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  • Review Article
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Hydrogel interfaces for merging humans and machines

Nature Reviews Materials volume 7, pages 935–952 (2022)Cite this article

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Abstract

The last few decades have witnessed unprecedented convergence between humans and machines that closely operate around the human body. Despite these advances, traditional machines made of hard, dry and abiotic materials are substantially dissimilar to soft, wet and living biological tissues. This dissimilarity results in severe limitations for long-term, reliable and highly efficient interfacing between humans and machines. To bridge this gap, hydrogels have emerged as an ideal material candidate for interfacing between humans and machines owing to their mechanical and chemical similarities to biological tissues and the versatility and flexibility in designing their properties. In this Review, we provide a comprehensive summary of functional modes, design principles, and current and future applications for hydrogel interfaces towards merging humans and machines.

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Fig. 1: Current applications of hydrogel interfaces.
Fig. 2: Functional modes of hydrogel interfaces.
Fig. 3: Design principles for hydrogel interfaces with desired properties.
Fig. 4: Next-generation hydrogel interfaces.

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Acknowledgements

This work is supported by the National Institute of Health (1-R01-HL153857) and the National Science Foundation (EFMA-1935291). This work is additionally supported by the National Research Foundation, the Prime Minister′s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise programme, through the Singapore MIT Alliance for Research and Technology: Critical Analytics for Manufacturing Personalized-Medicine Inter-Disciplinary Research Group. H.Y. acknowledges financial support from Samsung Scholarship. X.Z. acknowledges the George N. Hatsopoulos (1949) Faculty Fellowship from the Massachusetts Institute of Technology and the Humboldt Research Award.

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    Present address: SanaHeal, Inc., Cambridge, MA, USA

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  1. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

    Hyunwoo Yuk, Jingjing Wu & Xuanhe Zhao

  2. Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

    Xuanhe Zhao

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Glossary

Capacitance

C=q/V, where q is the charge held on the conductor and V is the electric potential of the conductor. Unit: F.

Electrical conductivity

For an ideal conductor, the electrical conductivity is \(\sigma =L/RA\), where L is the length, A is the cross-sectional area and R is the electrical resistance of the material. The reciprocal of electrical conductivity is electrical resistivity. Unit: Sm-1.

Refractive index

n=c/v, where c is the speed of light in a vacuum and v is the speed of light in the material. Unitless.

Charge injection capacity

Amount of charge that the electrode can inject per unit area without causing irreversible electrochemical reactions or tissue damage.

Young’s modulus

The Young’s modulus of a material in the linear elastic region is E=S/ε, where S is the engineering stress and ε is the engineering strain of the material. Unit: Pa.

Fracture toughness

\(\Gamma ={G}_{c,{\rm{bulk}}}=-\,{\rm{d}}{U}_{{\rm{bulk}}}\,/\,{\rm{d}}A,\) where Gc,bulk is the critical energy release rate that drives bulk crack propagation in the material, Ubulk is the total potential energy of the material and A is the crack area measured in the undeformed state33,159. Unit: Jm-2.

Interfacial toughness

\({\Gamma }^{{\rm{inter}}}={G}_{c,{\rm{inter}}}=-\,{\rm{d}}{U}_{{\rm{inter}}}\,/\,{\rm{d}}A,\) where Gc,inter is the critical energy release rate that drives interfacial crack propagation, Uinter is the total potential energy of the adhered materials and A is the crack area measured in the undeformed state33,162. Unit: Jm-2.

Friction coefficient

\(\mu =f\,/\,N,\) where f is the measured friction force and N is the applied normal force to the material. Unitless.

Acoustic impedance

For a homogeneous material, the acoustic impedance is \(Z=\sqrt{{\rho }_{{\rm{eff}}}{K}_{{\rm{eff}}}},\) where \({\rho }_{{\rm{eff}}}\) is the effective density and Keff is the effective bulk modulus of the material. Unit: Pa∙sm−3.

Transmittance

T=I/I0, where I0 is the intensity of incident light and I is the intensity of transmitted light through the material. Unitless and often denoted in percentage.

Electrical double layer

Accumulation of charged ions around the electrode within electrolytic medium under the applied electric potential.

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Yuk, H., Wu, J. & Zhao, X. Hydrogel interfaces for merging humans and machines. Nat Rev Mater 7, 935–952 (2022). https://doi.org/10.1038/s41578-022-00483-4

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