Development of hydrophilic silicone-based ink for the 3D vat photopolymerization printing of biomedical devices

Abstract

Three-dimensional (3D) printing is a layer-by-layer additive manufacturing technique that continues to gain interest due to its ability to fabricate customized structures at low setup cost and quick turnaround time. In this thesis, advanced ink materials are developed for the fabrication of elastic biomedical devices using vat photopolymerization (VP) printing.

In Chapter 1, an overview of various 3D printing techniques is presented, including their respective advantages, disadvantages, and requirements for ink material. Compared to other major 3D printing techniques, VP printing offers high printing accuracy, resolution, and superior surface quality. However, the fabrication of elastic structures using VP printing has long been a challenge due to the high viscosity and tackiness of elastomeric material. A review of various elastic materials and their current applicability in VP printing is also presented. Finally, recent materials and strategies for fabricating biomimetic implants and fluidic devices via VP printing are discussed.

In Chapter 2, a VP-printable hydrophilic silicone-based material is developed, using aminosilicone methacryloyl (SilMA) incorporated with acrylamide (AA) and poly(ethylene glycol) dimethacrylate (PEGDMA) as reactive diluents. The incorporation of AA and PEGDMA addresses the issues of high pre-gel viscosity and slow curing rate of SilMA. Furthermore, the formation of a SilMA/AA/PEGDMA interpenetrating network (IPN) upon curing is novel as it differs from the existing acrylate and thiol-ene silicone network. By integrating hydrogel components, the material displayed distinct characteristics compared to conventional silicone, including hydrophilicity and good swelling properties. Additionally, compared to regular hydrogels, the material shows improved strength, elasticity, and durability suitable for the fabrication of biomimetic implants.

Despite its excellent VP printability, the developed material exhibits signs of overcuring, which hinders the printing of ultra-fine features. Hence, in Chapter 3, cellulose nanocrystal (CNC) is used to improve printing accuracy and resolution. The use of CNC to tune photocuring depth is novel and, to the best of our knowledge, has not been reported in literature previously. Upon the integration of 1 wt% of CNC, the developed material exhibits a high printing accuracy and resolution down to 100 μm with a near-zero deviation in the X and Y direction. Most importantly, the incorporation of CNC results in a printed fluidic device with excellent surface detail, good fluid processibility, and minimal colour staining.

Yet, with the SilMA-based material, it remains challenging to achieve one-step printing of fluidic devices with embedded channels. Therefore, in Chapter 4, ink formulation with siloxane oligomer instead of polymer is developed for an even lower pre-gel viscosity. In this ink formulation, amphiphilic siloxane oligomer (silmer) is complemented with AA and glycidyl methacrylate (GMA). The use of silmer as the primary component in resin formulations is uncommon due to the challenges in dissolving high concentrations of silmer. Herein, a novel approach using a solvent blend is introduced as a critical strategy for formulating the amphiphilic silicone-based ink materials for VP printing. Silmer conformation is solvent-dependent, resulting in tuneable pre-gel viscosity, transparency, and surface properties. Upon ink optimization, a silicone-based fluidic device with embedded channel is successfully produced with VP printing, and the printed device shows excellent capability in synthesizing drug-encapsulated hydrogel beads, demonstrating its feasibility for real-world biomedical applications.

Taken all together, this thesis presents the formulation of VP-printable hydrophilic silicone-based resin material with two different strategies: (1) the addition of reactive diluents and (2) the use of lower-molecular-weight siloxane oligomers; offering new perspectives on the formulation of hydrophilic elastomeric resin material for VP printing. Furthermore, the successful fabrication of biomimetic scaffold implants and biomedical fluidic devices with VP printing reveals a significantly simpler and more cost-effective method for the fabrication of silicone-based biomedical devices, moving beyond the conventional method of soft- lithography and moulding.