Preparation of medical implants by stereolithography: photo-crosslinked networks and structure
Scaffolds are porous implants which provide support for cells and formed tissues. Ideally, these scaffolds are prepared from materials which are biocompatible, biodegradable and have mechanical properties which match those of the tissues that are replaced. Synthetic biodegradable polymer networks are excellent candidates for such materials due to the ease of tailoring the material properties. This thesis describes the preparation of three-armed, methacrylate functionalized oligomers (macromers) based on trimethylene carbonate, their formulation into processable stereolithography resins and the characterization of the obtained photo-crosslinked networks for medical implants.
The degradation of poly(trimethylene carbonate) (PTMC) networks was investigated in vitro and in vivo. It was shown that the degradation rates of these networks can be tuned by changing the crosslink density or by changing the macromer composition by preparing copolymeric macromers. Networks with lower crosslink densities and networks from copolymeric macromers prepared from TMC and either D,L-lactide or Ɛ-caprolactone showed an increased rate of degradation.
Designed 3D network structures with excellent mechanical properties can be prepared by stereolithography using resin formulations based on relatively high molecular weight macromers. Building with resins based on such high molecular weight macromers is a challenge. A sub-total, porous meniscus implant with mechanical properties close to that of a natural meniscus was designed and prepared. A study in an cadaveric knee showed that such an implant reduces the mean and peak pressure on the tibial condyle.
Networks with good mechanical properties and high toughness can also be prepared using mixtures of low molecular weight PTMC macromers forming bimodal networks. This allows for the preparation of implants with excellent mechanical properties by stereolithography without the challenges presented by building with resins based on macromers with relatively high molecular weight.
Hydrogel mixed-macromer network structures with excellent mechanical properties could be prepared using resins based on mixtures of PTMC, PEG, PCL and PDLLA macromers with two different molecular weights. It was shown that these networks are phase-separated. Cylindrical structures with gyroid pore architecture and a range of pore sizes and porosities could readily be prepared. These mixed-macromer network structures had compression moduli between 27 and 120 kPa in the hydrated state.
It was further shown that hydrogels prepared from (methoxy)-PEG grafted onto PTMC networks decreased the friction coefficients of these networks.
The excellent mechanical properties of these network films and structures, their degradability and applicability of the macromers in additive manufacturing techniques such as stereolithography makes these structures interesting candidates for clinical use as medical implants.
