An implantable, wireless, battery-free system for tactile pressure sensing

Abstract

The sense of touch is critical to dexterous use of the hands and thus an essential component of efforts to restore hand function after amputation or paralysis. Prosthetic systems have addressed this goal with wearable tactile sensors. However, such 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 pressure 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 assessment, and testing in a primate hand. The sensor implanted in the fingertip accurately measured applied 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.

Keywords: Electrical and electronic engineering; Electronic devices.

Fig. 1. Implantable, wireless, battery-free tactile sensing system.

a Illustration of the tactile sensing system wirelessly monitoring normal 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 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), an ASIC with antenna and electronic components, and a lower silica plate. c Images of the electronics assembly of the system. The ASIC is connected to the antenna, capacitors, and pads of the sensor. d Image of the system on a human index finger. e Side view of the hermetic microsystem with fused silica plates. f System-level block diagram. g Illustration of our previously published tactile sensing system and the current system

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

a Sensor equivalent circuit diagram: two semicircular 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 is 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 a silicone layer mimicking skin overlying the sensor. b Sensor response to static forces applied directly to the sensor membrane (blue) or through the silicone layer (red). Boxes indicate the force range used for subsequent dynamic loading. c, d Sensor response to sinusoidal dynamic loading at 1 Hz on the sensor membrane when the force is from 1.5 N to 2 N (blue) or through the silicone layer when the force is from 2 N to 7 N (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 a 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

Fig. 5. In vivo pilot study of sensor package implantation in a nonhuman primate fingertip.

a A nonfunctional silica package (inset) was implanted. Photographs show wound healing at 1 week a, b, 2 weeks c, and 3 weeks d. By the end of week 1, the tissue sealant (Vetbond, 3 M) used during implantation had peeled off, taking the outer epidermal layer with it. The skin was fully healed by week 3

Fig. 6. Illustration of envisioned closed-loop hand reanimation treatment for paralysis.

The implantable wireless tactile sensors developed in this work are intended to be part of an artificial sensory feedback pathway (green). The tactile signals acquired by the sensors would be wirelessly conveyed to a brain stimulator that activates a somatosensory area in proportion to the sensor output, resulting in a sense of touch. In addition, volitional movement would be restored by stimulating paralyzed muscle in proportion to neural activity recorded in a motor brain area (blue)