3D Printing Mechanically Robust and Transparent Polyurethane Elastomers for Stretchable Electronic Sensors

Abstract

Advanced stretchable electronic sensors with a complex structure place higher requirements on the mechanical properties and manufacturing process of the stretchable substrate materials. Herein, three kinds of polyurethane acrylate oligomers were synthesized successfully and mixed with a commercial acrylate monomer (isobornyl acrylate) to prepare photocurable resins with a low viscosity for a digital light processing three-dimensional (3D) printer without custom equipment. Results showed that the resin containing poly­(tetrahydrofuran) units (PPTMGA-40) exhibited optimal mechanical properties and shape recoverability. The tensile strength and elongation at break of PPTMGA-40 were 15.7 MPa and 414.3%, respectively. The unprecedented fatigue resistance of PPTMGA-40 allowed it to withstand 100 compression cycles at 80% strain without fracture. The transmittance of PPTMGA-40 reached 89.4% at 550 nm, showing high transparency. An ionic hydrogel was coated on the surface of 3D-printed structures to fabricate stretchable sensors, and their conductivity, transparency, and mechanical performance were characterized. A robust piezoresistive strain sensor with a high strength (∼6 MPa) and a wearable finger guard sensor were fabricated, demonstrating that this hydrogel-elastomer system can meet the requirements of applications for advanced stretchable electronic sensors and expand the usage scope.

Rubber Seed Oil-Based UV-Curable Polyurethane Acrylate Resins for Digital Light Processing (DLP) 3D Printing

“…The parameters of penetration depth ( D p ) and the threshold dose of energy ( E c ) were employed to assess the curing properties of UV-curing printing resins [ 44 ]. The two values could be calculated according to the following equations derived from the Beer–Lambert law [ 45 , 46 ]: where Z d is the cure depth, t 0 is the critical cure time, t d is the exposure time, and I 0 is the intensity at Z d = 0. By measuring and plotting the cure depth of the resin as a function of the different exposure times, the data of D p and t 0 was obtained from the slope and the intercept of the fitted line, respectively [ 46 ].…”

Vat Photopolymerization 3D Printing of Advanced Soft Sensors and Actuators: From Architecture to Function

“…Reproduced with permission. [ 48 ] Copyright 2020, American Chemical Society. c) Printed silicone elastomer coated with CNT‐doped hydrogel layer (left), and a stretchable electric switch (right).…”

“…successfully synthesized three kinds of photocurable resins with great stretchability and elongation (15.7 MPa and 414.3%, respectively) for DLP. [ 48 ] To fabricate a stretchable sensor, a photocurable ionic hydrogel (monomer acrylamide/LiCl) film was dip‐coated with the printed elastomer PPTMGA as a conductor and maintain good transparency, as shown in Figure 3b. [ 48 ] Similarly, Zhao et al.…”

“…[ 48 ] To fabricate a stretchable sensor, a photocurable ionic hydrogel (monomer acrylamide/LiCl) film was dip‐coated with the printed elastomer PPTMGA as a conductor and maintain good transparency, as shown in Figure 3b. [ 48 ] Similarly, Zhao et al. developed a series of silicone elastomers with tunable mechanical properties, and a 3D‐printed cellular was coated with a layer of CNT‐doped hydrogel by dip coating to form a stretchable electric switch, as shown in Figure 3c, which can adjust the intensity of the LED by pressure applied.…”

Three-dimensional printing of tactile sensors for soft robotics

“…Three-dimensional printing techniques can also be modified or integrated with other techniques to produce more complex structures for piezoresistive tactile sensors; some examples are the core-shell 3D printing with a specially designed syringe to produce both a protective sheath layer and inner sensing material, 17 plasma treatment after printing to create microstructure for both strain and gas sensing application, 18 casting with 3D printed mold to create porous structure, 19 and dip coating with ionic conducting hydrogel after 3D printing to achieve high transparency and mechanical properties such as large elongation at break, and good shape recovery. 20…”