Extended Data Fig. 2: Fabrication process of the micro-LED array.
From: Wireless closed-loop optogenetics across the entire dorsoventral spinal cord in mice
a, Main steps of the microfabrication of the micro-LED array: Step 1: Schematic illustration of the micro-LED array microfabrication process. A Ti/Au/Ti film is sputtered on a polyimide substrate and subsequently patterned by photolithography and wet etching. Next, the device interconnects are covered by a 2nd layer of polyimide, and the whole polyimide stack is patterned by photolithography and reactive ion etching (RIE). The preparation is covered with a thin layer of PDMS. The silicone superstrate is patterned to the device layout by photolitography and RIE, exposing the micro-LED integration sites. Then, micro-LEDs are precisely interfaced with the device interconnects. Finally, the micro-LED array is encapsulated with PDMS and released from the silicon carrier. Step 2: Photograph of the 4-inch wafer following the micro-LED array microfabrication process. Step 3: Colorized scanning electron micrograph (45° tilted view) of the micro-LED array surface, highlitghling the fine patterning of the PDMS superstrate and integration of the bare dies. Schematic cross-section of the device, showing three interconnects encapsulated in PDMS, top left inset. Step 4: Photograph of the optoelectronic device laminated on a fingertip. The array hosts 2 independent micro-LED channels, connected via serpentine interconnects that accommodate physiological motion. Step 5: Stress-strain curves for bulk PI, a fully functional implant and bulk PDMS measured under a displacement rate of 100μm/s. In the panel referring to the fully functional implant, the y-axis on the right (blue) also reports the relative resistance measured at the current input of 1 mA. The grey box highlights the strain range under in vivo conditions. b, Downconversion of light to desired wavelength. Step 1: Schematic illustration of the downconversion process using a phosphor-silicone matrix. Blue photons are converted to the desired wavelength (1), transmitted through the matrix (2) or back-scattered (3). Step 2: Photographs of the optoelectronic devices with the introduction of the phosphor-silicone matrix. The phosphor peak emission wavelengths are indicated below the corresponding photographs. Step 3: Optical characterization following downconversion of blue light. Emission spectra of the micro-LED arrays depending on their respective phosphor-silicone matrix implementation. Note the leakage of blue light at 𝜆 = 470 nm (left). Total optical power produced by one micro-LED channel covered with phosphor-silicone matrices with emission peaks at 𝜆 = 590 nm or 𝜆 = 620 nm. The respective optical power of the leaked blue light is depicted at 𝜆 = 470 nm. For reference, the optical power of bare blue LEDs is plotted. Step 4: Characterisation of different wavelength implants in Thy1-ChR2 and vGlut 2 ChrimsonR mice. Only 470 nm wavelength light results in muscle responses in Thy1-ChR2 mice. Only 590 nm and 650 nm wavelengths result in muscle responses in vGlut 2 ChrimsonR mice.
