The demand for lightweight, robust, and smart wearable sensors in sports, medicine, and aerospace is driving research into bioelectronics. However, current wearable electronics often lack protective capabilities, while existing protective armors typically lack sensing functionality. This necessitates the development of multifunctional wearable sensors that integrate sensing and protection. This research addresses this challenge by creating a novel composite material that combines the strength of a bio-inspired 3D-printed structure with the piezoelectric properties of Rochelle salt crystals. The cuttlebone structure, with its unique chambered wall-septa microstructure, provides high stiffness and energy absorption, while the Rochelle salt crystals enable piezoelectric sensing. This integrated approach offers a promising solution for next-generation smart monitoring devices.

Existing literature highlights the individual advancements in lightweight, robust bioelectronics and high-strength protective materials. Wearable bio-monitoring technologies often utilize soft piezoelectric materials or flexible printed circuit boards, which are lacking in protective capabilities. Conversely, advanced armors, utilizing strong fibers, metals, or ceramics, are typically not equipped for sensing applications. The integration of these functionalities remains a significant challenge. Bio-inspired designs, particularly those mimicking the microstructure of natural materials, have gained attention for creating functional structural materials with low density and high strength. Cuttlefish bone, with its unique chambered wall-septa microstructure, is a prime example of such a structure. Additive manufacturing (3D printing) offers a significant advantage in fabricating complex bio-inspired structures that traditional methods cannot achieve. Rochelle salt (RS), an eco-friendly material with piezoelectric and ferroelectric properties, presents a suitable option for piezoelectric sensing due to its ease of synthesis, low melting temperature, and recrystallization capabilities. The combination of these elements – bio-inspired design, 3D printing, and Rochelle salt – forms the basis of this novel composite.

The research involved the 3D printing of cuttlebone-inspired structures using stereolithography. A photocurable resin was used for the printing process, resulting in high-resolution structures. Rochelle salt (RS) crystals were then grown within the 3D-printed structures by immersing them in a synthesized RS medium. The growth process was systematically investigated using optical microscopy, scanning electron microscopy (SEM), and computerized tomography (CT) scanning. The microstructure and composition of the 3D-printed Rochelle salt cuttlebone composite (RSC) were analyzed using SEM and energy-dispersive X-ray spectroscopy (EDS). Piezoelectric performance was assessed by measuring the output voltage under various weight loadings and cyclic impact tests. The effective piezoelectric coefficient d33 was determined. The converse piezoelectric effect was also evaluated using a laser vibrometer. Mechanical properties, including compressive strength, fracture toughness, and flexural strength, were investigated using a universal testing machine and three-point bending tests. Finite element method (FEM) simulations using COMSOL Multiphysics were conducted to model the piezoelectric response and mechanical behavior of the composite. The recyclability and healing behavior of the composite were evaluated by dissolving the RS crystals through heating and subsequent regrowth. Finally, the composite's application in smart armor for football players and fall alarm knee pads was demonstrated.

The 3D-printed RSC composite exhibited significant piezoelectric and mechanical performance. The output voltage was proportional to the magnitude of the applied force, reaching up to 8 Vpeak-peak under a 20g weight drop. Cyclic impact tests showed stable output voltage up to 6800 cycles, after which fracture occurred due to RS crystal detachment. The effective piezoelectric coefficient d33 was approximately -30 pC/N. The converse piezoelectric effect was also confirmed. The cuttlebone-inspired structure significantly enhanced the mechanical properties of the composite, with a 24-hour crystal growth showing an 8.4-fold increase in flexural strength and a 1044.6% increase in fracture toughness compared to the pure polymer structure. The composite demonstrated excellent recyclability and healability, retaining 95% of its original piezoelectric performance after repair and recycling. The application of the composite in a 4x4 array smart armor for football players demonstrated accurate detection of impact force magnitude and location. The smart knee pad successfully detected falls at different angles and heights, triggering an alarm and providing data for injury assessment.

The findings demonstrate the successful integration of bio-inspired design, 3D printing, and eco-friendly piezoelectric materials to create a multifunctional composite with superior mechanical and piezoelectric properties. The cuttlebone-inspired structure successfully enhanced the toughness and strength of the composite, while the Rochelle salt crystals provided effective piezoelectric sensing capabilities. The recyclability and healability characteristics significantly enhance the sustainability and lifespan of the sensor. The applications in smart armor and fall detection knee pads showcase the potential of this technology to improve safety and provide valuable data for performance analysis and medical diagnosis. The results advance the development of next-generation smart monitoring devices for various applications.

This research successfully fabricated a novel recyclable and healable piezoelectric composite by growing Rochelle salt crystals within a 3D-printed cuttlebone-inspired structure. The composite exhibits superior mechanical and piezoelectric performance, and its applications in smart armor and fall detection devices were demonstrated. Future research could focus on exploring other bio-inspired structures and piezoelectric materials, optimizing the crystal growth process, and expanding the applications to other fields, such as healthcare monitoring and structural health monitoring.

The current study focused on Rochelle salt crystals, which may have limitations in temperature and humidity stability compared to other piezoelectric materials. The long-term durability and stability of the composite under prolonged use still require further investigation. The scalability of the 3D printing process and the cost-effectiveness of the composite material should also be considered for wider applications.