Optimization of photocrosslinkable resin components and 3D printing process parameters

Abstract

The role of 3D printing in the biomedical field is growing. In this context, photocrosslink-based 3D printing procedures for resorbable polymers stand out. Despite much work, more studies are needed on photocuring stereochemistry, new resin additives, new polymers and resin components. As part of these studies it is vital to present the logic used to optimize the amount of each resin constituent and how that effects printing process parameters.

The present manuscript aims to analyze the effects of poly(propylene fumarate) (PPF) resin components and their effect on 3D printing process parameters. Diethyl fumarate (DEF), bisacylphosphine oxide (BAPO), Irgacure 784, 2-hydroxy-4-methoxybenzophenone (HMB) and, for the first time, in biomedical 3D printing, ethyl acetate (EA), were the resin components under investigation in this study. Regarding printing process parameters, Exposure Time, Voxel Depth, and Overcuring Depth were the parameters studied. Taguchi Design of Experiments was used to search for the effect of varying these resin constituent concentrations and 3D printing parameters on the curing behavior of 3D printable PPF resins.

Our results indicate that resins with higher polymer cross-link density, especially those with a higher content of PPF, are able to be printed at higher voxel depth and with greater success (i.e., high yield). High voxel depth, as long as it does not sacrifice required resolution, is desirable as it speeds printing. Nevertheless, the overall process is governed by the correct setup of the voxel depth in relation to overcuring depth. In regards to resin biocompatibility, it was observed that EA is more effective than DEF, the material we had previously relied on. Our preliminary in vitro cytotoxicity tests indicate that the use of EA does not reduce scaffold biocompatibility as measured by standard cytotoxicity testing (i.e., ISO 10993-5). We demonstrate a workpath for resin constituent concentration optimization through thin film tests and photocrosslinkable process optimization.

Statement of Significance

We report here the results of a study of photo-crosslinkable polymer resin component optimization for the 3D printing of resorbable poly(propylene fumarate) (PPF) scaffolds. Resin additives are initially optimized for PPF thin film printing. Once those parameters have been optimized the 3D printing process parameters for PPF objects with complex, porous shapes can be optimized. The design of experiments to optimize both polymer thin films and complex porous resorbable polymer scaffolds is important as a guess and check, or in some cases a systematic method, are very likely to be too time consuming to accomplish. Previously unstudied resin components and process parameters are reported.

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.