Battery-free, wireless sensors for full-body pressure and temperature mapping

Abstract

Thin, soft, skin-like sensors capable of precise, continuous measurements of physiological health have broad potential relevance to clinical health care. Use of sensors distributed over a wide area for full-body, spatiotemporal mapping of physiological processes would be a considerable advance for this field. We introduce materials, device designs, wireless power delivery and communication strategies, and overall system architectures for skin-like, battery-free sensors of temperature and pressure that can be used across the entire body. Combined experimental and theoretical investigations of the sensor operation and the modes for wireless addressing define the key features of these systems. Studies with human subjects in clinical sleep laboratories and in adjustable hospital beds demonstrate functionality of the sensors, with potential implications for monitoring of circadian cycles and mitigating risks for pressure-induced skin ulcers.

Fig. 1. Concept illustrations, exploded view schematic diagrams, and photographs of wireless, battery-free epidermal sensors used for full-body monitoring

(A) Illustration of a collection of thin, conformable skin-mounted sensors distributed across the body, with continuous, wireless transmission of temperature and pressure data in a time-multiplexed fashion. (B) Top-view photograph (scale bar, 8 mm) of a representative sensor [red, near-field communication (NFC) microchip and temperature sensor; blue, designed silicon membrane pressure sensor; green, external resistor; black, polydimethylsiloxane (PDMS) for encapsulation of sensor]. (C) Exploded view schematic illustration of the device structure. (D) Illustration of 65 wireless sensors mounted across the body, with corresponding photographs of devices at representative locations in insets. (E) Photographs of sensors at different locations on the front and back of the body. Red and green dashed boxes correspond to (D). (F) Photograph of 65 sensors that were used for experiments (scale bar, 16 mm).

Fig. 2. Physical properties and measured responses of the sensors

(A) Infrared (IR) photograph of several sensors on the forearm of a human subject for measurement of temperature response time between the skin and sensor. (B) Measured and computed temporal responses of devices constructed with different thicknesses of an insulating elastomeric support, with enlarged view (right) of a region highlighted by the red dashed box. (C) Photograph of a device mounted on the upper lip of a human subject during respiration. (D) Temperature fluctuation wirelessly recorded (sampling rate, 6 Hz) with the device shown in (C), with enlarged view (right) of a region highlighted by the red dashed box. Cycles of inhalation (green arrow) and exhalation (red arrow) are evident. (E) Schematic diagram of the mechanics and finite element analysis (FEA) results for the maximum principal strain (enlargement of red dashed box, right) across the spiral-shaped thin silicon pressure sensor with and without the polyethylene terephthalate substrate. (F) Photographs of a sensor mounted on left forearm (left) and pressed with a fingertip (right). The inset shows a magnified view to highlight the conformal contact with the skin. (G) Equivalent circuit diagram of the pressure sensing part of the device. (H) Pressure fluctuation wirelessly recorded (sampling rate, 6 Hz) with a device on the left forearm during application of various forces with the fingertip (green dashed box, poking; black dashed box, touch; red dashed box, holding). The frame on the right corresponds to the red dashed box on the left, with inset photograph (scale bar, 4 cm).

Fig. 3. Electromagnetic considerations in operating range and area coverage

(A) Sequence of photographs showing short-range readout from the skin-mounted sensor using a smartphone. Inset photograph is a diagram of the operational principles. (B) Photograph of dual-antenna system configured for full-body readout on a mattress, with inset of a subject lying on top of a ~5-cm-thick pad that covers the antennas. Subject: 27 years of age, male, 90 kg. (C) Diagram of use of such a system for time-multiplexed readout of a large collection of wireless sensors. (D) Graph of experimental measurements of operating range for an antenna (yellow rectangle in the XY plane) with dimensions of 800 mm × 580 mm × 400 mm, at radio frequency (RF) powers of 4, 8, and 12 W. (E) Computed magnetic field strength as a function of vertical distance (z) away from the XY plane at various RF powers. (F and G) Magnetic field distribution in XZ plane (F) and YZ plane (G).

Fig. 4. Wireless, full-body thermography on a human subject in a clinical sleep laboratory

(A) Diagram of the locations of 65 sensors on the human body. (B) Photograph of the bed in the sleep laboratory, with a pair of readout antennas (red dashed boxes) located underneath a soft pad on the mattress. (C) Photograph of a subject lying on the mattress. Subject: 27 years of age, male, 90 kg. (D to F) Graphs of temperature averaged over local body regions during the 7 hours of the study. The gray shaded sections indicate sleep. The black dashed boxes indicate changes in temperature occurring 2 to 3 hours before waking. Number of sensors for average neck, 4; forehead, 3; behind the ears, 4, thigh, 10; arm, 4; leg, 10; forearm, 6; chest, 5; back, 8; waist, 7; shoulder, 4. (G) Maps of temperature distributions across the body just before the subject falls asleep, (H) 2 hours before waking, and (I) shortly after waking.

Fig. 5. Wireless, full-body pressure mapping on a human subject in a hospital bed

(A and B) Diagram and photographs of the locations of 29 sensors on the back side of the body. (C and D) Photograph of an angle-adjustable bed in a hospital, with dual-antenna setup for continuous pressure monitoring. (E) Photograph of a subject (27 years of age, male, 90 kg) lying on the bed in the supine position. (F) Corresponding results of pressure measurements averaged over the body region. Number of sensors for average arm, four; leg, four; shoulder, four; buttock, three; dorsum, four; lumbar, three. Error bar: SD, one set. (G and H) Photograph of a subject and pressure measurements for the supine angle of 60°. (I) Maps of pressure distributions across the body in supine position 0° after 1000, (J) 2000, and (K) 3000 s.

Fig. 6. Summary of comparative studies of temperature measurements on a human subject in a clinical sleep laboratory: first night

(A) Schematic illustration and photographs of the locations of sensors for temperature measurement, the associated reader equipment, and the subject lying on the bed in the supine position. (B) Thermal IR photograph of the subject. (C) Rectal probe equipment as a reference. (D) Temperature in the shoulder region captured using wireless sensors. The graph on the right shows temperature measured using the rectal probe (data with individual sensor). (E and F) Temperature in the thoracic and lumbar regions captured using wireless sensors (data with individual sensor).

Fig. 7. Summary of comparative studies of pressure measurements on a human subject in a clinical sleep laboratory

(A) Schematic illustration and photographs of the positions for measurements of pressure using wireless sensors and a commercial, wired device (reference). (B) Photograph of the subject lying on the mattress with antenna embedded. (C) Pressure measured from the shoulder regions using wireless sensors and a reference device (measured at intervals of 1 min for 3 hours, data with individual sensor; error bar: SD, three sets). (D) Pressure measured from the dorsum region using wireless sensors and a reference device (measured at intervals of 1 min for 3 hours, data with individual sensor; error bar: SD, three sets). (E) Pressure measured from the lumbar region using wireless sensors and a reference device (measured at intervals of 1 min for 3 hours, data with individual sensor; error bar: SD, three sets).