(37) Liquid Crystal Polymer as an Encapsulating Layer for Implantable Electronics

Introduction: Long-term implantable devices for neural recording are essential for understanding brain function and developing effective therapies for neurological disorders. Existing implantable devices, however, often include bulky packages for electronics. There is a need for more compact encapsulation strategies that can accommodate high-density feed-throughs to high-channel-count electrode arrays. Liquid crystal polymer (LCP) is a flexible, biocompatible polymer that has shown promise as a long-lasting material for micro-electrocorticographic (μECoG) and stereotactic-electroencephalographic (sEEG) electrodes. However, there has been limited longevity testing of actively powered circuits using LCP encapsulation. Here we show accelerated aging of encapsulated active circuits using a wirelessly powered near-field communication (NFC) integrated circuit (IC).

Methods: We developed a heat press procedure for laminating LCP sheets into a monolithic material around the NFC device. We assessed lamination using micro-CT to detect air bubbles and material heterogeneities. To assess longevity, we submerged the LCP-encapsulated NFC device in phosphate-buffered saline (PBS) and monitored device functionality through continuous, wireless communication between the NFC IC and a Python module. Device failure due to saline ingress is indicated by a short in the antenna interrupting communication. We maintained samples in PBS at 60°C and 87°C to obtain accelerated aging factors of ~5x and 32x, respectively. We also monitored samples for 1 week in 150mM H2O2/PBS at 37°C for reactive accelerated aging. We compared the longevity of LCP to thermoplastic polyurethane (TPU)-encapsulated NFC devices in identical aging conditions.

Results: Micro-CT and visual inspection of the LCP lamination indicated a monolithic structure. LCP and TPU-encapsulated devices survive reactive accelerated aging tests. LCP-encapsulated devices survive for at least 4 months at 60°C (equivalent to ~ 1.7 years in the human body). LCP and TPU tests at 60°C and 87°C are ongoing.

Conclusion: LCP is a novel and promising material for implantable devices, but fabrication and testing methods are limited for LCP encapsulated devices. We developed a procedure for encapsulating wirelessly powered active circuits in LCP and assessing longevity in an accelerated aging environment using soak testing. Preliminary results demonstrate survival of LCP-encapsulated circuits in accelerated aging for an equivalent of ~1.7 years in the body (ongoing at the time of writing).