Implantable multi-modal physical sensor for nerve monitoring

Abstract

Traumatic peripheral nerve injury is a common disease in public health. 3% of patients in all multi-trauma cases were found to suffer from various peripheral nerve injuries. In spite of advances in the surgical technologies for nerve repair and advances in theoretical understanding of mechanism of nerve degeneration /nerve regeneration, successful outcomes of nerve recovery cannot be guaranteed after the reconstructive surgeries. Therefore, a real-time post-surgical monitoring of the physical states of the regenerated nerve during restoration of the patients is medically/clinically important. In this work, as a basic study for the achievement of this technology, a battery-free wireless, biocompatible, and implantable multi-modal physical sensing system is proposed. To the best of my knowledge it is the first research on the integration of multi-modal physical sensors into implantable medical devices for the nerve monitoring. The implantable multi-modal physical (IMMP) sensor is basically a flexible and ultra-thin film cuff type in order to easily wrap the nerve with a conformal contact. The sensor was designed to have a multi-layered encapsulation structure, a [Elastomer/Polyimide (PI)/gold (Au)/polyimide/Elastomer] structure, where a gold metal thin film active layer is embedded in a polyimide passivation film and soft elastomer matrix. This sensor structure provides mechanical/electrical stability and biocompatibility, which are the most important factors for the successful development of the implantable medical device. Furthermore, the combination of stretchable design of the [PI/Au/PI] layer and the softness of the elastomer matrix minimizes restriction of the natural motion of the target nerves. The physical sensors array is composed of a temperature sensor and a strain sensor. Each sensor was designed to be capable of independent signal sensing without interference by other stimuli, by introducing different structural patterning of the polyimide and a temperature compensation electrical circuit design. Several sensor devices could be mass-produced in a single fabrication cycle due to MEMS process-based fabrication methods. The temperature sensor exhibits high linearity (R$^2$ = 0.99), low hysteresis, reversible response and sufficient resolution (0.1°C). Also, the strain sensor displays favorable strain sensing performance with high linearity (R$^2$ = 0.99), low detection limit (0.2%), fast response time (< 50 msec), negligible hysteresis and temperature-independent detection capability. In addition, by combining the IMMP sensor and a near field communication (NFC) platform, a battery-free wireless IMMP sensing system was developed. This sensing system can communicate the measured temperature and strain information wirelessly and in real-time. Also, the entire sensing system could become simplified (wires are not required), and is possible to operate continuously and in a long-term even in a completely implanted state. Finally, in order to prove the feasibility of the implantable nerve monitoring of the wireless IMMP sensing system, in-vivo animal tests are demonstrated. The temperature and strain changes due to the sciatic nerve injuries of rats are successfully detected. In conclusion, it is expected that the proposed sensing system can monitor the physical states of the regenerated nerves in real-time after the nerve recovery surgery and it can be used for rehabilitation and diagnosis of the patients, so that it would contribute to advances in new treatment technologies in the medical field.