Kirigami-Based Stretchable Interconnects for Robust, Soft Neural Interfaces

Recording or modulating neural activity using implanted electrode arrays has enabled advancements in the restoration of sensory-motor pathways and brain-computer interfaces. Long-term reliability of these interfaces (&gt;6 months) is paramount to their deployment in neuroprosthetic medicine. Furthermore, hermetic encapsulation of conductive components within electrode arrays is required for long-term protection against the moisture and ions from the physiological environment to avoid electrical failure over time.<br/> <br/>We have developed a kirigami-based technology to engineer elasticity within thin films of metal and thermoplastic polymer. The periodic cuts (kirigami) patterned and replicated across a thin stack of Polyimide/Platinum/Polyimide of 1/0.1/1 µm thicknesses program stretchability within interconnect tracks as narrow as 52 µm width. The Polyimide layer is patterned with a 1 µm width offset to that of the Platinum layer, such that the metallization is fully encapsulated, top, bottom, and sideways with Polyimide, thereby providing a built-in long-term hermetic barrier.<br/> <br/>We assessed the robustness of the kirigami-based interconnects through a set of <i>in vitro</i> accelerated aging tests and repeated mechanical loading tests. We monitored the evolution of leakage current between two interdigitated tracks (IDE) biased at 5 V and soaked in phosphate-buffered saline (PBS) at 67 °C for 20 days. The elastic regime and plastic onset of the interconnects were measured during incremental uniaxial tensile stretching cycles. The electro-mechanical fatigue was assessed through uniaxially stretching linear interconnects for 1 million cycles at 10% strain, within the elastic regime.<br/> <br/>The results of the accelerated aging showed that the leakage current remained below 0.2 μA after 20 days. These findings indicate that the interconnects maintain a high level of hermeticity, equivalent to &gt;25 MΩ after 160 days at 37 °C, in accordance with Arrhenius law.<br/>The electro-mechanical characterization demonstrated that the Polyimide-encapsulated kirigami interconnects can deform reversibly up to 15% of applied strain. Moreover, cyclic stretching for 1 million cycles at 10% strain resulted in a slight increase of 5% in relative resistance, indicating minimal mechanical fatigue.<br/> <br/>The <i>in vitro</i> characterization demonstrated that the kirigami micro-patterning strategy used to fabricate stretchable interconnects can achieve a high level of hermeticity and exhibit low mechanical fatigue. Ongoing research efforts are focused on integrating materials with superior moisture barrier properties, such as silicon carbide, and further tuning the kirigami geometry to achieve a higher range of stretchability. These advancements aim to design functionally stable and longer-lasting neural interfaces that can withstand the mechanical challenges associated with chronic applications.