Feature: Rebuilding site
3 January 2006. By Chrissie Giles
"Tissue engineering is a combination of engineering, biology, chemical engineering and materials science," says Julian Jones, from Imperial College London. "Its aim is to stimulate the body's own regenerative system to restore tissues and organs to their original state. Scaffolds are used to provide a framework for regenerating tissue."
The first kinds of implant to be developed - still used for devices such as heart valves, joint replacements and in-eye lenses - were made of non-biodegradable synthetic materials, and did not allow cells to adhere. The latest biomaterials, however, are based on porous scaffolds, which not only allow cells to adhere to them, but can be engineered to have biological functions too. "A bioactive scaffold is an artificial extracellular matrix [ECM] that you can engineer to signal to cells and achieve a specific biological outcome, such as cell proliferation or differentiation," says Professor Samuel Stupp, who is developing bioactive scaffolds for tissue repair at Northwestern University, USA.
Cell differentiation
As scientists have begun to understand better how cells behave in the body, scaffold design has become more sophisticated. "A key step was the realisation that there's a two-way communication between a cell and its environment," says Martin Humphries, Director of the Wellcome Trust Centre for Cell-Matrix Research at the University of Manchester. "Cells don't just float around and randomly anchor to their surroundings. Anchorage, differentiation, growth and movement are all influenced by the niche the cell finds itself in, and creates for itself. Tissues aren't like jigsaws, staying fixed in place. They need a framework which allows them to grow and develop over time."
But how is bioactivity conferred on a scaffold? First, signals can be attached to targets. "Short peptide sequences known to play a role in cell adhesion can be used to create sticky sites on scaffolds. They can be masked with a chemical such as polyethylene glycol and linked via a cleavable sequence to the scaffold," says Professor Humphries. "When a particular cell approaches the scaffold it might secrete an enzyme which cuts the sequence, thereby revealing the adhesion site and allowing the cell to bind."
The signal can also influence the cell's biology. A scaffold displaying a short part of an ECM protein called laminin, for example, causes neural stem cells to differentiate only into neurons. This has huge potential in central nervous system repair and neurodegeneration disorders such as Parkinson's, Alzheimer's and stroke. In unpublished work by Professor Stupp's team, it was found that nanofibres induce regeneration of motor and sensory axons in vivo in mice and rats.
Another way to cause biological effects is to deliver proteins on scaffolds. Certain growth factors can be attached to scaffolds, triggering stem cells to turn into bone cells.
Finally, scaffolds can be used to activate proteins that are already present in the body. Antonio Mikos from Rice University, USA, has used scaffolds to trigger the growth of blood vessels (angiogenesis) both in a test-tube and in rodents - something vital for cells to survive and grow. "We showed that you could jump-start feeding of a huge number of cells, especially if the tissue you're growing has a macroscopic structure," says Professor Mikos.
Nanotechnology
Coupled to these biological developments, recent advances in nanotechnology are transforming tissue engineering. "The introduction of nanoscale structure is important as it happens to be the same scale as a cell receptor," says Professor Stupp. "You can take design to a deeper level."
Moreover, it alters the entire approach to scaffold design. "What nanotechnology teaches us is that we ought to use bottom-up design, working from the nano- to the micro- to the macro-level. In the pre-nano era most efforts were focused on making different kinds of materials, starting big then getting smaller."
The key to bottom-up design in biology is self-assembly - designing molecules that spontaneously come together to create a scaffold with known properties. While some groups work on self-assembling fibres that are purely supportive, Professor Stupp is combining self-assembly and bioactivity, creating scaffolds with precisely defined biological properties.
First created in 2001, the scaffold is made of nanofibres, linear polymers comprising multiple copies of a 'peptide amphiphile' monomer. The monomers are designed such that they spontaneously form cylindrical nanofibres in an aqueous environment - as found inside the body.
Stem cells
But scaffolds themselves are of little use to tissue engineers without cells to seed on them. And the most exciting prospect is the use of stem cells. Because of their developmental flexibility, banks of stem cells could be used in all kinds of tissue regeneration - the properties of the scaffold dictating which type of cell they turn into.
"Stem cells are incredible promising as a therapeutic tool," says Professor Humphries. "It may take longer than people expect, and it is unlikely that simply injecting a stem cell into a brain or liver will accurately reproduce the tissue… that's not how organs form." Indeed, the environment of a stem cell is turning out to be key. "All basic work with stem cells shows that the presence of a matrix makes a difference to their behaviour," says Professor Stupp.
Unsurprisingly, stem cells featured heavily at the 2005 'Advances in Tissue Engineering' conference held at Rice University, Texas. "What a scaffold is made of is important as cells interact specifically with different materials," says Professor Stupp. "The delivery of stem cells was a critical issue at this year's meeting. Everything is tied in with how to create the correct environment."
As it is a field that draws on so many different specialisms, it's clear that effective research will require collaboration. "There's a big gap between researchers. Many people working on scaffolds come from the physical sciences, stem cell researchers come from biology. People will have to work together," says Professor Stupp.
The field looks set to continue to grow, with scaffolds also being tested for use in drug delivery and gene therapy. "Tissue engineering provides a new application to treat diseases and heal defects, and now we're seeing the first results in the form of product," says Professor Mikos. "It's not science fiction anymore, it's a reality."
What does the future hold?
Professor Julian Jones, Imperial College London
"For direct and rapid bone regeneration, a successful scaffold should have a highly interconnected porous structure, have controlled resorbability, be bioactive and be strong enough to survive under the stresses that bones suffer. Tailored inorganic-organic hybrids have the potential to fulfil these criteria."
Professor Martin Humphries, Wellcome Trust Centre for Cell-Matrix Research, Manchester
"The long-term future will probably focus on natural or biomimetic matrices because giving a stem cell the right environment controls how it develops, and this can only be done with biological scaffolds."
Professor Dame Julia Polak, Imperial College London
"Scaling up is the problem. At the moment, people are working on microscopic amounts, bioprocessing is necessary to scale up cells and scaffolds for use in the clinic."
Professor Samuel Stupp, Northwestern University, USA
"Future scaffolds will have to be even more complex, we will have to make scaffolds that can control spatial growth of different tissues in different positions, e.g. in different compartments."
Unsurprisingly, it is the tissue engineering applications with the biggest potential markets that are emerging in the clinic. Orthopaedic and cardiovascular approaches are two of the most successful:
Among many exciting developments, medical technology company Medtronic Sofamor Danek has marketed a device for spinal application that delivers bone morphogenetic protein-2 (BMP-2), a naturally occurring protein that induces bone and cartilage growth. A metal cage is filled with a collagen sponge soaked in recombinant BMP-2. As the bone grows to fuse the vertebrae together, the collagen sponge dissolves.
Much progress has been made in the cardiovascular field. In 2004, Professor Robert Langer and colleagues showed that rat heart cells grown on collagen sponges could be induced to align and couple electrically, beating in synchrony. In June, scientists from Duke University, USA, created robust arteries by culturing cells from elderly people on polyglycolic acid scaffolds. One of the major challenges remaining is to engineer narrow diameter vessels.
Further reading
- Silva GA et al. Selective differentiation of neural progenitor cells by high-epitope density nanofibres. Science 2004;303(5662):1352-5.
- Liu H, Roy K. Biomimetic three-dimensional cultures. Tissue Eng 2005;11(1-2_:319-30.
- Radisic M et al. Functional assembly of engineered myocardium by electrical stimulation of cardiac myocytes cultured on scaffolds. Proc Natl Acad Sci USA 2004;101(52):18129-34.