Digital micromirror device (DMD)-based 3D printing of poly(propylene fumarate) scaffolds

Abstract

Our recent investigations into the 3D printing of poly(propylene fumarate) (PPF), a linear polyester, using a DMD-based system brought us to a resin that used titanium dioxide (TiO2) as an ultraviolet (UV) filter for controlling cure depth. However, this material hindered the 3D printing process due to undesirable lateral or "dark" curing (i.e., in areas not exposed to light from the DMD chip). Well known from its use in sunscreen, another UV filter, oxybenzone, has previously been used in conjunction with TiO2. In this study we hypothesize that combining these two UV filters will result in a synergistic effect that controls cure depth and avoids dark cure. A resin mixture (i.e., polymer, initiator, UV filters) was identified that worked well. The resin was then further characterized through mechanical testing, cure testing, and cytotoxicity testing to investigate its use as a material for bone tissue engineering scaffolds. Results show that the final resin eliminated dark cure as shown through image analysis. Mechanically the new scaffolds proved to be far weaker than those printed from previous resins, with compressive strengths of 7.8 ± 0.5 MPa vs. 36.5 ± 1.6 MPa, respectively. The new scaffolds showed a 90% reduction in elastic modulus and a 74% increase in max strain. These properties may be useful in tissue engineering applications where resorption is required. Initial cytotoxicity evaluation was negative. As hypothesized, the use of TiO2 and oxybenzone showed synergistic effects in the 3D printing of PPF tissue engineering scaffolds.

Keywords: 3D printing; Bone tissue engineering; Digital micromirror device; Poly(propylene fumarate); Resorbable; UV filter.

Figure 1

Model of strategy undertaken to identify a working resin given the adjustable additives and build parameters based on the target cure depth, C_d, and 100% build completion (i.e., the pass condition).

Figure 2

Dimensions of the scaffold design used for benchmarking showing the key features: inner diameter (ID), outer diameter (OD), post diameter (PD), post height (PH), base height (BH), overall height (OH), support diameter (SD), and support height (SH). The build took place from left to right, or support end first.

Figure 3

M-type basement modified in order to reduce amount of resin required.

Figure 4

Depiction of the various user-defined build-style parameters in Table 1 (note that movements shown are exaggerated for effect). (1) A waiting time occurs before tilt separation following the completion of a layer of printing. (2) The basement hinges down at the tilt separation velocity to the tilt separation distance, Δz_tilt. (3) Waiting time before axis separation. (4) The build platform moves up at the axis separation velocity to the axis separation distance, Δz_axis. (5) Waiting time before axis positioning. (6) The build platform moves down at the axis positioning velocity to position the scaffolds just above the surface of the basement. (7) Waiting time before tilt positioning. (8). The basement hinges up at the tilt positioning velocity to its original level position. The distance between the bottom of the previous built layer of the scaffolds and the surface of the basement is the layer thickness. (9) Waiting time before exposure of the next layer.

Figure 5

Procedure carried out in ImageJ to analyze the area of dark cure.

Figure 6

A comparison of two CDTs without (left) and with (right) HMB demonstrating the power of HMB to eliminate dark cure.

Figure 7

Side-by-side comparison of the same hollow scaffold (Figure 2) built with BR (left) and WR (right). The elimination of particulate buildup through the use of WR is an example of how part accuracy has been increased. These particles have been eliminated through the use of HMB.

Figure 8

The average stress-strain response of scaffolds built with BR (n = 5) and WR (n = 4). The failure mode was buckling. It clearly shows the effects of high levels of HMB in WR, which make the scaffolds far more elastic and able to handle larger strains, but at lower stresses.

Figure 9

Effect of PPF to DEF ratio on C_d for WR. Error bars represent ± standard error and show that there is no significant change to C_d by changing the PPF to DEF ratio (n = 7).

Figure 10

Normalized dark cure area vs. exposure time for WR and BR. Error bars are ± standard error (n = 3).

Figure 11

Design space for C_d using resins with oxybenzone, TiO₂, and (a) 1% BAPO, (b) 2% BAPO, or (c) 3% BAPO. The gray zone (120–150 μm) is our target range for C_d to allow for adequate layer-to-layer stitching without compromising resolution. Filled-in markers indicate CDTs that resulted in dark cure. Error bars represent mean ± standard error (n = 5).

Figure 12

Live/dead staining for cytotoxicity of L929 cells: (a) direct contact with WR thin films, (b) normal culture on polystyrene negative control, and (c) 70% v/v methanol treated positive control taken at 100x.