Additive manufacturing (AM), or 3D printing, offers fabrication of complex structures beyond traditional methods. Emerging applications in deployable structures, soft robotics, flexible electronics, and biomimetic designs demand materials with vastly different properties. However, fabricating highly stretchable and stiff materials efficiently within a single AM process remains challenging. Inkjet printing (IJP), while popular for multimaterial parts, is expensive and often produces materials with low strain failure. Direct ink write (DIW) printing, with material mixing before extrusion or multiple nozzles, is slow and less accurate than other AM techniques. Multi-ink or multi-method AM approaches lack efficiency and the capability to fabricate complex multimaterial structures. Digital light processing (DLP) is a high-speed, high-resolution method, but single-vat DLP generally isn't suitable for multimaterial printing. Multi-vat approaches have limitations in speed and the number of materials. Grayscale digital light processing (g-DLP) offers potential for varied material properties from a single resin, but existing methods either involve a long second-stage thermal cure with low stretchability or struggle to achieve high property difference and high stretchability simultaneously. This research addresses this challenge by developing a new resin design strategy for single-stage g-DLP printing.

Existing multimaterial 3D printing methods, such as inkjet printing and direct ink write, suffer from high costs, low stretchability, slow printing speeds, or limited accuracy. Multi-vat DLP methods, while capable of printing multiple materials, are hampered by slow printing speeds due to vat switching and cleaning, as well as limitations in the number of materials that can be used. Previous single-vat multi-material printing methods using two different initiators and light sources are limited in property range and quality. While single-cured g-DLP has been used to modulate material properties, it hasn't achieved high property difference and high stretchability simultaneously. The literature lacks an efficient single-step printing method capable of achieving both.

A novel photocurable resin ink was formulated consisting of isobornyl acrylate (IBOA), 2-hydroxyethyl acrylate (2-HEA), and aliphatic urethane diacrylate (AUD). IBOA serves as a stiff monomer, while 2-HEA and AUD facilitate high stretchability through hydrogen bonding. The resin's viscosity is approximately 0.05 Pas. A bottom-up DLP printer with a 385 nm UV-LED light projector and an oxygen-permeable Teflon window was used. A MATLAB script controlled the grayscale (UV light intensity) of each pixel to modulate the degree of cure (DoC) and, thus, the mechanical properties. The printing speed was 1 mm/min (3 s per 0.05 mm layer). The DoC was controlled by varying the grayscale level from 0% (GO, full intensity) to 70% (G70, 70% darkness). A photopolymerization (PP) model was used to analyze the correlation between depth-dependent DoC and light doses. Mechanical properties were evaluated using uniaxial tensile tests and dynamic mechanical analysis (DMA). Fracture toughness was assessed via tearing tests. Gel fraction was determined using FTIR. Finite element analysis (FEA) was used to simulate hydrodynamic performance of printed heart valves and fish fin actuation.

The single-stage g-DLP printing method achieved a modulus range of 0.016 to 478 MPa (nearly 30,000 times difference) with stretchability up to 1500% in the soft state. The printed material demonstrated superior toughness (around 109 J/m³) and fracture toughness ranging from 650 to 10,000 J/m². The soft polymer exhibited excellent elastic properties and resilience even after 10,000 cycles of cyclic stretching at high strain. The stiff monomer IBOA increased Tg at high DoC, while hydrogen bonding with 2-HEA contributed to the high stretchability at low DoC. Composite structures with anisotropic behavior (fiber-embedded composites), airless tires, and biomimetic designs (collagen structures with helical fibers, mimicking artery tissues) were successfully 3D printed. The printed structures exhibited sequential deformation under increasing force levels. Biomimetic structures mimicking nacre and Bouligand structures demonstrated significantly improved fracture toughness (166 and 146 kPa m⁰·⁵ respectively). Complex inflatable structures, including a biomimetic pufferfish and various membrane structures with customizable inflation shapes, were fabricated. Soft pneumatic actuators capable of extension, bending, contraction, and twisting were created. A tentacle-like actuator capable of grasping objects was demonstrated. Strain and pressure sensors with liquid metal-filled microfluidic channels were successfully printed and exhibited good sensitivity.

The successful fabrication of structures with vastly different material properties in a single-step process addresses a major limitation in multimaterial 3D printing. The resin's ability to achieve both high modulus contrast and high stretchability at a high printing speed surpasses existing methods. The successful creation of complex structures, from biomimetic designs to functional sensors, demonstrates the versatility and potential of this technique for diverse applications. The ability to rapidly prototype complex functional parts opens up opportunities in soft robotics, biomedicine, and flexible electronics. The results validate the effectiveness of the proposed resin design strategy and its suitability for various advanced applications.

This research presents a novel resin formulation and printing technique for single-vat single-cure g-DLP 3D printing, achieving unprecedented control over material properties. The high modulus contrast, high stretchability, and high printing speed demonstrated significant advantages over existing multimaterial 3D printing methods. The successful fabrication of complex structures across various applications showcases the technique's broad potential. Future work could focus on optimizing the pixel scale transition layer between stiff and soft regions and exploring alternative resin formulations.

The organogel nature of the soft material makes it sensitive to high temperatures. A pixel-scale transition layer (50-100 µm) exists between stiff and rubbery regions, limiting the technique's suitability for micron-scale structures. Low grayscale levels cause size distortion in the x-y plane, affecting resolution for microstructures. The uncured acrylate groups make the printed structures UV-sensitive.