Fig. 1. Fundamental problem in the area of brain-machine interface: interaction between neurons and diodes. (a) Fundamental unit in the neural system (“brain”) is a single neuron; (b) fundamental unit in the information system (“machine”) is a semiconductor diode; (c) semiconductor diodes are utilized to modulate and detect neural activities: i. An electrically driven light-emitting diode (LED) can emit light to stimulate neural activities via optogenetics; ii. Under illumination, a photodiode (PD) can generate photoelectric signals to stimulate neural activities; iii. A photodiode can capture fluorescence signals generated from neurons expressing photosensitive protein, thus detecting neural activities; iv. The optical emission from an LED can be altered by the electrophysiological activities of neurons, which can be used for neural signal detecting

Fig. 2. Colocalized, bidirectional optogenetic modulation of neural activities with a wireless dual-color micro-LED probe^[34]. (a) Schematic view of probe inserted into target brain area to perform dual-color neural activation and inhibition; (b) schematic of cellular scale depiction of probe’s photo-modulation function: a red LED regulates cation channel ChrimsonR for depolarization, while a blue LED regulates anion channel stGtACR2 for hyperpolarization; (c) exploded view of dual-color LED probe made from vertically stacked blue and red thin-film micro-LEDs assembled on a flexible polyimide (PI) substrate, with a filter for spectrally selective reflection and transmission; (d) optical images of a micro-LED probe, showing dual-color LED can be independently controlled to realize blue and red emissions; (e) experimental photographs and simulated results illustrating blue and red light propagation in a brain phantom; (f) images of a micro-LED probe integrated with a wireless circuit for independent control of red and blue emissions; (g) cell activity traces presenting bidirectional spike activation and inhibition with alternating red and blue illuminations; (h) representative variations of dopamine signals in response to stimulation by red and blue LEDs; (i) representative heat maps showing real-time preference and aversion behavior following red or blue stimulation for mice co-expressing stGtACR2 and ChrimsonR; (j) photograph showing three free-moving mice implanted with wireless probes

Fig. 3. Thin-film silicon diodes for optoelectronic excitation and inhibition of neural activities^[35]. (a) Image of a freestanding Si film (thickness of ~2 µm) transferred onto a flexible substrate; (b) optical illumination generates polarity-dependent electrical signals by p⁺n Si and n⁺p Si film diodes; (c) electrical signals can depolarize (left) and hyperpolarize (right) neuron membrane potentials; (d) photoelectric signals from p⁺n Si and n⁺p Si films activate and inhibit neural activities; (e) a multichannel recording probe is guided into mouse brain to sample extracellular activities by illuminating Si films attached on the mouse cortex with a 473 nm laser; (f) p⁺n Si and n⁺p Si films selectively elicit (top) and suppress (bottom) electrical signals on mouse cortex; (g) cartoon diagram illustrating the modulation of mice’s peripheral nervous, with light illuminating Si films attached on sciatic nerve; (h) compound muscle action potentials (CMAPs) are recorded by EMG recording electrode inserted into hindlimb-related muscles under pulsed light with various intensities

Fig. 4. Wireless optoelectronic probe for monitoring calcium fluorescence signal in the deep brain^[36]. (a) Schematic exploded-view illustration of a wireless, injectable, ultrathin photometry probe with a InGaN micro-LED and a GaAs photodevice at flexible substrate, where thin-film filters serve as wavelength-selective coatings; (b) conceptual illustration of microprobe system for Ca²⁺ fluorescence sensing; (c) left image is optical micrograph of injectable photometry probe and right image is magnified colorized SEM image of probe tip; (d) photo of wirelessly operated probe system; (e) photograph of a mouse implanted with fluorescent probe; (f) injection schematic of virus AAV-DJ-CaMKII-GCaMP6f (green) into BLA and fluorescence photo of brain-area section expressed with calcium fluorescence indicator; (g) heatmap (upper) for Ca²⁺ fluorescence signals recorded by wireless device before and after shock, aligned with trace plotted as mean (curves) ± standard deviation (shading around curves) (lower)

Fig. 5. A semiconductor diode is utilized to optically sense biological activities^[37]. (a) Schematic of a thin-film and red-emitting micro-LED attached onto human skin for wireless optical sensing of biophysical signals based on its conductance dependent photon-recycling (PR) effect; (b) operational principle of the sensor, showing that the micro-LED’s photoluminescence (PL) intensity is dependent on its load resistances in different working conditions: short-circuit, open-circuit, and connected to different resistances (R₁>R₂); (c) schematic illustration of optoelectronic sensing of galvanic skin response (GSR); (d) microscopic images of micro-LED’s PL emission, showing different PL intensities of micro-LED under basal and deep breath conditions of a subject; (e) scheme showing optical readout of bioelectrical signals based on photon recycling of a semiconductor diode; (f) typical current-voltage characteristic measured for a red-emitting micro-LED; (g) absolute (red) and relative (blue) PL intensities in response to a voltage change of 1 mV, as a function of applied voltage