Speaker
Description
Introduction
Bone graft substitutes are typically provided as ceramic granules. Whilst they have undergone successful clinical implementation, they are not without limitations. These include brittleness, variable resorption rates and a lack of control over the microarchitecture, all of which can lead to poor integration of the graft and fibrous tissue formation at the interface. Porous polymer microparticles can overcome some of these limitations and have seen increased research interest recently due to their potential to deliver cells and therapeutics in a minimally invasive manner. As there is evidence that pore size and shape can be used to control mesenchymal stem cell (MSC) fate, defining the microarchitecture of a particle to include beneficial geometries may be a route by which a reliance on co-administration of exogenous growth factors can be reduced. Tightly controlling microparticle architecture at a length scale relevant to bone cells can only be achieved through high-resolution additive manufacturing techniques such as two-photon polymerisation (2PP). Therefore, here we describe the fabrication of multiple defined-geometry microparticles via 2PP from a range of materials and their subsequent evaluation for cellularisation and osteogenesis.
Method
By cross referencing data from a microarray screen of polymers that promote MSC attachment and the number of acrylate moieties on the precursor monomers, potential photosensitive materials for 2PP were ranked. Processing parameters of the top three candidates were optimised to permit rapid fabrication of complex structures with robust structural integrity. Using designs based on mathematical solids to allow precise definition of geometry, these materials were then fabricated into six different designs of 100 μm diameter defined-geometry porous microparticles, and a solid-sphere microparticle control. To assess amenability to cellularisation, each geometry/chemistry combination (21 total) was fabricated into tessellated 1×1 mm arrays to mimic the close packing of particles that would occur in vivo. Arrays were seeded with three different donors of human bone marrow-derived MSCs (N=3, n=4) and cultured for five days before examination by confocal microscopy. By integrating fluorescence intensity across the height of the arrays, cellularisation could be quantitatively compared.
Results and Discussion
Quantitative analysis of array cellularisation revealed variation in pore shape can modulate cell infiltration even with a constant material chemistry. Particular geometry/chemistry combinations outperformed the solid particle controls, and importantly, certain geometries were observed to have a high degree of cellularisation across all chemistries investigated, indicating an architecture with utility in regenerative medicine. Microparticles which encouraged cell infiltration are currently undergoing screening for synergistic osteogenic effects that may further enhance their performance as bone graft substitutes. In parallel, fabrication of 3D arrays (e.g. 1×1×1 mm) of microparticles for assessment in a small animal model are underway.
In summary, 2PP allows us to incorporate a defined, cell-scale internal geometry within porous polymer microparticles. This work revealed a mathematically definable geometry that promotes MSC infiltration in comparison to a solid polymer microparticle which may be of interest clinically and is currently undergoing further evaluation. </div>
Acknowledgements
We acknowledge funding from the United Kingdom Regenerative Medicine Platform 2 (UKRMP2) [MR/R015651/1] and Next Generation Biomaterials Discovery [EP/N006615/1].
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