Apr 22, 2008
Nano-engineering biocompatible materials
The ability to design and engineer scaffolds from biocompatible materials, which encourage living cells to repair and restore damaged tissues, is increasingly being exploited in tissue engineering applications.
Electrospinning of biomaterials for regenerative medicine applications is an active research area. The simple technique utilises a high voltage power supply coupled with a range of variable parameters to readily create, long, continuous fibres with diameters ranging from nanometres to microns.
The ability to vary the materials used and fibre diameter produced confers adaptability in the electrospinning process, and justifies its use within the field of tissue engineering for creating scaffolds capable of mimicking the structural properties of the tissue they are intended to restore.
At the University of Manchester, a number of research projects incorporating electrospinning to create engineered scaffolds are currently being undertaken. These include: a random network of fibres with incorporated phosphonates for bone restoration, highly aligned fibres to encourage correct formation of new muscle tissue and formation of 3D aligned fibrous bundles for tendon regeneration. This latter project will be discussed below.
The electrospinning apparatus (fig. 1) consists of a high voltage power supply connected to a needle-tipped syringe containing a polymer-solvent solution pumped under a known flow-rate. An earthed target collector is positioned at a measured distance from the needle-tip. Applying a sufficiently high voltage causes electrostatic charging of the polymer solution. The polymer is expulsed as a jet once the charge intensity of solution become high enough to overcome surface tension and viscoelastic forces. Due to the charges present, the polymer jet stretches and thins as it travels towards the collector. Upon impact, the charge of the fibres dissipates and the electrical circuit is completed.
Incorporation of polymeric nanofibres into tissue engineered constructs makes an ideal environment for cellular attachment, proliferation and maintenance of phenotypes. Furthermore, aligned nanofibres have been shown to aid cell orientation along fibre axes by 'contact guidance'.
Morbidity-related tendon injuries pose a personal and clinical burden, which as yet are untreatable. The rate of degeneration and spontaneous rupture of tendon tissue is increasing - mostly as the population ages and the fact that more and more people are taking part in sport. Current interventions often have a poor outcome, resulting in loss of function, further degeneration and rupture. Autografts (tissue taken from another part of the patient) and allografts (tissue taken from a donor patient) can cause additional morbidity and tissue rejection respectively.
Tendons connect muscle to bone allowing force transmission and ultimately joint movement. They are highly fibrous tissues with hierarchical organisation. Collagen type I is the main structural component, with its fibres lying in longitudinal arrays throughout the tendon. The diameter range of these collagen fibres is 50-500 nm, making electrospinning a suitable method for creating polymer fibres of replicable diameter, length and alignment.
To mimic fibre diameter, suitable electrospinning parameters have to be determined. Some of the many variables of this method include:
1. Polymer solution properties - viscosity, polymer molecular weight, conductivity and surface tension.
2. Electrospinning setup - voltage, solution flow rate, needle-tip to collector distance.
3. Ambient conditions - external temperature and humidity.
An investigation of parameters showed that a solvent with high dielectric constant yielded a dense fibrous network. Solutions of low polymer concentration, low polymer molecular weight, long needle-tip to collector distance, low voltage and high flow rate produced fibres of finer diameter.
Once fibres of desired size were produced, we focused our attention on directing fibre orientation. There are a number of methods for fabricating highly aligned, 2D mats of fibres, including: electrospinning between two parallel plates or onto the edge of a rotating mandrel. 3D constructs of aligned fibres can be produced - with increased difficulty - by electrospinning onto a liquid, within an earthed tray, and drawing fibres off the liquid surface causing fibres to coalesce and align (fig. 2). Using imaging software to measure bundle width, incorporated fibre diameters fabricated from this technique were found to measure 46 microns and 640 nm respectively.
Due to the difficulty in drawing uniform fibres, we further manipulated aligned 2D mats to form 3D fibrous bundles. This has allowed us to develop a more consistent bundle with controllable lengths.
Our research aims to develop a 3D scaffold appropriate for surgical implantation into damaged tendons. We envisage that this synthetic structure will perform the mechanical functions of a tendon whilst promoting cell migration and subsequent repair. The biodegradable properties of the polymer, in this case polycaprolactone, will allow scaffold degradation at the same rate as new tissue forms, allowing a smooth transfer of load and avoiding further invasive surgery with its associated risk of infection and recovery time. We hypothesise that implanting tendon scaffolds will facilitate appropriate tendon healing, leading to improved clinical outcomes for sufferers of tendon pathology.
We hope that constructs of different shape and size could be matched to the type and extent of tissue damage (fig. 3).
To date, our research has focused primarily on scaffold development and characterisation, coupled with in vitro cell studies. We shall now investigate scaffold performance once implanted using mouse Achilles as a model for pre-clinical trials. Depending on the success of this, further clinical trials are planned and it is anticipated that an "off-the-shelf" product will be available within five years.
About the author
Lucy Bosworth graduated in 2004 with a first class Masters degree in biomedical materials science from the University of Manchester, UK. Since then she has worked for DePuy CMW in R&D and Biocompatibles as a production scientist. She is currently undertaking a PhD in biomaterials for tendon regeneration at the University of Manchester under the direct supervision of Prof. Sandra Downes.