Fig. 1. Hydrogel optical waveguides. (a) Fabrication of slab hydrogel optical waveguide^[31]; (b) light guiding in slab hydrogel waveguide^[31]; (c) attenuation spectra of PEGDA hydrogels of different relative molecular masses^[31]; (d) fabrication of step-index, core-cladding hydrogel optical fiber^[41]; (e) light guiding of a hydrogel optical fiber in porcine tissue^[41]; (f) propagation losses of the hydrogel fibers^[41]; (g) microscope images of a highly stretchable, tough hydrogel optical fiber made of Alginate/PAAm^[33]; (h) stress-strain curve of Alginate/PAAm hydrogel optical fiber^[33]; (i) fabrication of core-cladding hydrogel optical fiber by dynamic wet spinning apparatus^[49]

Fig. 2. Hydrogel micro/nano photonic devices. (a) Fabrication of hydrogel photonic crystals (PhCs)^[53]; (b) mechanical flexibility of the hydrogel PhCs^[53]; (c) structural color of hydrogel PhCs with designed periodic nanoholes^[54]; (d) fabrication of hydrogel gratings by laser holographic lithography^[62]; (e) laser-induced reduction of Ag⁺ ions to prepare gratings^[61]; (f) photographs of the hydrogel gratings^[62]; (g) surface image of the hydrogel grating^[61]; (h) diffraction spectrum^[62]

Fig. 3. Elastomer optical waveguides. (a) Fabrication of PDMS waveguide array^[65]; (b) PDMS waveguide array integrated with light source and photodiodes^[65]; (c) effect of stretching on waveguide performance^[65]; (d) fabrication of step-index, core-cladding elastomer optical fiber^[67]; (e) cross-section image of a PDMS optical fiber^[67]; (f) light guiding in a PDMS optical fiber^[67]; (g) coextrusion fabrication of step-index TPE optical fibers^[69]; (h) photograph of 200 m TPE fibers^[69]; (i) stretchability of the TPE optical fibers^[69]; (j) fabrication of single-mode TPE optical fiber^[72]; (k) single-mode guidance in the TPE optical fiber^[72]

Fig. 4. Elastomer micro/nano photonic devices. (a) Fabrication of PDMS PhCs^[74]; (b) tunable structural color of PDMS PhCs upon stretching^[74]; (c) strain induced spectral shift of the PhCs in reflection^[74]; (d) fabrication of PDMS gratings by nanoimprint lithography^[76]; (e) microscope image of PDMS diffraction grating^[76]; (f) stretchable PDMS optical waveguide with nanograting structure^[77]

Fig. 5. Biodegradable photonic devices based on naturally polymers. (a) Direct ink printing of silk waveguides^[90]; (b) images of straight and wavy silk waveguides^[90]; (c) light guiding of a step-index silk waveguides in tissue^[91]; (d) silk microprism array^[93]; (e) silk PhCs^[92]; (f) silk microlens array^[95]; (g) fabrication of agarose optical fiber with porous structure^[85]; (h) photograph of a porous agarose optical fiber^[85]; (i) cross-section image of a double-core/cladding cellulose optical fiber^[87]

Fig. 6. Biodegradable photonic devices based on synthetic polymers. (a) PLA film fabricated by melt pressing^[30]; (b) waveguide of specific shape fabricated by laser cutting of PLA film^[30]; (c) time-dependent in vivo degradation of PLA waveguide^[30]; (d) average relative molecular mass of PLA chains versus degradation time^[30]; (e) fabrication of core-cladding citrate-based polymeric optical fiber^[35]; (f) cross-section image of a citrate-based optical fiber^[35]; (g) photograph showing light guiding of a citrate-based optical fiber^[35]; (h) biodegradability of the citrate-based optical fiber in vitro^[35]

Fig. 7. Implantable biomedical sensing. (a) Structure of the glucose-sensitive hydrogel optical fiber^[44]; (b) implantation of hydrogel optical fibers in porcine tissue^[44]; (c) time-dependent transmission changes as glucose binds to the hydrogel fiber^[44]; (d) photographs of the glucose-sensitive hydrogel fibers with/without gold nanoparticles (GNPs) modification^[32]; (e) transmission spectra^[32]; (f) implantation of glucose-responsive fluorescent hydrogel fibers in mouse ears^[113]; (g) glucose monitoring 140 days after implantation^[113]; (h) real-time monitoring of blood oxygen saturation by implanted hydrogel optical fiber in living mice^[41]; (i) readout of oxygenated and deoxygenated hemoglobin concentrations^[41]

Fig. 8. Light-based therapies. (a) Implantation of waveguide in porcine skin incision^[30]; (b) photochemical tissue bonding of skin incision with hydrogel waveguide^[30]; (c) hydrogel waveguide for noninvasive PDT in a mouse glioblastoma multiforme (GBM) model^[123]; (d) light guiding of near infrared light transmission^[123]; (e) mouse tumors were regressing after PDT as compared to other controls groups^[123]; (f) optogenetic hydrogel optical fiber probe^[108]; (g) mouse implanted with a hydrogel optical fiber in motor cortex for optogenetic modulation^[108]; (h) regulation of mouse behaviors under neuronal stimulation^[108]

Fig. 9. Wearable sensing applications. (a) Stretchable optical strain sensor based on PDMS optical waveguide coated with a thin gold reflective layer^[127]; (b) stretchable fiber-based strain sensor integrated on an athletic tape^[128]; (c) monitoring weight-bearing activities^[128]; (d) wrist pulse monitoring^[68]; (e) detection of muscle movement during speaking^[68]; (f) smart glove for finger motion detection^[68]; (g) activations in the motor cortex of a patient with Parkinson's disease while performing finger motions^[68]; (h) flexible and wearable fiber-optic temperature sensor^[67]; (i) subtle thermal signals generated by breathing^[67]; (j) wearable strain and temperature sensor based on photonic crystals^[134]; (k) schematic of the thermoresponsive plasmonic microgel films^[135]; (l) spatial skin temperature visualization and mapping^[135]

Fig. 10. Applications in human-machine interface and robotics. (a) Artificial prosthetic hand installed with flexible optical waveguide sensor^[24]; (b) photographs of flexible optical waveguide^[24]; (c) prosthetic hand actuated to perceive texture, shape, and softness^[24]; (d) flexible LSPR optical tactile sensor^[29]; (e) robotic hand integrated with LSPR sensor to perceive material hardness^[29]; (f) tactile force mapping of different hand motions^[29]; (g) smart glove integrated with elastomer optical fiber strain sensor for virtual model control^[69]; (h) polymer-waveguide-based flexible tactile sensor array^[138]; (i) microscope image of a waveguide taken at the surface of the sensing area^[138]; (j) a thin-film waveguide sensor array at 3×9 matrix^[138]

Fig. 11. Multimode tactile sensing. (a) Heterogeneous tactile sensor with multi-sensing elements^[139]; (b) optical waveguide composed of elastomer cladding and ionic liquid core^[139]; (c) estimation result for eight different combinations of multimode deformations^[139]; (d) light output of the stretchable distributed fiber-optic sensor under different deformation modes^[140]; (e) wireless glove integrated with distributed fiber-optic sensors^[140]; (f) real-time reconstruction of combined proprioception and exteroception^[140]