Resilient yet entirely degradable gelatin-based biogels for soft robots and electronics

Abstract

Biodegradable and biocompatible elastic materials for soft robotics, tissue engineering or stretchable electronics with good mechanical properties, tunability, modifiability or healing properties drive technological advance, and yet they are not durable under ambient conditions and do not combine all the attributes in a single platform. We have developed a versatile gelatin-based biogel, which is highly resilient with outstanding elastic characteristics, yet degrades fully when disposed. It self-adheres, is rapidly healable and derived entirely from natural and food-safe constituents. We merge all the favourable attributes in one material that is easy to reproduce and scalable, and has a low-cost production under ambient conditions. This biogel is a step towards durable, life-like soft robotic and electronic systems that are sustainable and closely mimic their natural antetypes.

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Fig. 1: A resilient yet fully degradable biogel.
Fig. 2: Tunability, stability and extreme mechanics of gelatin biogels.
Fig. 3: Resilient biogels for soft actuators.
Fig. 4: Soft and degradable electronic sensor patches.

Data availability

All the data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Information. All source files and experimental data are freely and publicly available at www.gel-sys.eu. Additional data related to this paper may be requested from the authors.

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Acknowledgements

This work was supported by the European Research Council Starting Grant ‘GEL-SYS’ under grant agreement no. 757931, the Austrian Research Promotion Agency GmbH (FFG) within the COMET-project TextileUX under grant agreement no. 865791 and through start-up funding of the Linz Institute of Technology (LIT) ‘Soft Electronics Laboratory’ under grant no. LIT013144001SEL. M.B. received support from Borealis and the Borealis Social Scholarship, VDI and the Dr Maria Schaumayer Foundation. G.B. acknowledges financial support from the European Commission within the ‘LiNaBioFluid’ project within the scope of H2020-FETOPEN-2014-2015-RIA. We thank Ewald-Gelatine, especially T. Hilt, for material support and fruitful discussions. We dedicate this work to S. Bauer.

Author information

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Authors

Contributions

M.B., S.B. and M.K. conceived the research project; M.B. developed the materials with input from G.B. and S.H.; F.H., M.B. and M.D. designed the experiments; M.B., L.L., D.P., F.H. and L.R. prepared the materials; F.H., M.B., D.W., L.R., L.L., F.G., M.S. and E.B. conducted the mechanical materials characterization; M.B., R.G. and D.P. performed the BOD and dissolution tests; S.D. recorded the SEM images; G.M. and F.H. conducted the mechanical simulations and modelling; M.B., D.P. and F.H. developed and characterized the soft actuator; F.H., R.P., M.D. and T.S. developed and characterized the degradable sensor patches; R.P. and D.S. developed the flex, PCB and software; S.K. performed the bacterial growth tests; F.H., D.W. and M.D. analysed the data; M.K. gave input at all the stages; F.H., M.B., M.D., D.W. and M.K. wrote the manuscript; all the authors contributed to editing the manuscript. M.K. supervised the research.

Corresponding author

Correspondence to Martin Kaltenbrunner.

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The authors declare no competing interests.

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Extended data

Extended Data Fig. 1 Degradation of biomaterials.

a, A bioinspired soft actuator consisting of wood, cotton-yarn, and gelatin-based biogel. b, Degradation reaction and end product of cellulose. c, Degradation and end product of gelatin. Single polypeptide strands are split by microorganisms found in waste water, gut, or soils.

Extended Data Fig. 2 Biodegradable soft robot demonstrator “Percy the Gellyfant”.

a, The manually actuated elephant trunk in its relaxed state and b, actuated u-shaped state. c, U-shaped movement allows grabbing of various objects. Scale bar, 2 cm.

Extended Data Fig. 3 Assembly of sensor skins.

Assembly process of the sensor skin consisting of degradable e-skin and reusable PCB. 1, Gels of different mechanical properties are joined by laser assisted rapid healing (LARH). 2–3, A zinc metal sheet is then applied to the gel and structured by a fiber laser. 4, After the structuring process the zinc residues are peeled off. 5, A flexible reusable PCB is mounted on the gel and soldered to the zinc foil of the e-skin. In the last fabrication step a temperature sensitive paste is placed on the gel to finalize the temperature sensor.

Extended Data Fig. 4 Biodegradable sensors characterization.

a, Stretchable meanders of zinc foil serve as conductors on biogels (G1730). b, Repeated stretching and increase of the maximum strain does not affect the conductivity of the zinc meanders. Mean resistance changes (solid line) during stretch-release cycles (data envelope) are shown. c, Those are durable for over 1000 stretching cycles, when stretched to a maximum strain of 20 %. d, Humidity sensors are realized with structured zinc foil on biogels (G2430). e, The magnitude of the impedance Z is measured as function of frequency at different climatic conditions, showing a change of two orders of magnitude. f, The sensor response is below 1 % when stretched repeatedly to a maximum strain of 10 %. g, Temperature sensors are realized with a conductive paste between two stretchable conductors on a biogel (G3030). h, The conductive pastes, fabricated on glass substrates, show a change larger than a factor of 2 over a range of 30 °C. Error bars, standard deviation for a measurement period >10 min. i, Strain sensors are designed with 5 fingers total to allow displacement of the electrodes. j, The sensor signals follow the applied strain profile linearly. Repeated cycles are tested between 20 % and 40 % strain, to account for irreversible mechanical deformation of the substrate during the first stretch-release cycle. Scale bars, 5 mm.

Extended Data Fig. 5 Adhesion tests.

a, Schematic of the 90° peel-test used to measure the debond energy between a biogel slightly pressed against diverse substrates. A 6 µm thick PET foil serves as stiff backing to prevent elongation of the peel arm, whereas a linear guiding ensures a constant angle of 90°. b, Measured debond energy for aluminum (blue), paper (purple), porcine skin (turquoise), PDMS (orange) and Ecoflex (red). c, Photographs of a biogel-porcine skin peel-sample taken during a peel-test showing the propagation of the peel-front between biogel and substrate. Mean values (solid line) and standard deviation (shadow) for n > 4 samples each. Scale bar, 1 cm.

Extended Data Fig. 6 On skin test – long time adherence.

a, Two gel discs (G1730) with a thickness of 1 mm and 2 mm and a diameter of 2 cm were applied on the skin of the upper arm with little pressure. Scale bar, 2 cm. b, The test started at 9am, c, and showed no signs of gel disintegration during 4 h of wearing. Scale bars, 1 cm. d, Even after sports activities (13 pm to 14 pm) that involved higher mechanical load and sweating, there are no signs of detachment. Scale bar, 1 cm. e, After 7 h the biogel still adheres to the skin, even at the edges of the gel. Scale bar, 1 cm. f, The gel can be removed with no visible irritations after removal. Scale bar, 2 cm. g, Rectangular stripes were applied on the skin for up to 4 days (5–12 h each day) and compared to reference samples of the same batch. Mean values (solid line) and data envelope (gray shadow) for n > 3 samples each are shown. The biogel mechanics do not change upon wearing, even after 38 h wear-time on skin, resulting in constant h, Young’s modulus (ref. A: 170 kPa ± 4 kPa, ref. B: 207 kPa ± 5 kPa), i, ultimate stress (ref. A: 830 kPa ± 60 kPa, ref. B: 1070 kPa ± 26 kPa), j, and ultimate strain (ref. A: 373 % ± 26 %, ref. B: 423 % ± 22 %). Error bars, standard deviation for a n > 3 measurements.

Extended Data Fig. 7 Pressure skin.

a–d, A weight of 20 g is applied to single pixels of the pressure skin sensor matrix. The measurement shows an impedance increase of 7–15 % compared to the initial value. Scale bar, 2 cm.

Supplementary information

41563_2020_699_MOESM2_ESM.mp4

Dissolution of biogel. The video shows dissolution of a disk of G1620 biogel in DI-water.

41563_2020_699_MOESM3_ESM.mp4

Dissolution of biogel foam. The video shows dissolution of a disk of G1215f biogel foam in DI-water.

41563_2020_699_MOESM4_ESM.mp4

Biodegradable encapsulation. The video shows dissolution of a disk of a G2430 biogel in DI-water in comparison to an equal disk with a biodegradable encapsulation. The initial color change of the coating is typical for shellac immersed in water. The biodegradable encapsulation significantly delays biogel dissolution also in acidic solutions (citric acid, pH 2.1). Degradation is triggered by basic solutions (Tris buffer, pH 8.1).

41563_2020_699_MOESM5_ESM.mp4

Inflation of biogels. The video shows inflation of three different biogels (G2820, G2420, and G1620). The comparison of the biogels shows that G2820 biogel allows for the largest strains.

41563_2020_699_MOESM6_ESM.mp4

Motion of the biogel actuators. The video shows u-shaped and s-shaped motion of the biogel actuators. An actuator tube made from fluorescent biogel is barely visible in relaxed state, but fully visible when actuated due to increased surface area of the biogel.

41563_2020_699_MOESM7_ESM.mp4

Biogel actuators operating underwater. Pneumatic actuators are dip-coated in cooking oil and operate in 20 °C DI-water. Actuator reaches 900 actuation cycles after 45 min of operation. Action continues over 1.5 h with more than 1500 actuation cycles before failure.

41563_2020_699_MOESM8_ESM.mp4

Lifting of objects with the s-shaped actuator. The video shows an actuator tube with a suction cup on its tip lifting objects. Vacuum is applied when an object is in contact with the suction cup and turned off when the lifting movement is over. Different sizes of objects can be lifted. Even objects that are smaller than the suction cup itself. Small dolls are equally lifted despite different weights ranging from 5.5 g to 30 g. The actuator (actuator weight ~35 g) lifts objects up to 119 g.

41563_2020_699_MOESM9_ESM.mp4

Interaction between two soft biodegradable robots. 1, In this video two biodegradable robots perform a manually actuated ball transfer. At the beginning the smaller degradable robot “Percy the Gellyfant” with inflated u-shaped trunk holds a sphere shaped object. The s-shaped actuator of the larger plywood soft robot has a suction cup attached to its front end and is also in its inflated state. 2, While Percy is still holding the table tennis ball the plywood elephant slowly deflates and approaches the round object until the suction cup completely touches it. 3, Vacuum is manually applied to the suction cup while the actuator of Percy is deflated. 4, As the actuator of Percy is deflated the plywood elephant actuator is inflated and lifts the table tennis ball. 5, The plywood elephant is then deflated and approaches the now inflated trunk of Percy. 6, When the biogel-actuator of Percy is bent to a curve, the lowered suction cup of the plywood elephant actuator places the table tennis ball back in Percys trunk. “Percy the Gellyfant” is holding an object. 1, The manually actuated u-shaped trunk is holding a cylindrical rigid object. 2, When the trunk is deflated the object is released 3, until it is dropped.

41563_2020_699_MOESM10_ESM.mp4

Pressure sensor enhanced smart soft actuator. 1, The video shows the movement of an u-shaped biogel actuator against a rigid object. On the tip of the soft actuator a pressure sensor is mounted, which is triggered when the sensor is compressed by contact to an object. The inflation and deflation is automated by a program. 2, At first the actuator passes by a screwdriver after inflation 3, until it reaches its maximum bending angle. At this point the deflation is actuated by the program and not by the sensor. 4, In the second case the biogel-actuator is again inflated by the program with the screwdriver in its trajectory. 5, The actuator approaches the object. After contact the foam sensor is compressed and initiates the deflation. 6, The soft actuator retreats from the screwdriver until it is completely deflated. 7, Via automated inflation of the actuator the pressure sensor at top of the actuator tip approaches a sharp rose thorn. 8, Contact with the sharp thorn compresses the pressure sensor without harming it and triggers retracting from the pointed obstacle. 9, The actuator retreats fully from the obstacle.

41563_2020_699_MOESM11_ESM.mp4

Autonomous soft electronic sensor skin. The video shows an electronic skin that intrinsically adheres to the human body. The e-skin sticks to the skin under shaking and twisting of the arm. Wireless communication and signal processing is placed on a flexible PCB, whereas the rest of the e-skin is biodegradable and stretchable. The impedance change is monitored during aspiration of the e-skin. Temperature sensors monitor hot objects like a cup with hot water. The e-skin is peeled off from the body after use.

41563_2020_699_MOESM12_ESM.mp4

Powder coating for reduced adhesion. The video shows biogels covered with talcum powder. 1, Powder covered biogels loose their stickiness. 2, The powder is removed from the surface by wiping it away with water. 3, With the powder removed, the biogel’s adhesion to arbitrary surfaces is restored.

41563_2020_699_MOESM13_ESM.mp4

Biogel thin film. 1, The video shows a biogel thin film (G1730) of 0.58 mm thickness, prepared with doctor blading. 2, The film is coated with talcum powder to make handling easier. 3, The thin film is reversibly highly stretchable and 4, deformable.

41563_2020_699_MOESM14_ESM.mp4

Soft pressure sensor array. The video shows a 4 ×4 pressure sensor array monitoring different loads. Weights (10 g, 20 g, 50 g) are placed on a single sensor to demonstrate its response. A 20 g weight is placed on different sensors. The position of the weight is detected throughout the whole skin.

Supplementary Information

Supplementary Discussion, Methods, Tables 1–3, Figs. 1–24 and Video captions 1–13.

Supplementary Video 1

Dissolution of biogel. The video shows dissolution of a disk of G1620 biogel in DI-water.

Supplementary Video 2

Dissolution of biogel foam. The video shows dissolution of a disk of G1215f biogel foam in DI-water.

Supplementary Video 3

Biodegradable encapsulation. The video shows dissolution of a disk of a G2430 biogel in DI-water in comparison to an equal disk with a biodegradable encapsulation. The initial color change of the coating is typical for shellac immersed in water. The biodegradable encapsulation significantly delays biogel dissolution also in acidic solutions (citric acid, pH 2.1). Degradation is triggered by basic solutions (Tris buffer, pH 8.1).

Supplementary Video 4

Inflation of biogels. The video shows inflation of three different biogels (G2820, G2420, and G1620). The comparison of the biogels shows that G2820 biogel allows for the largest strains.

Supplementary Video 5

Motion of the biogel actuators. The video shows u-shaped and s-shaped motion of the biogel actuators. An actuator tube made from fluorescent biogel is barely visible in relaxed state, but fully visible when actuated due to increased surface area of the biogel.

Supplementary Video 6

Biogel actuators operating underwater. Pneumatic actuators are dip-coated in cooking oil and operate in 20 °C DI-water. Actuator reaches 900 actuation cycles after 45 min of operation. Action continues over 1.5 h with more than 1500 actuation cycles before failure.

Supplementary Video 7

Lifting of objects with the s-shaped actuator. The video shows an actuator tube with a suction cup on its tip lifting objects. Vacuum is applied when an object is in contact with the suction cup and turned off when the lifting movement is over. Different sizes of objects can be lifted. Even objects that are smaller than the suction cup itself. Small dolls are equally lifted despite different weights ranging from 5.5 g to 30 g. The actuator (actuator weight ~35 g) lifts objects up to 119 g.

Supplementary Video 8

Interaction between two soft biodegradable robots. 1, In this video two biodegradable robots perform a manually actuated ball transfer. At the beginning the smaller degradable robot “Percy the Gellyfant” with inflated u-shaped trunk holds a sphere shaped object. The s-shaped actuator of the larger plywood soft robot has a suction cup attached to its front end and is also in its inflated state. 2, While Percy is still holding the table tennis ball the plywood elephant slowly deflates and approaches the round object until the suction cup completely touches it. 3, Vacuum is manually applied to the suction cup while the actuator of Percy is deflated. 4, As the actuator of Percy is deflated the plywood elephant actuator is inflated and lifts the table tennis ball. 5, The plywood elephant is then deflated and approaches the now inflated trunk of Percy. 6, When the biogel-actuator of Percy is bent to a curve, the lowered suction cup of the plywood elephant actuator places the table tennis ball back in Percys trunk. “Percy the Gellyfant” is holding an object. 1, The manually actuated u-shaped trunk is holding a cylindrical rigid object. 2, When the trunk is deflated the object is released 3, until it is dropped.

Supplementary Video 9

Pressure sensor enhanced smart soft actuator. 1, The video shows the movement of an u-shaped biogel actuator against a rigid object. On the tip of the soft actuator a pressure sensor is mounted, which is triggered when the sensor is compressed by contact to an object. The inflation and deflation is automated by a program. 2, At first the actuator passes by a screwdriver after inflation 3, until it reaches its maximum bending angle. At this point the deflation is actuated by the program and not by the sensor. 4, In the second case the biogel-actuator is again inflated by the program with the screwdriver in its trajectory. 5, The actuator approaches the object. After contact the foam sensor is compressed and initiates the deflation. 6, The soft actuator retreats from the screwdriver until it is completely deflated. 7, Via automated inflation of the actuator the pressure sensor at top of the actuator tip approaches a sharp rose thorn. 8, Contact with the sharp thorn compresses the pressure sensor without harming it and triggers retracting from the pointed obstacle. 9, The actuator retreats fully from the obstacle.

Supplementary Video 10

Autonomous soft electronic sensor skin. The video shows an electronic skin that intrinsically adheres to the human body. The e-skin sticks to the skin under shaking and twisting of the arm. Wireless communication and signal processing is placed on a flexible PCB, whereas the rest of the e-skin is biodegradable and stretchable. The impedance change is monitored during aspiration of the e-skin. Temperature sensors monitor hot objects like a cup with hot water. The e-skin is peeled off from the body after use.

Supplementary Video 11

Powder coating for reduced adhesion. The video shows biogels covered with talcum powder. 1, Powder covered biogels loose their stickiness. 2, The powder is removed from the surface by wiping it away with water. 3, With the powder removed, the biogel’s adhesion to arbitrary surfaces is restored.

Supplementary Video 12

Biogel thin film. 1, The video shows a biogel thin film (G1730) of 0.58 mm thickness, prepared with doctor blading. 2, The film is coated with talcum powder to make handling easier. 3, The thin film is reversibly highly stretchable and 4, deformable.

Supplementary Video 13

Soft pressure sensor array. The video shows a 4 ×4 pressure sensor array monitoring different loads. Weights (10 g, 20 g, 50 g) are placed on a single sensor to demonstrate its response. A 20 g weight is placed on different sensors. The position of the weight is detected throughout the whole skin.

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Baumgartner, M., Hartmann, F., Drack, M. et al. Resilient yet entirely degradable gelatin-based biogels for soft robots and electronics. Nat. Mater. (2020). https://doi.org/10.1038/s41563-020-0699-3

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