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Skeletal muscle is considered as a near-constant volume system, and the contractions of the muscle are related to the changes in tissue thickness. Assessment of the skeletal muscle contractile parameters such as maximum contraction thickness ( T h ), contraction time ( T c ), contraction velocity ( V c ), sustain time ( T s ), and half-relaxation ( T r ) provides valuable information for various medical applications. This paper presents a single-element wearable ultrasonic sensor (WUS) and a method to measure the skeletal muscle contractile parameters in A-mode ultrasonic data acquisition. The developed WUS was made of double-layer polyvinylidene fluoride (PVDF) piezoelectric polymer films with a simple and low-cost fabrication process. A flexible, lightweight, thin, and small size WUS would provide a secure attachment to the skin surface without affecting the muscle contraction dynamics of interest. The developed WUS was employed to monitor the contractions of gastrocnemius (GC) muscle of a human subject. The GC muscle contractions were evoked by the electrical muscle stimulation (EMS) at varying EMS frequencies from 2 Hz up to 30 Hz. The tissue thickness changes due to the muscle contractions were measured by utilizing a time-of-flight method in the ultrasonic through-transmission mode. The developed WUS demonstrated the capability to monitor the tissue thickness changes during the unfused and fused tetanic contractions. The tetanic progression level was quantitatively assessed using the parameter of the fusion index (FI) obtained. In addition, the contractile parameters ( T h , T c , V c , T s , and T r ) were successfully extracted from the measured tissue thickness changes. In addition, the unfused and fused tetanus frequencies were estimated from the obtained FI-EMS frequency curve. The WUS and ultrasonic method proposed in this study could be a valuable tool for inexpensive, non-invasive, and continuous monitoring of the skeletal muscle contractile properties.

Skeletal muscle monitoring provides valuable information about the muscle contractile properties which could be useful in various clinical applications. A wearable ultrasonic sensor (WUS) was constructed using a 110-µm thick polyvinylidene fluoride polymer piezoelectric film to perform a continuous and hands-free muscle monitoring. The lightweight and flexible properties of WUS enables the stable attachment to the skin surface without affecting the tissue motion of interest which results in less motion artifacts. In order to perform reliable estimation of tissue thicknesses by the WUS in an ultrasonic pulse echo mode, the accuracy of selected signal analysis techniques was evaluated using a laser displacement measurement result as a reference. In addition, the effects of moving averaging on the signal-to-noise ratio of the ultrasonic signals and the estimation accuracy of the tissue thickness were investigated. In order to demonstrate the feasibility of the proposed ultrasound technique for in-vivo measurements, continuous monitoring of the lateral head of triceps muscle of a healthy male subject was performed using the WUS. The tissue thickness changes due to the evoked muscle contraction by an electrical muscle stimulation (EMS) were monitored. It was observed that the muscle was unable to fully relax at the EMS frequency at 16 Hz, indicating the tetanic contraction.

Skeletal muscle is considered as a near-constant volume system and the contractions of muscle are linked to the changes in tissue thickness. Assessment of muscle contractile properties provides valuable information for various medical applications. The Ultrasound method provides a high spatial and temporal resolution measurement of the architectural changes of skeletal muscle during contractions. However, one of the challenges in ultrasound methods is motion artifacts due to the weight, size, and attachment instability of the conventional ultrasonic probe employed. A flexible, lightweight, thin, and small size ultrasonic sensor would provide a secure attachment to the skin surface without affecting the muscle contractions dynamics. Thus, in this study, a wearable ultrasonic sensor (WUS) made of a polyvinylidene fluoride (PVDF) film was employed to measure skeletal muscle contractile properties as a quantitative assessment tool. The isometric contractions of gastrocnemius (GC) muscles at a lower leg were electrically stimulated. The tissue thickness changes were measured by means of an ultrasound time-of-flight method in the through-transmission mode. Maximum contraction thickness, contraction time, contraction velocity, sustain time, and half-relaxation time were successfully extracted from the monitored thickness changes. The results demonstrated the potential of the developed WUS in measuring skeletal muscle contractile parameters.

Monitoring of the physical properties of the tissues provides valuable information for the clinical diagnosis and evaluation. However, one of the challenges of an ultrasonic method for continuous monitoring of a tissue motion using a conventional clinical ultrasonic image system could be motion artifacts due to the weight and size of its handheld ultrasonic probe employed. A wearable ultrasonic sensor, made of a polyvinylidene fluoride (PVDF) polymer piezoelectric film, may be able to reduce the motion artifacts due to its lightweight and flexible properties. However, the PVDF ultrasonic sensor has a relatively weak transmitting acoustic signal strength which causes poor signal-to-noise ratio of the ultrasonic signal acquired in pulse-echo measurements, particularly for the signal reflected from deeper tissue. This paper investigated an improvement of the ultrasonic performance of the WUS using double-layer PVDF films. The sensor was constructed using two 52-μm thick PVDF films. The developed double-layer WUS showed the 1.7 times greater ultrasonic signal amplitude compared to the WUS made of a single-layer PVDF having the equivalent PVDF film thickness. Thus, the developed double-layer PVDF WUS improved the depth of the ultrasonic penetration into the tissue. The developed WUS successfully demonstrated to monitor the contractions of biceps muscles in an upper arm. In addition, a cardiac tissue motion was clearly observed in the M-mode measurement corresponding with the cardiac cycles obtained from the ECG measurement.

One of the long-term complications of diabetes is peripheral neuropathy, where insensitive nerves can lead to serious foot ulceration. Studies have shown that the mechanical properties of plantar soft tissues change with diabetes. Ultrasound methods could be used to measure plantar tissue thickness and mechanical properties, as a quantitative assessment tool. Diabetic shoes or orthoses are often custom-designed for each person to prevent foot injury during daily activities. This study developed an ultrasonic method and sensors that can be use between the foot and a custom insole to continuously monitor the mechanical properties of plantar soft tissue during physical activities. In the proposed sensor design, the protection layer on the sensing side (bottom electrode) was eliminated and the top side was electrically shielded to reduce environmental noises. A developed wearable ultrasonic sensor was used in a preliminary experiment to measure plantar tissue thickness. Plantar heel tissue thickness was successfully measured, decreasing from 16.2 mm to 10.1 mm with pressure changes from 0 to 190 kPa. Stress-strain pressure application and release curves and the heel tissue hysteresis parameter were also successfully calculated.

A wearable and flexible ultrasonic sensor was developed for monitoring of skeletal muscle contraction. The sensor was constructed using a PVDF piezoelectric polymer film without a matching layer or backing material. Due to its lightness (less than 1 gram), thinness (200 μm) and flexibility, this sensor was wearable and enabled non-invasive and continuous muscle monitoring without restricting muscle movement, which is not feasible using a conventional handheld ultrasonic probe. The developed sensor was used to monitor muscle contractions in the index finger by ultrasonic pulse-echo measurements and in the forearm by through-transmission measurements. It was successfully demonstrated that the tissue thickness variations were measured in accordance with the muscle contraction performed.