Additive Manufacturing of Laser-Induced Graphene with Piezoelectric Polymer for Wearable Ultrasound Transducers

Ultrasound technology has emerged as a versatile and indispensable tool with applications in various specialized fields, such as non-invasive diagnostic imaging, therapeutic approaches for targeted treatments, and non-destructive testing of material structures. Ultrasound imaging relies on transmitting ultrasound pulses into the medium and detecting reflected echoes from different tissue interfaces. This process requires an ultrasound transducer (UST) that efficiently converts electrical energy into mechanical (acoustic pressure) energy and vice versa, using piezoelectric materials. Traditional USTs are designed for hand-held use and made of rigid piezoelectric ceramics, which are not suitable for wearable applications and long-term monitoring. To address the need for a new class of wearable and flexible USTs, we propose a novel approach by integrating piezopolymers with graphene, which offers exceptional mechanical strength, flexibility, and electrical conductivity. We present an innovative approach for developing disposable UST patches using laser-induced photothermal patterning of graphene electrodes on flexible polyimide substrates and additive manufacturing through 3D printing with Polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) ink. By utilizing a far-infrared laser, localized photothermal irradiation causes a temperature increase within the laser's focused area. This rise in temperature breaks the covalent bonds between carbon atoms in the polyimide precursor, resulting in the formation of porous structures as the gaseous molecules in the polyimide evaporate. Subsequently, a composite piezoelectric material is created by integrating PVDF-TrFE with 3D porous graphene. To complete the fabrication process, the transducer is subjected to a poling process. This poling process facilitates the piezoelectric domains' alignment within the PVDF-TrFE composite, enhancing its overall piezoelectric properties. The fabrication was completed by gold sputter-coating and deposition of a 1µm thick parylene film for passivation. The piezoelectric coefficient (D33), electromechanical coupling coefficient (k), and signal-to-noise ratio (SNR) of the UST were characterized by applying compressional force, using LCR, and pulse-echo measurements, respectively.<br/>Deposition and infiltration of PVDF into graphene pores increased surface area interaction and produced thin and durable wearable UST patches with enhanced piezoelectric performance and high imaging resolution (D33=99 pm/V, k=0.29, and SNR=109.45). To demonstrate the versatility of our approach, we fabricated graphene-based USTs of different geometries and configurations, which included a single circular element 3.5 mm in diameter, an M-shaped UST 7-mm across, a dual-element doppler transducer, and a 32-element array with 400-µm pitch. The PVDF deposition could be tuned to achieve a center frequency ranging from 5 MHz for the doppler transducer to 21 MHz for a single-element device. The patterning of graphene-based electrodes produced the desired sensor configuration with uniform PVDF deposition across the entire device. This illustrates the suitability of our novel technique for applications requiring special ultrasound geometries. The method is also amenable to producing transducer arrays for B-mode imaging. The production cost of our UST is estimated to be under $5 per unit, making them a low-cost solution for flexible USTs.