The implant is fabricated on a silicon wafer in a low-temperature process that is described in detail in Supplementary Material Fig. S4. Various properties of the implant are highlighted in the pictures shown in Fig. 4. The pictures are taken before step (I) in the fabrication process shown in Supplementary Material Fig. S4. Polydimethylsiloxane (PDMS) ϵr=3 forms the main substrate (100 μm) on top of which gold traces interconnect the implant circuit. A parylene interface layer (8 μm) is added between the PDMS and the gold layer to enhance the adhesion of gold to the PDMS substrate while maintaining flexibility and transparency. This combination of two dielectric materials was chosen because parylene alone is not transparent at a thickness that provides strong structural support. On the other hand, PDMS cannot be solely used as a main substrate because it does not exhibit good attachment to gold. While other methods exist for enhancing the adhesion of gold to PDMS, we rely on parylene because it offers other desirable properties, such as low permeability to moisture (Dimer C). In our previous work [38 (link)], we found that the combination of parylene/PDMS maintains excellent transparency with a parylene layer thickness of up to 20 μm. Clear observation of cultured human-derived cardiomyocytes was possible using an inverted fluorescence microscope (Axio Observer Z1, Zeiss). A parylene passivation layer (2 μm) is added to protect and electrically isolate the electronics. A soft silicone elastomer pedestal is then attached to the bottom of the implant, underneath the positive electrode to fill the gap between the skull and the dura substitute for testing in a rat model. The pedestal has a thickness that is similar to the rat’s skull (1 mm) and it allows the implant to be positioned on the bone during in vivo testing. Electrical stimulation is delivered from the top layer through a stainless steel VIA to a gold-plated disk electrode with a diameter of 1.2 mm that is attached using silver epoxy to the silicone elastomer pedestal. Detailed steps describing the electrode fabrication, and photographs of the implant with the integrated electrode are shown in Supplementary Material Fig. S5. The assembly process for the human implementation may not require the silicone elastomer (see Fig. 1) if the implant can entirely fit on the dura substitute. Instead, only a VIA and a similar electrode with possibly a larger diameter can be used.
The antenna is formed of two coated and flexible stainless wires each with a length of 25 mm and diameter of 127 μm, as seen in Fig. 4(a). Measurement wires are connected to the implant for data acquisition. The implant’s circuit is miniaturized as shown in Fig. 4(b), and it occupies an area of 15.6 × 6.6 mm2 which allows in vivo testing in a rat model. Combining soft dielectric materials with gold, a malleable metal, results in good tolerance to bending as shown in Fig. 4(c). The implant maintains transparency, allowing clear observation of the text underneath, as shown in Fig. 4(d). These mechanical properties make the implant a suitable tool for biomedical applications where flexibility and small thickness are paramount to avoid complications. Additionally, maintaining transparency and miniaturized overall dimensions ease handling, in vivo aligning the implant to target a specific cortical region during the surgery.