Expanding the Usage of Lignin in DLP 3D Printing by Optimized Synthesis and Processing Parameters

Digital light processing (DLP) has gained substantial interest in recent years as a versatile additive manufacturing technique across different disciplines. However, many materials used in DLP 3D printing are from non-sustainable sources. The use of renewable resources like lignin, a complex aromatic polymer derived from plant biomass and an industrial byproduct produced in ton-scale, in resins promotes the development of sustainable and environmentally friendly additive manufacturing processes and applications. Therefore, we first present a cost-effective and reproducible acrylation synthesis route that enables the modification of lignin using acryloyl chloride as a functionalization reagent. Further, we systematically investigate the effects of different contents of unmodified lignin up to 20 wt % and modified lignin with acrylate groups up to 30 wt % in resins using a time-efficient, self-developed step test to achieve optimal printing parameters with the smallest possible feature size. These parameters are used for high-resolution DLP 3D printing of microneedles for potential medical applications, which is why the cytotoxicity of the lignin resins is determined as well. The addition of lignin to resins influences their rheological properties and printability remarkably as well as the mechanical performance of the corresponding 3D-printed objects. Notably, higher lignin concentrations are found to enhance the mechanical strength of 3D-printed parts. Furthermore, we assess the effects of lignin acrylation in photopolymer formulations leading to improved solubility, substantial change in rheological properties from thixotropic to non-thixotropic behavior, and an enhanced E-modulus of 3D-printed materials. As a result, we showcase the feasibility to 3D-print with up to 30 wt % of modified lignin and present a 3D-printed material with 15 wt % of modified lignin, which is classified as non-cytotoxic according to EN ISO 10993-5. However, further increasing lignin content generally adversely affects the printability due to increased resin viscosity as well as light scattering and pronounced UV light absorption.