Textile-based shape-conformable and breathable ultrasound imaging probe

The development of wearable devices for continuous, non-invasive health monitoring is gaining significant traction. Daily monitoring offers the promise of early disease detection, a critical advancement over conventional medicine's reactive approach. For effective daily monitoring, devices must possess high flexibility and breathability. High flexibility ensures close skin contact for optimal signal quality, while breathability prevents skin irritation and inflammation caused by trapped sweat. Electronic textiles (e-textiles) are promising substrates for wearable devices due to their inherent flexibility and breathability. However, applying e-textiles to ultrasound (US) imaging probes presents a challenge because the air gaps within textiles significantly attenuate or reflect US waves. Previous wearable US probe designs, while improving conformability, often lacked breathability or employed methods that could cause patient discomfort. This research aims to overcome these limitations by developing a textile-based US probe that maintains both conformability and breathability, enabling comfortable, long-term monitoring of internal tissues. The challenge lies in developing a method to allow the ultrasound waves to pass through the textile material without sacrificing the breathability of the fabric.

Existing wearable health monitoring devices utilize e-textiles for electrocardiogram (ECG) and temperature sensing. However, these lack the capability of internal tissue imaging, crucial for detecting subtle morphological and dynamic abnormalities not readily apparent in vital signs. Ultrasound (US) imaging is a minimally invasive method for visualizing internal tissues; however, conventional US probes require firm pressure against the skin for effective signal transmission, resulting in discomfort during prolonged use. Several approaches have been explored to create wearable US probes: adapting handheld probes with fixators, using thin, rigid probes with hydrogel coatings, and developing flexible probes on plastic or elastomer substrates. While flexible probes reduce pressure on the skin, their non-breathable nature can lead to skin irritation. No prior studies have successfully created a breathable, flexible US probe using textile substrates due to the significant impedance mismatch between air and textile materials, leading to high reflection and attenuation of US waves.

The researchers fabricated a textile-based US imaging probe by sandwiching US elements between two woven polyester e-textiles. Copper electrodes and wirings were formed on the textiles using electroless plating, a process that fills the air gaps between fibers in the electrode areas, improving US wave penetration. The non-electrode areas retain air gaps, maintaining breathability. The US elements were 1-3 piezoelectric composites (PZT with epoxy filler), selected for better acoustic impedance matching with human tissues. The top and bottom textiles had signal and ground electrodes, respectively, connected via low-temperature solder. To enhance flexibility, cut-outs were made between electrodes on the top textile. An adhesive hydrogel sheet was applied to the bottom textile for better adhesion to the skin. The wearability of the textile-based probe was compared with polyimide and PDMS-based probes using air permeability and bending tests. Imaging performance was evaluated using pulse-echo waveform measurements, imaging resolution tests with a wire phantom, and blood vessel visualization using a blood vessel phantom. Human neck imaging was performed to assess the probe's ability to monitor common carotid artery (CCA) and internal jugular vein (IJV) dynamics. A 24-hour continuous monitoring experiment was conducted to evaluate the probe's long-term stability and performance. The electromechanical coupling coefficient (EMCC) was used to assess the probe's performance under various bending conditions. Hadamard-encoded synthetic aperture (SA) method was used for RF data acquisition, combined with delay-and-sum (DAS) image reconstruction weighted with a generalized coherence factor (GCF) to improve image quality.

The fabricated textile-based probe exhibited low flexural rigidity (0.066 × 10⁻⁴ N m² m⁻¹) and high air permeability (11.7 cm³ cm⁻²s⁻¹), significantly higher than polyimide and PDMS-based probes. It showed high stability against large and repeated deformations, with minimal changes in EMCC after extensive bending tests. Pulse-echo waveform analysis revealed a -6 dB wave width of 0.64 µs (0.96 mm) and a -6 dB bandwidth of 3.8 MHz, comparable to a commercial linear probe. Imaging resolution tests demonstrated lateral and axial resolutions of 0.31 mm and 0.64 mm, respectively, at 10 mm depth. The probe successfully visualized simulated blood vessels (6 mm diameter) in both longitudinal and transverse planes. Human neck imaging demonstrated clear visualization of CCA and IJV pulsations. The 24-hour monitoring study showed stable visualization of CCA and IJV over the entire duration, with observable changes in IJV diameter corresponding to changes in body posture and CCA pulsation rate changes associated with sleep. No skin irritation was observed after 24-hour use. The study also included detailed analyses of pulse-echo waveforms, frequency spectra, and the effectiveness of Hadamard encoding and generalized coherence factor weighting in improving image quality.

The successful visualization of blood vessels and the ability to monitor CCA and IJV dynamics over 24 hours demonstrate the potential of the textile-based US probe for long-term health monitoring. The high breathability and flexibility of the probe minimize skin irritation and allow for comfortable prolonged use. The ability to detect changes in IJV diameter, correlated with circulating blood volume, opens possibilities for early dehydration detection. Similarly, monitoring CCA pulsation could aid in early atherosclerosis detection. The partial filling of air gaps in the textile with copper is crucial for efficient US wave transmission, overcoming the limitations of using textiles as US probe substrates. While the study demonstrated successful 24-hour monitoring, potential long-term effects of metal exposure to moisture and sweat need further investigation. Biocompatibility testing and potentially a protective coating of metal parts should be considered before clinical applications. The use of textiles offers superior stability against deformation compared to plastic or elastomer films, crucial for wearable devices subjected to repeated bending. The independent movement of individual fibers in the textile allows for high-density element arrangement without resorting to less efficient serpentine wiring designs.

This research successfully demonstrated a textile-based ultrasound imaging probe for long-term health monitoring. The probe shows superior breathability, flexibility, and stability compared to existing designs. The successful visualization of blood vessels and monitoring of clinical information highlight the potential for early disease detection through continuous monitoring. Future work should focus on improving image quality by addressing grating lobes through smaller element pitch and exploring a two-dimensional element array for improved FOV and robustness against positional variations. Further investigation into biocompatibility and long-term durability is also necessary before clinical translation.

The image quality of the textile-based probe was lower than commercially available probes, primarily due to grating lobes resulting from the relatively large element pitch (1.0 mm). This limited the imaging of complex organs and necessitated the use of longitudinal imaging in human subject experiments to avoid misalignment issues. The use of an adhesive hydrogel sheet, while beneficial for adhesion, might slightly reduce the breathability of the probe. The 24-hour monitoring involved intermittent imaging sessions, as continuous imaging with the current system limited the subject’s daily activities. The need for a stationary US acquisition system during imaging and the potential positional variations of the probe during the long-term study also presented challenges. The study used a single healthy subject, and further studies with larger participant numbers and various health conditions are needed for more comprehensive validation.