In recent years, the advancement of 3D printing has brought new technologies to the world of medical devices and tissue engineering scaffolds, among others. 3D printing processes enable the production of complex structures and reduce material waste versus the traditional manufacturing approaches. One of these methods, vat photopolymerization, utilizes a light source to selectively crosslink a photocurable resin into the desired architecture.1 Digital light processing (DLP) and continuous liquid interface production (CLIP), in particular, are methods that are able to produce strong, complex parts with high print fidelity at relatively high throughputs.1 However, as the field of 3D printing grows in interest, there is a constant demand for the development of new printing technology and printable materials across fields.
The synthesis and subsequent fabrication of biodegradable materials is of particular interest, particularly the medical device industry. Polymers such as polylactic acid (PLA), polycaprolactone (PCL), and polyglycolic acid (PGA) have dominated this space.2 Within the vat photopolymerization methods, acrylate monomers and acrylate-functionalized polymers remain the gold standard, owing to fast reaction rates and stability.1,3 However, this stability resulting from carbon–carbon bonds produced in the 3D printing process inherently limits the potential for degradation of the printed construct. Poly(propylene fumarate) is a biodegradable polyester which is easily photochemically 3D printable via the alkene bond in its backbone.2,4 It has been previously investigated for a variety of applications using SLA, DLP, and CLIP techniques to fabricate the respective constructs. PPF is easy to modify architecturally, with a collection from blocks to stars5–7 to gradient copolymers. Star PPF has been shown to have a lower viscosity at higher molecular weights than linear PPF,5 and it has been printed using a rapid thiol–ene reaction which increases degradability and enables a high degree of mechanical tunability within a biologically relevant regime.7
Integrating particles or fibers into a polymer matrix enables the formulation of a polymer composite. Traditionally fabricated using molding or casting, polymer composites possess enhanced mechanical performance or additional functionality when compared to the unaltered polymer alone.8 Combining 3D printing and novel polymer composite materials is promising for the fabrication of complex parts with highly functional and tunable properties and performance. Polymer composites can be produced for a variety of target applications in augmenting the matrix polymer’s properties in a multitude of ways. Bioactive ceramics such as hydroxyapatite (HAp), calcium phosphates, and bioactive glass can be used to improve cell adhesion and viability and promote cell differentiation in 3D-printed tissue engineered scaffolds,9–13 while growth factors can also be incorporated into the polymer in order to enhance bioactive behavior.11 Carbon nanotubes and graphene offer mechanical enhancement as well as improved electrical and thermal properties.11 Additionally, certain nanoparticles have been utilized for strain sensors, piezoelectric composites, and magnetic-sensitive materials.11,14
In tissue engineering, hydroxyapatite (HAp) is a commonly used filler that is the primary inorganic constituent of bone and teeth15 and is commonly used for bone defect repair and bone tissue engineering.15,16 Hydroxyapatite is frequently manufactured through selective laser sintering methods to bind the ceramic powder together into the desired product shape,15 but it has also previously been used some in DLP printing to produce scaffolds. However in most cases, rather than acting as a true composite, the ceramic constitutes a large volume fraction of the resin which is in turn printed into a structure, and the polymer in these cases acts as a binder and is removed in a sintering process which leaves only the ceramic behind.15–17
Microfillers and nanofillers can be added to a photopolymer resin for mechanical enhancement, additional functionality, and reduced part shrinkage.11,18 However, fillers affect the printing of the photopolymer, and this effect should be considered. As light penetration is critical to the curing of the polymer, fillers must not contribute significantly to light scattering and still must enable cure depth into the resin.19 Further, reducing viscosity is a crucial part of vat photopolymerization. The effect of fillers on the resin’s viscosity must be minimized.11 Nanoparticles have high surface-to-volume ratios which results in property enhancement at low loading, which would not increase the viscosity too much, and are small enough to not affect the layer height during layer-by-layer printing.11,20 Vat photopolymerization methods also require post-processing and curing to complete the chemical reaction fully following the initial print. Steps to maximize properties using post-processing conditions can include heat, post-cure irradiation as well as printing conditions such as layer thickness and layer curing time for vat photopolymerization techniques.21–23 However, there is little research investigating DLP 3D printing with nanofillers and how ideal post-processing conditions are influenced by the incorporation of these nanofillers.24
In this work, a previously explored UV-curable and bioresorbable PPF star polymer resin7 has been augmented with a nanofiller of hydroxyapatite, and composite structures, including porous gyroid scaffolds, were 3D printed. Previous investigations of the unaltered resin and the nanocomposite resin have demonstrated the need for optimization and exploration of the post-processing curing conditions and their respective effects on the printed part. In the effort to maximize mechanical behavior, various post-printing processing conditions and their influence on the tensile and compression behavior of the printed structures with and without HAp were explored, such as post-curing time under UV, vacuum oven temperature during drying, and vacuum oven time. As a result, specific conditions were pinpointed that led to the maximized mechanical performance of the printed structures both under tensile and compression loading.
PPF stars with a degree of polymerization (DPn) of 120 were synthesized according to methods described previously.5–7meso-Erythritol was utilized as a tetra-functional initiator to produce four-arm PPF stars via a ring-opening copolymerization of maleic anhydride and propylene oxide, facilitated by Mg(BHT)2(THF)2 as a catalyst. The scaled up polymerization (300 g) was conducted at 80 °C in toluene (Fig. S1†) and further isomerized using 0.15 M equivalents of diethylamine at 60 °C in chloroform (Fig. S2†), upon which complete isomerization was determined with 1H NMR by confirming a move from the resonance shift from δ = 6.3 ppm (the cis-vinyl proton of the maleate unit) to δ = 6.8 ppm (the trans-vinyl proton of the fumarate unit).
Pre-dried polymers were dissolved in ethyl acetate (60 wt% PPF : 40 wt% ethyl acetate) resulting in resins well-below the empirical upper limit viscosity of 10 Pa s required for high resolution printing in CLIP.26 The viscosity of the resin with and without nanoparticles was determined via rheology (Fig. S6†), with the hydroxyapatite nanoparticles increasing the viscosity of the resin by an average of 1.5 Pa s. A radical scavenger and photoinitiator were added to the resins at 0.3 wt% relative to the mass of the polymer and 0.5% relative to the mass of the polymer, respectively. Previous reports have shown a lack of cytotoxic response with photo-initiated PPF systems.27 The addition of thiol at a 10 : 1 alkene : thiol molar ratio was completed immediately before printing for each resin. For the nanocomposite resin, HAp nanoparticles were added before the addition of the thiol such that 5 wt% of the final product would contain nanoparticles (all but solvent were considered in this calculation) (Fig. 1). Gyroid triply periodic minimal surface scaffolds (unit cell 6 × 6 × 6 mm, porosity 65%), tensile bars, and DMA bars were 3D printed using both resins and subjected to post curing irradiation (λ = 390–420 nm) for either 5, 15, or 30 min following washing with 2, 4, or 6 days in a vacuum oven at 50 °C or 60 °C, resulting in 36 unique conditions for post-treatment. DMA bars were used to analyze the high and low extremes of the conditions for pure resin and the nanocomposite. Triply periodic gyroid designs were utilized in this study rather than a solid structure since PPF is often used as a bone tissue engineering scaffold material.28,29 Triply periodic structures are often utilized due to their interconnected pore structure for nutrient transfer and their inherent mean curvature of zero, which is close to the mean curvature of trabecular bone (also close to zero).30–32 Therefore, evaluating the optimization of these materials within an intended structure served to evaluate their potential as composite scaffolds for regenerative medicine. Samples were examined following printing with SEM and μCT. Aggregates of nanoparticles could be observed in both the SEM and μCT images (Fig. 2A–C) and also confirmed consistent HAp dispersion throughout the scaffolds. The clusters do provide some surface roughness which is helpful for cell adhesion, but too much aggregation would limit the mechanical property augmentation effect and create a non-uniform material. Aggregation is common among nanoparticle fillers in general. van der Waals interactions lead to their aggregation, especially at the nanoscale. Future investigations varying filler type, size, or concentration could warrant the analysis of the zeta potential of the filler systems, which may help with understanding the concentration for repellence. Additives could potentially be added to facilitate better particle dispersion within the resins, but these must be biocompatible.
