Full length articleOptimization of photocrosslinkable resin components and 3D printing process parameters
Graphical abstract
Introduction
Current tissue engineering strategies to repair tissue and organs can be broadly categorized into three main approaches: (I) entirely inert devices, (II) cell-based therapy, and (III) the implantation of constructs laden with cells, coatings of those cell’s ECM (extracellular matrix), and/or growth factors (cytokines) [1]. Much work is currently going into the development of new biomaterials and manufacturing techniques, especially the fabrication of three dimensional (3D) resorbable and cell-laden scaffolds. One of the better known biomaterials for this work is poly(propylene fumarate) (PPF). Indeed, PPF resin and process parameter optimization for stereolithography (SLA; a.k.a. Stereolithographic Assembly) and/or digital light processing (DLP; a.k.a. mask projection) 3D printing are longstanding areas of biomedical device fabrication research [2]. Different photocrosslinkers, co-crosslinkers, solvents, dyes, and functionalization components have been investigated in the attempt to modify 3D scaffold manufacturing, mechanical properties, and/or biological properties. Unlike other photocrosslinkable resin systems, PPF has long shown its relevance to preparing solid-cured scaffolds that are appropriate for bone tissue engineering, including the preparation of cytokine-surface-functionalized scaffolds [3].
In 2003, Cooke et al. [4] reported on the first scaffold fabrication via 3D printing of a resorbable material developed for tissue engineering. They used the SLA 3D printing technique with a resin consisting of PPF as the polymer, DEF as a solvent, and BAPO as a photoinitiator. This resin mixture was successfully crosslinked resulting in scaffolds that accurately represented the computer aided design file that was guiding the 3D printer. In so doing, they demonstrated SLA as a viable process to prefabricate custom-fitting, tissue engineered scaffolds for critical-size bone defects. In 2007, Lee et al. [5] also studied the effect of resin formulation and SLA parameters to obtain bone tissue engineering scaffolds. They employed a resin mixture based on PPF, DEF, and BAPO. They observed that PPF/DEF mixtures resins with viscosity up to 1.8 Pa·s are the upper limit for the stereolithography process. These authors, therefore, chose a PPF/DEF 60:40 ratio with 1 wt% BAPO content. In 2009, Lan et al. [6] designed and fabricated 3D porous scaffolds (65% of porosity) by SLA with a PPF:DEF ratio of 70:30 and 1% wt% BAPO as a photoinitiator. Scaffolds were well fabricated with a line width of 90 µm, pore size of 250 µm, and layer thickness of 110 µm. Samples showed 25% isotropic shrinkage compared to the design. These scaffolds were coated with a hydroxyapatite layer. MC3T3-E1 cells showed good cell attachment in the coated scaffold group. In 2012, Dean et al. [7] used digital light processing (DLP) to manufacture bone tissue engineering scaffolds with high accuracy (60 µm layer thickness). The resin used in that study included PPF, titanium dioxide (TiO2) as a dye, BAPO as a photoinitiator, and DEF as a solvent. Results showed an increase in “green strength” (i.e., strength during and just after 3D printing and before post-printing exposure to heat or additional light) with respect to PPF scaffolds previously 3D printed by SLA. More recently, the significant reduction in polydispersity offered by ring opening polymerization (ROP) synthesis [8] of PPF has allowed better control over 3D printed scaffold accuracy [9] and resorption kinetics [10]. However, despite the longstanding research into PPF 3D printing, the recent availability of low polydispersity PPF has provided an unprecedented opportunity to improve control over scaffold manufacturing accuracy and mechanical and biological properties including resorption kinetics.
Understanding how resin component concentrations and 3D printing process parameters effect a photocrosslink-based 3D printing process is crucial. This study aimed to analyze the effect PPF resin component concentration and printing process parameters have on 3D printed scaffold yield. Diethyl fumarate (DEF), bisacylphosphine oxide (BAPO), Irgacure 784, and 2-hydroxy-4-methoxybenzophenone (HMB) and for the first time in biomedical 3D printing, ethyl acetate (EA), were the resin components under investigation. Regarding printing process parameters, Exposure Time, Voxel Depth, and Overcuring Depth were the parameters under investigation. Taguchi’s method for optimizing the Design of Experiments [11] was used to search the effect these components and parameters have on the curing behavior of PPF resins.
Section snippets
Methodology
Photocrosslink-based 3D printing processes work by focusing an Ultraviolet (UV) light or Visible Light onto a vat of photopolymerizable resin. These processes are governed by curing thickness, which is also known as cure depth (Cd). Cd mainly depends on the resin components (Rc), light intensity (Li), and exposure time (Et). With relatively low concentrations of non-polymer components, Rc, and relatively low Li, Cd curve can be approximated by an exponential equation (Fig. 1a) as follows:
Cure depth results
Results showed the strong influence the resins’ components and, especially, the exposure time have on the final Cd. Fig. 4 shows the increase in Cd as Et increases, according to Eq. (1). This same trend was observed with non-ROP PPF in previous works [2].
Although every resin showed different curing behaviors, it was possible to perform the curing depth tests with all of them. This fact proves that every resin that we tested, is predicted to be to be 3D printable with high yield (i.e., within
Conclusions
The present manuscript has analyzed the effects of the following resin components: poly(propylene fumarate) (PPF), diethyl fumarate (DEF), ethyl acetate (EA), bisacylphosphine oxide (BAPO), Irgacure 784, and 2-hydroxy-4-methoxybenzophenone (HMB), and the most important 3D printing process parameters, namely, Exposure Time, Voxel Depth, and Overcuring Depth, have on Cure Depth (Cd) and 3D printability.
Our results show the strong effect that the concentration of resin components have on the
Disclosures
Drs. Dean and Becker have founded a company, 3DBioResins [21MedTech, LLC, Akron, OH], that produces Ring Opening Polymerization-synthesized PPF. Both Drs. Dean and Becker hold founders equity in this company.
Acknowledgements
AG acknowledges the financial support from the Ministry of Economy and Competitiveness (MINECO) and the Government of Spain for a PhD scholarship and grants DPI2016-77156-R and EEBB-I-18-12797. Also, the authors are grateful for the financial support received from the University of Girona (Spain) MPCUdG2016/036. DD and MLB acknowledge partial support was provided by the Akron Functional Materials Center. DD and LHC acknowledg partial support from FAPESP-OSU Global Gateway grant #2015/50241-0.
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