3D fabrication methods – polymers
Electrospun fibre networks
Above: Electron micrographs of random and aligned electrospun fibres
Electrospinning allows us to prepare ordered single polymer fibres, or random fibre networks according to application need. Fibres can be smooth or porous, depending upon the preparation conditions offering a range of micro and nano-architectures for cells to colonise.
Open networks formed by fibrous scaffolds have been found to be ideal for the growth of many cell and tissue types. Electrospinning synthetic polymers offers control over fibre chemistry and morphology, allowing an empirical optimisation process for a given tissue type.
Right: Phase separation in copolymer gels can give rise to intricate internal maze-like structures.
Hydrogels are synthetic biological polymers that trap a large volume of water within their structure. They are an extremely diverse group of materials, which vary in terms of their water content and structure, phase morphology, polymer structure, biofunctionality and surface science. All of these parameters have been shown to influence their biological performance.
One of the important problems in tissue engineering is that cells towards the centre of any 3D tissue grown in vitro are starved of nutrients, and suffer problems with the build-up of waste products. One goal of polymer engineering work at Sheffield is to develop gels with hydrophilic channels that improve circulation through the bulk of the scaffold block.
This can, in principle, be done using amphiphilic gels formed by polymer molecules that have both hydrophilic and hydrophobic block sections. Hydrophobic blocks group together in water, to form denser areas, while the hydrophilic sections can be mostly water. Control over phase separation during gelation allows us to control both porosity and permeability of the gel block.
Right: Cells growing within a 10% (w/v) biocompatible thermo-responsive gel (gelation temperature 37°C), after 72 hours.
The tri-block copolymer NIPAM-MPC-NIPAM can form a biocompatible, thermo-responsive gel. It is composed of chain-like molecules, the centre section of each chain being the biocompatible MPC (2-methacryloyloxyethyl phosphorylcholine) oligomer. The two polyNIPAM blocks at either end of the chain allow the molecule to dissolve in water at low temperatures.
At higher temperatures hydrogen bonds between the NIPAM blocks and water are disrupted, causing them to collapse and group together. This causes a gel to form, and careful tuning of the relative proportions of MPC and NIPAM allows polymer scientists to control the temperature at which this occurs. Preliminary studies have shown that cells can grow and multiply within the gel.
Above: Preferential patterning of cortical neurons (A & B, green¼ MAP2 stain, blue ¼ DAPI), Dorsal root ganglion cells (C & D, red ¼ b-III tubulin, blue ¼ DAPI) and human neural progenitors (F & G) along P:DLC tracks. Scale bars are 90 µm.
We are developing 3D microstructuring techniques based on UV microstereolithography. We have been developing a laser based printing technique for producing biomolecule arrays and biosensors. This technique is able to print viscous fluids containing DNA, proteins and even living cells.