED with a constant current up to a 21% strain (Figure 5e). Consequently, the luminance remained constant as well, as the variations can be assigned to a slightly varying orientation of the LED with respect to the photosensor.

Fully Biodegradable Light-Emitting Device under Isotropic Bending Strain
In order to take full advantage of the biodegradability of the PGSA-19 and gelatin substrates and the biocompatibility of the Galinstan interconnects, the LED was replaced with a printed and partially biodegradable light-emitting electrochemical cell (LEC) pixel (Figure 6a). [16][17][18] The LEC comprised a printed gold cathode, a printed PEDOT:PSS anode, and a blade-coated active layer, i.e., a mix of a biodegradable solid polymer electrolyte and a yellow polymer emitter. A biodegradable thermoplastic foil served as a substrate for the pixel (Figure 6b). [19] The entire LEC stack was tested for biodegradability under the same conditions as the PGSA-19 substrate and revealed partial biodegradation in aqueous media of 62% theoretical oxygen demand in 82 d ( Figure S8a, Supporting Information), corresponding to a 46% relative degradation compared to the cellulose standard ( Figure S8b, Supporting Information details in Section S5). A portion of the incomplete degradation of this device can be attributed to the biologically inert PEDOT:PSS. Metals, such as gold, are not considered for the theoretical oxygen demand. However, certified tests in a compost environment have revealed full biodegradability for a device made of the same cellulose acetate, a gelatin-based electrolyte, printed gold ink, and PEDOT:PSS. [97] Hence, the cellulose acetate likely constitutes the degradation bottleneck in water and such electronic devices require compost conditions to degrade properly.
The bendable LEC pixel was attached to the stretchable substrate ( Figure S9a-c, Supporting Information). As the size of the contact spot (2 × 2 mm) was relatively small compared to the substrate, the presence of the pixel did not interfere with the mechanical properties of the elastic substrate. The Galinstan directly contacted the gold and PEDOT:PSS electrodes of the LEC pixel and linked it to the external power supply (Figure 6d). Printing the LEC layers directly onto an elastic substrate resulted in excessive swelling and nonfunctional devices ( Figure S9d, Supporting Information).
Again, isotropic stretching by semispherical objects up to a strain of 21% ( Figure S9e, Supporting Information) emulated the strains in wearable applications (Figure 6e). Upon increased bending, the current density of the pixels remained independent of the isotropic strain (Figure 6f). The decrease in current density for the gelatin substrate beyond 14% can be attributed to a gradual loss of contact with the power supply. The luminance of the pixel's yellow emission (Figure 6c,f) was equally independent of the applied strain and followed the current density fluctuations. These variations were a property of the pixel itself rather than a result of the bending ( Figure  S9f,g, Supporting Information).
In essence, the combination of a bendable pixel, liquid metal interconnects and a stretchable substrate material provided fully biodegradable light-emitting devices for isotropic bending strains of up to 21%. Further small biodegradable devices could be attached to the soft platforms by state-of-the-art printed circuit board placing technology. This concept decouples the device properties from the mechanical properties of the rubber-elastic substrate. Hence, the failure of the circuit is limited by the performance of the interconnects rather than the devices themselves.

Conclusion
We fabricated soft electronics platforms on the basis of the biodegradable synthetic elastomers in combination with the liquid metal alloy Galinstan. The mechanical properties of the synthetic polymer PGSA were adjusted by the degree of acrylation and compared to state-of-the-art biodegradable and nonbiodegradable rubber-elastic materials. The PGSA elastomer with a 19% degree of acrylation was selected as the best performing biodegradable elastic substrate for optoelectronics, on which light-emitting devices were mounted. The certified biodegradability of PSGA and the evident ecofriendliness of gelatin renders them more suitable than the commonly used PDMS for applications requiring bio-and ecofriendliness.
Embedding Galinstan conductive traces into PGSA and gelatin created elastic electronic platforms that function for linear strains of up to 90% and were successfully applied to the body. These embedded interconnects supplied a standard LED and a biodegradable LEC pixel with power, revealing a stable performance for isotropic bending strains of up to 21%. Ultimately, the maximum strain of these stretchable devices is limited by the elastic substrate material, since the functionality of the Galinstan electrode materials and the mounted pixel is strain independent. Fabricating biodegradable and fully stretchable pixels will remain challenging as long as emitter polymers and PEDOT:PSS electrodes do not offer rubber-elasticity, form cracks after a few duty cycles and limit the maximum linear strain to 30%. [53,98] These fully stretchable biodegradable devices emphasize the possibility to transfer biodegradable technology to applications where high elasticity is required. The employment of biodegradable and body-compatible materials highlights the ecofriendly potential of stretchable electronic devices and points out design routes beyond silicone and polyurethane elastomers. Applications in agriculture, healthcare, packaging, and disposable electronic consumer goods would greatly benefit from this unique combination of rubber-elasticity, biodegradability, and conductivity.

Experimental Section
Synthesis and Origin of Elastic Materials: The synthesis of PGS and PGSA is described in great detail in Section S1 of the Supporting Information. The source of gelatin, Ecoflex, PDMS, natural rubber, latex, Terratek Flex, and PEO:TMPE is detailed in Section S2 (Supporting Information).

Biodegradation Tests:
The certified Fraunhofer UMSICHT Institute conducted the biodegradability test according to DIN EN ISO 14851 (2004-10) "Determination of the ultimate aerobic biodegradability of plastic materials in an aqueous medium-Method by measuring the oxygen demand in a closed respirometer." The testing volume of 250 mL, filled with an optimized mineral media and with the test and reference substance at a concentration of 400 mg L -1 , was seeded with 140 mg L -1 activated sludge from a sewage plant and kept at an incubation temperature of 20 ± 1 °C. Microcrystalline cellulose (20 µm, Sigma Aldrich, product number 310697, total organic carbon 44%, theoretical CO 2 amount 118.5 mg/100 mg) served as the reference substance. The blind control was prepared with the mineral media and inoculum (seeding material, 0.28% dry mass, 20% of dry mass are organic) without any reference or test substance. The test substances were small chunks of crosslinked PGSA (19% DA), flakes of dried PEDOT:PSS (Clevios VPAI 4083, Heraeus), and shreds of a printed LEC stack on cellulose acetate (printed PEDOT:PSS FHC Solar, printed ZnO x , super yellow, P(CL-TMC), TBABOB, PEDOT:PSS FHC Solar). Three parallel runs were conducted for the reference, blind, and test samples. The validity criteria of a maximum oxygen demand of the seeding material of 2 mg per mg dry mass (0.21 mg mg -1 dry mass after 82 d), as well as a biodegradation degree of the reference substance of >60% (60% after 19 d) were fulfilled. The theoretical oxygen demand of the reference and test substance was calculated from a chemical elemental analysis (elemental analyzer, "Elementar") of the dry mass according to appendix A of DIN EN ISO 14851.
Tensile Tests and Interconnect Resistance Measurement: An Alluris FMT-310BU tensile tests machine was employed, where the force was measured either with an FMT-310FUC5 500N (precision of 0.1 N) or an FMT-318FUB5 50N (precision of 0.005 N) unit at a speed of 10 mm s -1 . The Young's modulus was calculated from the stress-strain slope of the initial 10% strain, with an error of about 2%. The Young's modulus, ultimate tensile strength, and elongation at break exhibit a variation of up to 20% between samples. The fabrication of the plain thin films for all materials and embedded interconnect films for PGSA-19 and gelatin, including the elbow-mounted interconnect, is detailed in Section S2 (Supporting Information). The thickness of the elastomer films was measured with a digital micrometer (Shut geometrical Metrology, 908.750), using two Teflon sheets to avoid the torsion of the film by the micrometer. The resistance of the interconnects was measured by a Keithley 2612B SourceMeter continuously at 50 Hz through a custom sample clamp. The Galinstan (Ga:In:Sn = 62:22:16 by weight, eutectic, ≥99.99% purity) was acquired from VWR or metallpulver24.de. The experiments with elbowmounted devices were performed in compliance with the KIT's policy on ethics, the informed consent of the human subject was obtained.
Mounted LEDs on Biodegradable Soft Platforms and Isotropic Bending Test: The yellow LEDs were purchased from Everlight and stuck directly into the outlets of the embedded Galinstan interconnects in the PGSA-19 and gelatin substrates. The luminance-current-voltage characteristics were recorded with a Botest Source Meter Unit (Botest LIV LED SMU, Asys Group). In order to apply an isotropic bending strain onto the suspended elastic film, differently sized half-spheres were placed underneath the film, stretching the suspended film in an isotropic manner. The bending strain was calculated from the curvature and peak of the half-spheres. In contrast to the dynamic linear tensile tests, the isotropic bending test was static.
Light-Emitting Electrochemical Cell Fabrication: The LEC was fabricated by printing and spincoating onto a 500 µm thick sheet of cellulose acetate (Rachow Kunststoff-Folien), which was flattened by a silicon wafer in the nanoimprinter (NIL CNI V1.0) at 120 °C at 5.5 bar for 10 min, as previously published, [19] and treated with a 5 min oxygen plasma. The bottom electrode was inkjet printed (Fujifilm Dimatix DMP 2831) with gold ink (DryCure AU-J without binder, C-Ink Japan) and sintered in the nanoimprinter at 120 °C from 30 min. The active layer combined the polymer emitter super yellow (PDY-132, Merck), the ion-conducting polymer poly(caprolactone-co-trimethylene carbonate) (80:20, synthesized) and the salt tetrabutylammonium bis(oxalato) borate (synthesized). Each component was dissolved at 10 g L -1 in anisole and mixed at a volume ratio of 1:0.1:0.075. The mixed solution was spincoated at 2000 rpm, 1000 rpm s -1 for 60 s and dried at room temperature in vacuum for 5 h. The top electrode was printed with the same inkjet printer from a PEDOT:PSS dispersion (Clevios F HC Solar, Hereaus Germany) with 0.25 wt% zonyl surfactant and dried at room temperature in vacuum. The LEC samples were cut into individual pixels of 10.5 × 7 mm size with an active area of 4 × 6 mm.
Mounted LEC Pixels on Biodegradable Soft Platforms: The LEC pixels were attached to the PGSA-19 or gelatin substrate (25 × 25 mm) with embedded stretchable Galinstan interconnects by a biodegradable adhesive film (NatureFlex Silver-30 with BioTak S100). A small hole in each contact pad of the LEC pixel acted as a via to connect the Galinstan outlet from below the pixel with the electrode on the planar pixel's surface. This via hole is essential to keep the Galinstan bead attached to the pixel's underside when the pixel's edge gets lifted up from the elastic substrate upon increasing isotropic strain. The Galinstan inlets were connected to the pins of the luminance-current-voltage measurement system with copper tape. The LIV characterization under isotropic strain was performed just as with the mounted LEDs.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.