Development of a Wearable Ultrasound Transducer for Sensing Muscle Activities in Assistive Robotics Applications

Abstract

Robotic prostheses and powered exoskeletons are novel assistive robotic devices for modern medicine. Muscle activity sensing plays an important role in controlling assistive robotics devices. Most devices measure the surface electromyography (sEMG) signal for myoelectric control. However, sEMG is an integrated signal from muscle activities. It is difficult to sense muscle movements in specific small regions, particularly at different depths. Alternatively, traditional ultrasound imaging has recently been proposed to monitor muscle activity due to its ability to directly visualize superficial and at-depth muscles. Despite their advantages, traditional ultrasound probes lack wearability. In this paper, a wearable ultrasound (US) transducer, based on lead zirconate titanate (PZT) and a polyimide substrate, was developed for a muscle activity sensing demonstration. The fabricated PZT-5A elements were arranged into a 4 × 4 array and then packaged in polydimethylsiloxane (PDMS). In vitro porcine tissue experiments were carried out by generating the muscle activities artificially, and the muscle movements were detected by the proposed wearable US transducer via muscle movement imaging. Experimental results showed that all 16 elements had very similar acoustic behaviors: the averaged central frequency, -6 dB bandwidth, and electrical impedance in water were 10.59 MHz, 37.69%, and 78.41 Ω, respectively. The in vitro study successfully demonstrated the capability of monitoring local muscle activity using the prototyped wearable transducer. The findings indicate that ultrasonic sensing may be an alternative to standardize myoelectric control for assistive robotics applications.

Keywords: flexible ultrasound transducer; muscle movement; powered exoskeleton; robotic prosthesis; ultrasound imaging; wearable ultrasound transducer.

Figure 1

A schematic representation of wave transmission and reflection in the matching layer.

Figure 2

(a) Schematic demonstration of the wearable US transducer design. (b) The fabrication process for the wearable US transducer.

Figure 3

Demonstration of the wearability and customizability of the transducer: (a) size is about 1.5 cm, which can be used for single muscle measurements; (b) Size is about 10 cm, which can be used for multiple muscles measurements; (c) demonstration of flexibility for the wearable transducer.

Figure 4

Schematics of the experimental setups for transducer characterizations: (a) pulse/echo test for the transducer; electrical impedance measurements for the transducer element in (b) water and in (c) air.

Figure 5

Experimental setup for in vitro test: block diagrams of the experimental system: (a) side view and (b) top view; (c) photographs of the experimental setup.

Figure 6

The experimental setup used for the in vivo test: (a) block diagrams of the experimental system; (b,c) photographs of the experimental setup.

Figure 7

Transducer characterizations element #10: (a) measured electrical impedance and (b) pulse-echo response of element #10.

Figure 8

The variations of received RF signals from porcine tissue#1 at different pulling distances from T0 to T5 from (a) element #2 and (b) element #16. The location of selected elements is marked in red.

Figure 9

Muscle movement imaging for (a) porcine tissue#2 and (b) porcine tissue#1. The color bar represents the relative displacement of the muscle. The displacement was calculated using T0 as the reference.

Figure 10

Muscle average movement imaging during in vivo test. The color bar represents the relative displacement of the muscle. The displacement was calculated using the forearm at $0^{\circ }$ as the reference.