Developing new chemistry for high-resolution stereolithography (SLA) 3D printing

Stereolithography (SLA)-based additive manufacturing (“SLA 3D printing”) has gained significant interest with regard to the fabrication of microfluidic devices as a result of its ability to print bespoke 3D geometries, in addition to increased automation, cost-effectiveness, and throughput. Despite all these benefits, 3D printing cannot compete with the conventional microfluidic fabrication methods where biocompatible, transparent devices, with sub-10 μm resolution, are often demanded. Commercial photocurable resins for SLA 3D printing are not necessarily formulated to meet the requirements of microfluidics. Thus, the custom formulation of photocurable resins is necessary to overcome some restrictions in the use of SLA 3D printing of microfluidics. These limitations are mainly due to inherent trade-offs between layer thickness, exposure time, material strength, and optical penetration. As of today, SLA 3D printing is capable of fabricating closed microchannels as small as 15μm x 15μm by customising the resin formulation (by adding UV absorbers) and using a relatively high-resolution printer (7.6 μm pixel pitch in the image plane). Although this resolution can be regarded as an evolution with respect to other 3D printing technologies, it still cannot compete with the resolution of those obtained by so. lithography (sub-100 nm). Based on this state-of the-art, the main objectives of this research were (1) to develop a new class of SLA resin with improved shelf-life and oxygen resistance and (2) to introduce the fundamentals of a novel concept to realise features with truly micron and sub-micron sized Z resolution, without the need of any additives, delivering reasonably fast print times.

In Chapter 1, an overview of the most frequently used techniques for 3D printing microfluidics is presented. The reasons why SLA 3D printing has the potential to replace so. lithography for fabrication of microfluidics are elaborated. This review covers the current state of SLA 3D printing and its barriers, specifically related to the material chemistry of photocurable resins. Moreover, past and recent investigations into new SLA resin formulations for 3D printed microfluidics are reviewed.

In Chapter 2, three different types of photocurable resins based on thiol-acrylate, acrylate, and thiol-ene chemistry were developed, focusing on their shelf-life stability and oxygen resistivity for their use in SLA 3D printing. The thiol-acrylate based resin developed within this chapter of work exhibited significantly higher shelf-life stability compared to thiol-ene and commonly used acrylate-based resins. No premature dark gelation for the thiol-acrylate resin was observed a.er 30 days of storage at 4°C, whereas the thiol-ene resin rapidly underwent a dark premature gelation within 2 hr of storage at 4°C. The impact of oxygen inhibition was recognised at the early stage of induction time (from 30 to 60 sec) for thiol-acrylate resin. After irradiation for about 30 sec, an approximate curing depth of 0.65 mm was achieved using the thiol-acrylate chemistry, whereas the acrylate-based resin did not undergo any photopolymerisation. A real-time Fourier-transform infrared (RT-FTIR) spectroscopy method was developed to study the kinetics of the photopolymerisation reaction of the photoinduced thiol-acrylate resin during exposure at a given wavelength of light. In addition, the resin formulation was optimised and characterised based on the o.-stochiometric ratios of thiol and acrylate monomers and concentration of photo-initiator, free radical scavenger, and UV absorber. A.er optimisation, a resolution of 90 μm in X-Y plane with a build layer thickness of 5-200 μm was achieved. Furthermore, the 3D printability of polydimethylsiloxane (PDMS) based thiol-acrylate resin for fabrication of transparent horizontal microchannels with minimum dimensions of 500 μm was demonstrated.

In Chapter 3, a novel concept is described, which was developed to confine and localise the curing depth (Z resolution) at the liquid-monomer interface via interfacial photopolymerisation in the absence of additives (e.g., photo-blockers, UV absorbers). Here, a biphasic interfacial photo-initiation system (BIPS) for 3D printing was developed, where the two components of type II photo-initiation system (photo-initiator and co-initiator) were distributed into two immiscible phases. The polymer’s growth at the interface increases over time, but the reaction eventually stops because of the increased diffusion barrier. BIPS material chemistry is formulated to meet the requirements of an interfacial type II photopolymerisation where the H-atom abstraction and subsequent radical generation only occur at the liquid-monomer interface. The choice of monomer is limited to non-hydrogen donor monomers. Resins based on thiol-ene and thiol-acrylate cannot be used in BIPS given that the intermolecular hydrogen atom transfer to alkene and/or acrylate is facilitated by thiol monomer. BIPS can realise a spatially controlled formation of a polymeric network with various curing depths <100 μm without the need of any additives. Furthermore, the printability of a horizontal microchannel with a minimum curing depth of 40 μm and a build layer thickness of 20 μm is demonstrated.

However, there is a trade-off. between high resolution and printing time. BIPS can resolve 40 μmZ resolution, however, print time is sacrificed because type II photoinitiation systems undergo a bimolecular reaction. In Chapter 4, the fundamentals of an innovative concept, ‘to tether the photo-initiator via surface activity to the liquid-monomer interface’, are introduced and developed to realise truly sub-10 μm Z resolution without sacrificing the print time in the absence of any additives. A fluorinated photo-initiator (Irgacure 819-F) is aggregated near the liquid-monomer interface because of its inherent ability to migrate, due to the low surface tension and surface energy of fluorine atoms. Irgacure 819-F was successfully synthesised and characterised, based on Friedel- Crafts acylation, which exhibits a high absorption peak at 365-405 nm. However, the synthesis of Irgacure 819-F was not reproducible via Friedel-Crafts acylation due to the decomposition of Irgacure 819 in reaction with anhydrous aluminium. This novel concept has not come to SLA 3D printing yet, a possible reason being the lack of fluorinated photo-initiator with an absorption peak near UV or visible light (at 365-405 nm). Future developments on synthesis of fluorinated photo-initiator at 365-405 nm can advance this concept from theory to practice.

Finally, in Chapter 5, future research paths are discussed based on the results obtained in this thesis, which can enable further improvements of the Z resolution of SLA 3D printed microfluidic devices at reasonably fast printing times.