Wireless sensors for continuous, multimodal measurements at the skin interface with lower limb prostheses

Abstract

Precise form-fitting of prosthetic sockets is important for the comfort and well-being of persons with limb amputations. Capabilities for continuous monitoring of pressure and temperature at the skin-prosthesis interface can be valuable in the fitting process and in monitoring for the development of dangerous regions of increased pressure and temperature as limb volume changes during daily activities. Conventional pressure transducers and temperature sensors cannot provide comfortable, irritation-free measurements because of their relatively rigid construction and requirements for wired interfaces to external data acquisition hardware. Here, we introduce a millimeter-scale pressure sensor that adopts a soft, three-dimensional design that integrates into a thin, flexible battery-free, wireless platform with a built-in temperature sensor to allow operation in a noninvasive, imperceptible fashion directly at the skin-prosthesis interface. The sensor system mounts on the surface of the skin of the residual limb, in single or multiple locations of interest. A wireless reader module attached to the outside of the prosthetic socket wirelessly provides power to the sensor and wirelessly receives data from it, for continuous long-range transmission to a standard consumer electronic device such as a smartphone or tablet computer. Characterization of both the sensor and the system, together with theoretical analysis of the key responses, illustrates linear, accurate responses and the ability to address the entire range of relevant pressures and to capture skin temperature accurately, both in a continuous mode. Clinical application in two prosthesis users demonstrates the functionality and feasibility of this soft, wireless system.

Fig. 1. Schematic illustrations, optical images, and overview of a wireless multimodal sensing system for measurements at the interface between a residual limb and a prosthesis.

(A) Schematic illustration of the overall system. (B) Illustration of wireless, battery-free multimodal sensors on a residual limb and near-field communication (NFC)/Bluetooth low energy (BLE) modules on the outer surface of a prosthetic socket. (C) Picture of NFC/BLE modules on a socket worn by a person with amputation. (D) Picture of an NFC/BLE module attached to a socket. (E) Optical image of a multimodal sensor highlighting its flexible and thin form factor. (F) Functional block diagram showing three parts of the system: wireless, battery-free multimodal sensors; NFC/BLE module; and portable electronic device with GUI. SPI, Serial Peripheral Interface.

Fig. 2. Design features and performance characteristics of 3D millimeter-scale, soft pressure sensors.

(A) Simulation results for the distribution of strain in a 2D precursor before (0% compression) and after (20% compression) transformation into a 3D shape for pressure sensing. (B) Optical image of the precursor, showing patterned thin metal traces as strain gauges. The area enclosed in the red dashed line is shown at higher magnitude in the right inset. (C) Image of the 3D mesostructure formed by compressive buckling. (D) Schematic illustration (top) and exploded view diagram (bottom) of a sensor with an encapsulation (Encap.) layer and a strain-limiting frame. (E) Picture of a pressure sensor on the tip of an index finger. (F) Fractional change in resistance (|ΔR|/R) of a sensor with a strain-limiting frame as a function of applied pressure. (G) Responses of three different sensors to pressure loading and unloading. Orange: encapsulation with modulus of 2 MPa; assembly with 20% prestrain. Green: modulus of 2.6 MPa; prestrain of 20%. Blue: modulus of 2.6 MPa; prestrain of 15%. (H) Fractional change of resistance of a sensor under four cycles of loading/unloading compared to measurements using a force gauge. (I) Fractional change of resistance of a sensor during unloading of external pressure using a linear motion stage moving at a speed of 20 mm/s, compared to measurements using a force gauge.

Fig. 3. Battery-free sensors for wireless monitoring of pressure and temperature at the interface of a residual limb and a prosthesis.

(A) Exploded view schematic illustrations of the sensing device. The structural support, electronic components, circuits, antenna, and adhesive layer are abbreviated to Struct. support, Elec. comp., Circ., ant., and Adh., respectively. (B) Picture of the device. (C) Response of the wireless sensor under pressure loading and unloading. (D) Fractional change of the analog-to-digital converter (ADC) value transmitted by the device under three cycles of loading/unloading as compared to force gauge measurements. (E) Fractional change of the ADC value under a constant load as compared to force gauge measurements. (F) Response of the wireless temperature sensor at different temperatures. (G) Fractional change of the ADC value of the wireless temperature sensor under three cycles of temperature increase and decrease as compared to infrared (IR) camera measurements. (H) Fractional change of the ADC value at a constant temperature as compared to IR camera measurements.

Fig. 4. Wireless measurements of pressure and temperature from a non-amputee individual.

(A) Schematic illustrations of a non-amputee participant with three sensors on the right leg and NFC/BLE modules on the outside surface of a prosthesis simulator. (B) Pictures of a prosthesis simulator with attached NFC/BLE modules. (C) Representative pressure data collected while in a sitting position. (D) Representative pressure data collected while in a standing position. (E) Representative pressure data collected while walking on a treadmill (0.67 m/s). (F) Comparison of pressure data from the wireless sensor and from a wired pressure sensor array as a reference during leg flexing (90°). (G) Bland-Altman plot for pressure data from the wireless sensor and from the wired reference during leg flexing. (H) Temperature data from the wireless sensor and from a wired thermocouple as a reference collected while sitting, standing, and walking on a treadmill (0.67 m/s). All data were collected on two individuals, separately (n = 2).

Fig. 5. Wireless measurements of pressure and temperature from a participant with transtibial amputation.

(A) Schematic illustrations of an individual with transtibial amputation with three multimodal sensors on the residual limb and NFC/BLE modules on the prosthesis. (B) Pictures of the transtibial residual limb with wireless sensors adhered to different locations. (C) Picture of a prosthesis with attached NFC/BLE modules. (D) Pictures of the participant during three different activities. (E) Representative pressure data collected while in a sitting position. (F) Representative pressure data collected while in a standing position. (G) Representative pressure data collected while walking on a treadmill at a self-selected slow speed (0.31 m/s). (H) Representative pressure data collected while walking on a treadmill at a self-selected fast speed (0.81 m/s). (I) Temperature data collected throughout the testing protocol. All data were collected on a single participant (n = 1).

Fig. 6. Wireless measurements of pressure and temperature from a participant with transfemoral amputation.

(A) Schematic illustrations of an individual with transfemoral amputation with three multimodal sensors on the residual limb and NFC/BLE modules on the prosthesis. (B) Pictures of the transfemoral residual limb with wireless sensors adhered to different locations. (C) Pictures of a prosthesis with attached NFC/BLE modules. (D) Pictures of the participant during three different activities. (E) Representative pressure data collected while in a sitting position. (F) Representative pressure data collected while in a standing position. (G) Representative pressure data collected while walking on a treadmill at a slow self-selected speed (0.39 m/s). (H) Representative pressure data collected while walking on a treadmill at a fast self-selected speed (0.88 m/s). (I) Temperature data collected throughout the testing protocol. All data were collected on a single participant (n = 1).