Functional Gradient Materials for Wearable Devices: Overcoming Tear Resistance Challenges with Multi-Material 3D Printing

Currently, commercial wearable devices are primarily prepared using silica-based materials as substrates. Among them, PDMS has attracted attention due to its unique advantages, including low cost, excellent stretchability, biocompatibility, high and low-temperature resistance, high transparency, and ease of processing [8-11]. However, PDMS has a disadvantage compared to other flexible substrates, as it exhibits poor tear resistance [12]. Under standard conditions, its tear strength is only 2 KN/m, making it prone to fracture as a flexible substrate. This limitation hinders its development and application in flexible electronics.

Common methods to improve the tear resistance of silicone rubber include adjusting the rubber formulation [13-15] and blending it with other copolymers [16-18]. For instance, Peng et al. [19] introduced PDMS into laminated MMT-PVA scaffolds, resulting in PDMS-MMT composites with a toughness of 4.6 kJ/m2, which is 12 times higher than that of pure PDMS. Kazem et al. [20] blended liquid metal with Ecoflex material to increase tear resistance, achieving a 50-fold increase in toughness, from 250ﾱ50J/m2 to 11900ﾱ2600J/m2. Zhang et al. [21] used siloxane-grafted EVM as a compatibility additive between EVM and silicone rubber, leading to a tear strength of 17.9 KN/m in the blends, 44% higher than the original. Furthermore, Zuwei Fu et al. [22] improved the self-healing property of PDMS to resist external tearing, achieving a maximum self-healing efficiency of 100% and complete restoration of the state before tearing. However, previous research has shown that enhancing PDMS tear resistance often comes with drawbacks such as sacrificing other properties, low enhancement efficiency, and complex manufacturing processes. Therefore, exploring novel, facile, and efficient enhancement methods is necessary.

Traditional elastomeric substrates are prone to external damage at their edges, resulting in defects like notches and microcracks. Crack growth caused by these defects is among the main causes of failure in flexible electronic devices [23]. While using high tear strength elastomers as substrates can effectively prevent device failure, their high modulus may lead to an imbalance in modulus matching with the skin, compromising wearability. Selecting crack-insensitive elastomeric materials like Ecoflex for the substrate can prevent failure, as they possess excellent crack propagation inhibition capabilities. However, Ecoflex's low cross-linking density results in highly nonlinear response, high cycling hysteresis, and significant stress softening [24-25], making it unsuitable as a standalone flexible substrate material.

Functional gradient material structures offer a solution to the issues faced by traditional elastic material substrates. These structures can be designed to possess a variety of superior properties, including reduced in-plane stress, improved residual stress distribution, enhanced fracture toughness, and reduced crack sensitivity [26-28], thereby greatly improving tear resistance. Traditional methods for fabricating functional gradient materials, such as gas phase deposition [29], electrostatic spinning [30], and magnetic field actuation [31], have limitations in accurately controlling the distribution of gradient materials. However, the emergence of additive technologies, particularly multi-material 3D printing, addresses this limitation [32-35]. Multi-material 3D printing technology provides a new method for designing and fabricating functional gradient structures, ensuring accuracy, reliability, and reproducibility in the resulting gradient properties [36-39].