Tijna Alekseeva

Alignment hierarchies: engineering architecture from the nanometre to the micrometre scale

Natural tissues are built of metabolites, soluble proteins and solid extracellular matrix components (largely fibrils) together with cells. These are configured in highly organized hierarchies of structure across length scales from nanometre to millimetre, with alignments that are dominated by anisotropies in their fibrillar matrix. If we are to successfully engineer tissues, these hierarchies need to be mimicked with an understanding of the interaction between them. In particular, the movement of different elements of the tissue (e.g. molecules, cells and bulk fluids) is controlled by matrix structures at distinct scales. We present three novel systems to introduce alignment of collagen fibrils, cells and growth factor gradients within a three-dimensional collagen scaffold using fluid flow, embossing and layering of construct. Importantly, these can be seen as different parts of the same hierarchy of three-dimensional structure, as they are all formed into dense collagen gels. Fluid flow aligns collagen fibrils at the nanoscale, embossed topographical features provide alignment cues at the microscale and introducing layered configuration to three-dimensional collagen scaffolds provides microscale- and mesoscale-aligned pathways for protein factor delivery as well as barriers to confine protein diffusion to specific spatial directions. These seemingly separate methods can be employed to increase complexity of simple extracellular matrix scaffolds, providing insight into new approaches to directly fabricate complex physical and chemical cues at different hierarchical scales, similar to those in natural tissues.

Development of Conical Soluble Phosphate Glass Fibers for Directional Tissue Growth

One of the challenges of tissue engineering is the regulation of vascularization and innervations of the implant by the host. Here, we propose that using soluble phosphate glass (SPG) fibers, incorporated in dense collagen constructs will allow us to control the rate and direction of tissue ingrowth. The idea here was to generate channels with tailored direction using conical phosphate glass fibers. The changing surface area-to-mass ratio of conical fibers will make them to dissolve faster from their narrow ends opening up channels in that direction ahead of any ingrowing cells. In this study, we show that SPG fibers can be manipulated to produce conical shape fibers using graded dissolution. Our result shows that 40 µm fibers of composition ratio 0.5 (P2O5):0.25 (CaO):0.25 (Na2O) and dissolution time of 8–10 h have a mean reduction in fiber diameter of 8.85 ± 2.8 µm over 19.5 mm fiber length, i.e., a mean rate of 0.5 µm/mm (n = 20) change. These conically shaped fibers can also be manipulated and potentially used to promote uniaxial cell–tissue ingrowth for improved innervations and vascularization of tissue engineered constructs.

Roofed grooves: Rapid layer engineering of perfusion channels in collagen tissue models

Surface patterning (micro-moulding) of dense, biomimetic collagen is a simple tool to produce complex tissues using layer-by-layer assembly. The aim here was to channelise three-dimensional constructs for improved perfusion. Firstly, collagen fibril accumulation was measured by comparative image analysis to understand the mechanisms of structure formation in plastically compressed collagen during µ-moulding. This showed that shape (circular or rectangular) and dimensions of the template affected collagen distribution around moulded grooves and consequently their stability. In the second part, this was used for effective fabrication of multi-layered plastically compressed collagen constructs with internal channels by roofing the grooves with a second layer. Using rectangular templates of 25/50/100 µm widths and 75 µm depth, grooves were µ-moulded into the fluid-leaving surface of collagen layers with predictable width/depth fidelities. These grooves were then roofed by addition of a second plastically compressed collagen layer on top to produce µ-channels. Resulting µ-channels retained their dimensions and were stable over time in culture with fibroblasts and could be cell seeded with a lining layer by simple transfer of epithelial cells. The results of this study provide a valuable platform for rapid fabrication of complex collagen-based tissues in particular for provision of perfusing microchannels through the bulk material for improved core nutrient supply.