A low-profile electromechanical packaging system for soft-to-flexible bioelectronic interfaces

Abstract

Interfacing the human body with the next generation of electronics requires technological advancement in designing and producing bioelectronic circuits. These circuits must integrate electrical functionality while simultaneously addressing limitations in mechanical compliance and dynamics, biocompatibility, and consistent, scalable manufacturing. The combination of mechanically disparate materials ranging from elastomers to inorganic crystalline semiconductors calls for modular designs with reliable and scalable electromechanical connectors. Here, we report on a novel interconnection solution for soft-to-flexible bioelectronic interfaces using a patterned and machined flexible printed circuit board, which we term FlexComb, interfaced with soft transducing systems. Using a simple assembly process, arrays of protruding "fingers" bearing individual electrical terminals are laser-machined on a standard flexible printed circuit board to create a comb-like structure, namely, the FlexComb. A matching pattern is also machined in the soft system to host and interlock electromechanically the FlexComb connections via a soft electrically conducting composite. We examine the electrical and electromechanical properties of the interconnection and demonstrate the versatility and scalability of the method through various customized submillimetric designs. In a pilot in vivo study, we validate the stability and compatibility of the FlexComb technology in a subdural electrocorticography system implanted for 6 months on the auditory cortex of a minipig. The FlexComb provides a reliable and simple technique to bond and connect soft transducing systems with flexible or rigid electronic boards, which should find many implementations in soft robotics and wearable and implantable bioelectronics.

FIG. 1.

Assembly with FlexComb interconnections. (a) Photograph of a PDMS-based electrode array connected with FlexComb interconnections. (b) Microscope image of the connection area between FlexComb and the PDMS device. Inset: magnified portion of the finger area of the FlexComb. (c)–(h) Illustrated assembly process: (c) post-wafer fabrication of the electrode array on PDMS. (d) Application of the conductive Pt mesoparticles/PDMS composite serving as conductive adhesive over the interconnection area. (e) Alignment and placement of the FlexComb into the PDMS wells. The conductive paste is represented in transparency. (f) Assembly after the deposition of the FlexComb onto the wafer. (g) Removal of the PET stencil mask to pattern the conductive adhesive and isolate separate connections. (h) Dispensing of silicone sealant to mechanically fix the FlexComb onto the PDMS implant and isolate it electromechanically from the external environment. The conductive paste is represented in transparence.

FIG. 2.

Electrical characterization of FlexComb connections. (a) Diagram of the contact resistance test device. Electrical resistance is measured between each terminal on the distal end of the FlexComb (see supplementary material, Fig. 3). (b) Electrical measurement of the electrical test structure depicted in (a) for three different connection setups: silver paste + ZIF connector (ZIF) (in red), silver paste + FlexComb (in green), Pt-PDMS paste + FlexComb (in blue), respectively. Dotted line: linear regression. (c) Extracted contact resistance for the three connection setups.

FIG. 3.

Electromechanical characterization of FlexComb connections. (a) and (b) Relative change of contact resistance when stretched (R, resistance at maximum stretch, diamonds) up to 50% elongation (calculated on the length of the interconnect section) and at rest (R₀, resistance at rest, circles) after relaxation, for (a) silver paste with FlexComb (green) and (b) Pt-PDMS with FlexComb (blue). (c) and (d) Relative change of contact resistance when stretched (R, resistance at maximum stretch, diamonds) to 10% elongation (calculated on the length of the interconnect section) and at rest (R₀, resistance at rest, circles) after relaxation, upon cyclic stretching for (c) silver paste with FlexComb (green) and (d) Pt/PDMS with FlexComb (blue).

FIG. 4.

FlexComb packaging for soft electrode systems. (a) Comparison of the surface mounted connector (here, a ZIF connector) (left) and the FlexComb (right) interconnections for 32-channel soft electrode arrays. (b) Electrochemical impedance spectroscopy (top: impedance modulus, bottom: phase) of the electrodes connected with the SMT connector (red) and FlexComb (blue). (c) Impedance modulus values at 1 kHz extracted from (b). (d)–(g) FlexComb configurations for different soft electrode system designs targeting diverse in vivo applications. (d) Six-channel rat spinal cord implant with 0.5 mm connector pitch. (e) 16-channel minipig brain implant with 0.5 mm connector pitch and right-angle connection. Anchoring wings for bone screw fixation are shown on the left. (f) 32-channel non-human primate brain implant with 0.35 mm connector pitch and right-angle connection. (g) 60-channel electrode array for the human heart with 0.35 mm connector pitch. (h) and (i) Impedance modulus at 1 kHz (colored dots) and device yield (bar plot) for two consecutive assembled batches, respectively, of the FlexComb with the design presented in (d). An electrode was classified as nonfunctional when the impedance modulus at 1 kHz is >10 kΩ.

FIG. 5.

FlexComb as interconnection for in vivo neural interfaces. (a) Schematic of the subdural implantation of the neural interface with the FlexComb connector. (b) Photograph of FlexComb implanted subdurally in a minipig model, offering minimal tissue displacement. The dura mater can be sutured around the exiting fPCB cable. (c) Explanted minipig brain after 6-week implantation showing the position of the soft implant and the FlexComb connector. The position of the auditory cortex is highlighted. (d) Representative channel of auditory evoked potential recordings in response to auditory stimuli at 1 kHz compared to baseline after 2 months of implantation. The orange bar represents the timing of the auditory stimuli ON period (500 ms). (e) AEP recording map of the entire electrode array from acoustic stimuli at 1 kHz at 2 months implantation. The orange bar represents the timing of the auditory stimuli ON period. (f) Soft implant layout. Highlighted in color, three channels running in parallel at the connector and interconnect level. (g)–(i) Evoked potential recordings from 1 kHz auditory stimuli at 2 months' implantation from the three channels highlighted in (f). The orange bar represents the timing of the auditory stimuli ON period.