Human skin provides an interface that transduces external stimuli into electrical signals for communication with the brain. There has been considerable effort to produce soft, flexible, and stretchable electronic skin (E-skin) devices. However, common polymers cannot imitate human skin perfectly due to their poor biocompatibility, biofunctionality, and permeability to many chemicals and biomolecules. Herein, we report on highly flexible, stretchable, conformal, molecule-permeable, and skin-adhering E-skins that combine a metallic nanowire (NW) network and silk protein hydrogel. The silk protein hydrogels offer high stretchability and stability under hydration through the addition of Ca ions and glycerol. The NW electrodes exhibit stable operation when subjected to large deformations and hydration. Meanwhile, the hydrogel window provides water and biomolecules to the electrodes (communication between the environment and the electrode). These favorable characteristics allow the E-skin to be capable of sensing strain, electrochemical, and electrophysiological signals.

Photonic crystals (PhCs) efficiently manipulate photons at the nanoscale. Applying these crystals to biological tissue that has been subjected to large deformation and humid environments can lead to fascinating bioapplications such as in vivo biosensors and artificial ocular prostheses. These applications require that these PhCs have mechanical durability, deformability, and biocompatibility. Herein, we introduce a deformable and conformal silk hydrogel inverse opal (SHIO); the photonic lattice of this 3D PhC can be deformed by mechanical strain. This SHIO is prepared by the UV cross-linking of a liquid stilbene/silk solution, to give a transparent and elastic hydrogel. The pseudophotonic band gap (pseudo-PBG) of this material can be stably tuned by deformation of the photonic lattice (stretching, bending, and compressing). Proof-of-concept experiments demonstrate that the SHIO can be applied as an ocular prosthesis for better vision, such as that provided by the tapeta lucida of nocturnal or deep-sea animals.silk fibroin | photonic crystal | photo-cross-linking | ocular prosthesis | intraocular pressure sensor

Following the proof-of-concept experiment in the unit structure level, photonic crystal (PhC) phosphors-structurally engineered phosphor materials based on the nanophotonics principles-are integrated with a blue light-emitting diode (LED) chip to demonstrate a compact and efficient white light source. Red- or green-emitting CdSe-based colloidal quantum dots (CQDs) are coated on a Si N thin-film grating to fabricate PhC phosphors. The underlying PhC structure is designed such that the photonic band-edge modes at the zone center (k = 0) are tuned to the energy of the blue excitation photons. By progressively stacking the PhC phosphor plates on a blue LED chip, the blue, green, and red emission intensities can be tightly controlled to obtain white light with the desired properties. The chromaticity coordinates, (0.332, 0.341), and correlated color temperature, 5500 K, are obtained from a stack of 3 red and 11 green PhC phosphor plates; in contrast, a stack of 5 red and 16 green reference phosphor plates are required to generate a similar white light. Overall, the PhC phosphors produce 8% higher total emission intensity out of 33% less amount of CQDs than the reference phosphors.

Upconversion nanoparticles (UCNPs) convert near-infrared excitation into visible emission with efficiencies far greater than those of two-photon absorption or second harmonic generation, enabling upconversion with low intensity, incoherent light. For widespread applications, however, further enhancement of upconversion efficiency is desired. Photonic crystal (PhC) structure embedded with UCNPs provides a new way to engineer the photonic environment and enhance upconversion luminescence. We incorporate silica-coated UCNPs into a two-dimensional (2D) thin film PhC structure, which exhibits enhanced local electric field at the near-infrared (NIR) excitation wavelength of UCNPs. Thanks to the nonlinearity of the upconversion process, the local field enhancement is amplified and results in a significantly enhanced luminescence intensity. We observed approximately 130-and 350-fold enhancements for green and red luminescence, respectively, and present a detailed analysis of the enhancement mechanism. Unlike the plasmonic nanostructure, which tends to cause severe luminescence quenching, the purely dielectric photonic crystal structure generally shows little quenching and provides a good alternative for many applications.