3D printing of high-strength, porous, elastomeric structures to promote tissue integration of implants

Abstract

Despite advances in biomaterials research, there is no ideal device for replacing weight-bearing soft tissues like menisci or intervertebral discs due to poor integration with tissues and mechanical property mismatch. Designing an implant with a soft and porous tissue-contacting structure using a material conducive to cell attachment and growth could potentially address these limitations. Polycarbonate urethane (PCU) is a soft and tough biocompatible material that can be 3D printed into porous structures with controlled pore sizes. Porous biomaterials of appropriate chemistries can support cell proliferation and tissue ingrowth, but their optimal design parameters remain unclear. To investigate this, porous PCU structures were 3D-printed in a crosshatch pattern with a range of in-plane pore sizes (0 to 800 μm) forming fully interconnected porous networks. Printed porous structures had ultimate tensile strengths ranging from 1.9 to 11.6 MPa, strains to failure ranging from 300 to 486%, Young's moduli ranging from 0.85 to 12.42 MPa, and porosity ranging from 13 to 71%. These porous networks can be loaded with hydrogels, such as collagen gels, to provide additional biological support for cells. Bare PCU structures and collagen-hydrogel-filled porous PCU support robust NIH/3T3 fibroblast cell line proliferation over 14 days for all pore sizes. Results highlight PCU's potential in the development of tissue-integrating medical implants.

Keywords: 3D-printing; collagen; hydrogel; polycarbonate urethane; porous.

Figure 1.

Young’s modulus of various relevant biomaterials and tissues: cobalt chrome (CoCr) [10], titanium alloy (Ti) [10] polyether ether ketone (PEEK) [8], polylactic acid (PLA) [30], ultra-high-molecular-weight polyethylene (UHMWPE) [9] polycaprolactone (PCL) [31], polyvinyl acetate (PVA) [32], solid PCU (95a) [26], silicone [33], polyethylene glycol (PEG) [34], crosslinked hyaluronic acid (HA) [35], articular cartilage [36], intervertebral disc [37], tendon [38] [39], trabecular bone [40] and cortical bone [41].

Figure 2.

Production of PCU porous structures with a range of tightly controlled in plane pore size (IPPS) (100, 200, 400, 600 or 800 μm) by fused deposition modeling, a 3D printing technique. Cell proliferation, distribution and viability for every IPPS were studied for two different groups. One group was seeded with cells suspended in an aqueous solution. The other group was seeded with cells suspended in a collagen solution, which was thermally gelled inside the porous structure after seeding.

Figure 3.

Representative images of porous PCU with increasing IPPS. a) Image of porous structures taken with a light microscope. b) 3D reconstruction of the porous structures from a micro-CT scan. c) Cross section of the porous structures on the YZ plane.

Figure 4.

Illustration of how the theoretical void percentage compares (gray curve) to the experimentally measured void fraction (black dots).

Figure 5.

Representative stress-strain curves of 3D-printed samples created with the same crosshatched geometry as the porous structures. Number at the end of each curve represents the target IPPS.

Figure 6.

a) UTS compared to IPPS. b) STF of compared to IPPS. c) UTS compared to void volume fraction. Results are presented as the mean± standard error (S.E.) (n=3 per group).

Figure 7.

Cell proliferation assay of NIH/3T3 cells. DNA content in the porous PCU structures was measured for a) cells cultured on bare PCU or b) cells cultured on PCU combined with a collagen hydrogel. DNA content in the structures at days 0, 1, 3, 5, 7, 9, 11 and 14 was measured using a PicoGreen assay. Results are presented as the mean± S.D (n=4).

Figure 8.

Representative Live/Dead (Green = calcein AM in live cells, red = ethidium homodimer in dead cells)) staining images of NIH/3T3 cells, performed at days 1, 7 and 14 for a) bare and b) collagen filled porous structures, illustrating differences in cell distribution between groups. Scale bar = 400 μm.

Figure 9:

Comparison of experimentally determined Young’s modulus for 3D printed PCU constructs with varying porosity (solid dots) compared to the theoretical relationship between Young’s modulus and porosity for an ideal open cell foam with a scaling factor set at S=.3 (dotted line). Experimental results are presented as the mean± standard error (S.E.) (n=3 per group).