An implantable wireless tactile sensing system

Abstract

The sense of touch is critical to dexterous use of the hands and thus an essential component to efforts to restore hand function after amputation or paralysis. Prosthetic systems have focused on wearable tactile sensors. But wearable sensors are suboptimal for neuroprosthetic systems designed to reanimate a patient's own paralyzed hand. Here, we developed an implantable tactile sensing system intended for subdermal placement. The system is composed of a microfabricated capacitive force sensor, a custom integrated circuit supporting wireless powering and data transmission, and a laser-fused hermetic silica package. The miniature device was validated through simulations, benchtop testing, and ex vivo testing in a primate hand. The sensor implanted in the fingertip accurately measured skin forces with a resolution of 4.3 mN. The output from this novel sensor could be encoded in the brain with microstimulation to provide tactile feedback. More broadly, the materials, system design, and fabrication approach establish new foundational capabilities for various applications of implantable sensing systems.

Fig. 1.. Implantable wireless tactile sensing system.

(A) Illustration of the tactile sensing system wirelessly monitoring forces acting on the fingertip and palm. The implantable sensing system integrates capacitive force detection, signal processing, and customized wireless data and power transceiver interfaced to a battery-powered wearable base unit. (B) Exploded view of the implantable sensor, which contains five layers: an upper silica membrane, capacitive double plates, a middle silica plate (with vias, feedthroughs, and pads), ASIC with antenna and electronic components, and a lower silica plate. (C,D) Images of the front and back of the sensor. (E) Side view of the hermetic microsystem with fused silica plates. (F) System-level block diagram.

Fig. 2.. Sensor operation principle and finite element simulation results.

(A) Sensor equivalent circuit diagram: two semi-circular capacitors connected in series by the upper electrode with a floating potential. (B) Sensor in normal mode operation. (C) Sensor in touch mode operation. (D) Simulation model of static forces applied directly to the sensing membrane. (E) Direct force model results: sensor capacitance vs. applied load. (F) Simulation model of static forces applied to the surface of skin overlying the implanted sensor. Skin modeled as two elastic layers (dermis, hypodermis). (G) Simulated sensor response to tactile forces when implanted below skin (red) compared to the response to direct forces on the sensor membrane (black).

Fig. 3.. Benchtop sensor performance.

(A) Direct compression testing setup, shown here with silicone layer mimicking skin overlying the sensor. (B) Sensor response to static forces applied directly to sensor membrane (blue) or through silicone layer (red). Boxes indicate the force range used for subsequent dynamic loading. (C,D) Sensor response to sinusoidal dynamic loading at 1Hz on the sensor membrane (blue) or through the silicone layer (red). (E) Hydrostatic testing setup, with the sensor placed in water within a syringe. (F) Sensor response to hydrostatic pressures

Fig. 4.

Ex vivo sensor performance. (A) Image of cadaver monkey hand after the wireless sensor was surgically placed in the indicated fingertip. The base unit was placed on the fingernail and static and dynamic forces were applied to the skin overlying the implanted sensor. (B) The sensor response to static, light tactile forces, showing sensitivity and repeatability. (C) Dynamic forces applied to the fingertip over 20 s. (D) The sensor response to dynamic forces. (E) Estimated force based on the linear transformation of sensor output compared to the applied force.