In the last decade we have witnessed the booming of additive manufacturing (AM, also 3D printing), including its related fabrication technologies, materials, and applications. The biotechnology and bioprocessing fields have been significantly influenced by AM, with reports spanning upstream and downstream processing, including sorting and selection of cell strains (Lin et al., 2016), bioreactors (Saha et al., 2018), harvesting (Shakeel Syed et al., 2017), filtration (Tan & Franzreb, 2020), chromatography (Salmean & Dimartino, 2019), and extraction (H. Wang et al., 2017). One of the most popular AM methods employed in bioengineering is digital light processing (DLP) where a three-dimensional model is built, layer upon layer, by selectively curing a photo-sensible liquid resin. Reasons for the success of DLP in biotechnology include its relatively low cost, fast speed (litre sized objects can be printed overnight), and high resolution (generally in the order of 50 µm).
Historically, AM was primarily in the domain of the automotive, aerospace, and biomedical industries which favoured materials with mechanical properties over chemical characteristics and fabricated non-porous structures where strength and stiffness are key. On the other hand, bioprocess applications often require porous materials to maximize the total surface area available for cell adhesion, adsorption, and allow intraparticle mass transfer. Besides, materials in the biotechnology industry heavily exploit chemical characteristics such as electrostatic charge and hydrophobic behaviour to appropriately modulate their interactions with species as diverse as cells, proteins, and small metabolites. This requirement contrasts the status quo where the composition of commercially available AM materials is IP protected, making it impossible to rationally design material-species interactions of interest. Proprietary compositions also complicate compliance with FDA or EMEA requirements, hindering adoption of 3D printing in the biomanufacturing industry.
Here, we present a novel polymeric formulation for DLP 3D printing which can be easily tuned to adjust its chemical, porous, and mechanical properties of printed parts. The formulation consists of a few simple ingredients, including monomers and crosslinkers to create the polymeric network (Figure 1a), a UV photoinitiator to trigger the photopolymerization reaction, a photoabsorber to increase the resolution of the printed model, and porogenic components. The key feature of the proposed formulation lays in the use of bifunctional monomers bearing both a (meth)acrylate functionality for photopolymerization and a suitable chemical moiety for biomolecular interactions, e.g. charged groups, alkyl or aryl groups, or reactive groups for successive covalent immobilization of a desired ligand. By appropriate selection of the bifunctional monomers, different materials with different surface derivatizations to suit a range of applications in bioprocess engineering can be obtained. Furthermore, the nature and relative concentration of the components making up the overall formulation will directly impact on the propagation kinetics of the free-radical polymerization reaction, in turn affecting the morphology of the resulting polymeric network and its porous microstructure (e.g. surface area, average pore size, pore size distribution) (Barner-Kowollik et al., 2014; Buback, 2009). In this work, we manipulated these characteristics to produce different formulations for DLP printing, and obtain porous monoliths with different chemical and macroporous properties for use in chromatography, immobilized enzyme bioreactors, and biofilm bioreactors.