Bioresorbable
‘Tissue engineering is an interdisciplinary field that applies the principles of engineering and life sciences towards the development of biological substitutes that restore, maintain or improve tissue function ‘ Langer, R. and J. Vacanti, Tissue Engineering. Science, 1993. 260(14 May): p. 920-925
The field itself is still in its infancy and although the basic techniques are now well established, there is still much development work required to reliably produce functional tissue.
To engineer a neo-tissue in vitro requires appropriate cells, a scaffold and optimal culturing conditions (Fig. 1). Much of the current research is focused on creating scaffolds from bioresorbable materials.
Figure 1. Tissue Engineering
Scaffold design parameters
Scaffolds used in tissue engineering have three primary functions. First, the scaffold must facilitate the localization and delivery of cells to specific sites in the body. Second, it defines and maintain a three-dimensional space for the formation of new tissues with appropriate structures. Third, it guides the development of new tissue with appropriate function. In order for a scaffold to be able to fulfil this function it must possess certain qualities. These qualities include Biocompatibility?, porosity, permeability, mechanical strength and suitable surface for cell attachment. This section concentrates on scaffolds created from resorbable materials but similar qualities are required for non-resorbable scaffolds.
Figure 2 SEM images of fibrous scaffolds
Biocompatibility? of scaffold materials and FDA approval
In order for a material to be considered a suitable scaffold material it must be demonstratably biocompatible.
‘ A biocompatible material is one that possesses the ability to perform with an appropriate host response in a specific application’. Williams, D.F., Dictonary of Biomaterials. 1999, Liverpool: University Press. p.40.
This requires firstly that a scaffold material be non-toxic to the cells that it is supporting in vitro. If the material is to be used In vivo ?as well as in vitro it must be blood and tissue compatible. If the material is expected to degrade in vivo following implantation then the degradation products and leachables: oligomers; residual monomers; must also be non-toxic . If a large amount of a Biodegradable? polymer is implanted (in an Avascular 
site) it may overwhelm the body’s ability to remove the degradation products, leading to undesirable responses.
site) it may overwhelm the body’s ability to remove the degradation products, leading to undesirable responses.
The second condition for a scaffold material being considered biocompatible is that it must be able to perform the specific function that it was originally intended. It must have the necessary mechanical properties (e.g. strength), and these properties must last for the expected life of the material: if the material it expected to degrade there must be a smooth transition between mechanical properties of the implant and that of the new tissue. Finally it must be possible to sterilize the material.
With regards to biomaterials in general, no artificial material is ever completely inert in the human body so a suitable material is considered to be the one that causes the least reaction.
The U.S Food and Drug Administration (FDA) is responsible for protecting public health in the united states by assuring the safety, efficacy, and security of human and veterinary drugs, biological products, medical devices, food supplies, cosmetics, and products that emit radiation.
Bioresorbable Materials
For the tissue engineering process it is preferred that the scaffolding material is degradable.
Polymer degradation is a change in the properties of a polymer due to a change in its chemical structure, although faster loss of mechanical properties does not necessarily imply faster degradation. For tissue engineering scaffolds it is important to be aware of the difference between resorbable degradation and degradation by erosion. The degradation products of an eroded polymer will not necessarily be resorbed and removed from the body, and so may lead to other complications.
Synthetic resorbable polymers are mostly degraded via chemical hydrolysis. This has the advantage that in vivo degradation does not vary from patient to patient. Glassy aliphatic polyesters show little enzyme involvement in the early stages of degradation because of inaccessibility. While it is likely that enzymes can contribute at the later stages, especially following erosion, physical fragmentation and solubilization the hydrolysis of most scaffold materials is not enzyme related, so the present study will use the term ‘degradable’ rather than ‘Biodegradable?’.
Unlike synthetic materials natural biomaterials tend to be degraded by enzymes. It is possible to control the degradation rate by chemical crosslinking or other chemical modifications of some natural biomaterials.
An overview of bioresorbable materials can be found at https://www.orthoworld.com/orthosupplier/bonezone/online/2006/fall/editorial-capps.pdf
Natural materials
Natural polymers include collagens, glycosaminoglycan, starch, silk, alginate, chitin and chitosan. These polymers are either similar or identical to the natural extra cellular matrix they are expected to replicate. Compared to synthetic polymers there is far less of a problem with toxicity. On the other hand, natural polymers may still cause an Immunogenic? reaction when implanted in the body.
The benefit of using natural materials as biomaterials is that they function biologically at a molecular level, rather than a macroscopic level. The disadvantages of natural polymers are that there are large batch-to-batch variations as the substances must be isolated from biological tissue. It is also almost impossible to effectively sterilise natural material without changing its properties. Finally many natural materials tend to have poor mechanical properties, and as their chemical structure is often more complex than synthetic polymers they are difficult to manipulate into complex shapes. Below is a description of two of the natural materials that have been used in tissue engineering.
Collagen
There have been found to be twelve different types of collagen but biomedical applications tend to use type I. As many of the properties of collagen come from its native structure, it is important to disrupt this as little as possible during processing. Collagen can be made into films and sponges but this requires special solvent as it cannot be melted with a heat source without risk of denaturing the protein structure.
More details on the potential for producing collagen scaffolds can be found at http://www.seas.upenn.edu/sunfest/docs/papers/Tsing07.pd
Silk
In tissue-culture conditions silk maintains its mechanical integrity, but it is susceptible to degradation by a protease cocktail in vivo. Silk has the highest mechanical strength of any naturally degradable fibre, and so this gradual transfer of mechanical stability, through slow degradation, means that there has been renewed interest in using silk for tissue engineering.
Synthetic material
The development of synthetic degradable biomaterials is still a relatively new field. As a result the available polymers are still too limited to cover a diverse range of material properties required for engineering complex tissues.
Polylactic Acid (PLA) and Polyglycolic Acid (PGA)
PLA , PGA and copolymers (PLGA) are currently the most popular degradable polymers used in medical applications. They have an extensive U.S Food and Drug Administration (FDA) approval history. These are aliphatic polyesters and belong to the α-hydroxy group. The chemical structure of PLA has a methyl pendant group, and this contributes in its degradation rates. As a result, the degradation rate of their co-polymer PLGA depends on the exact ratio of PLA and PGA present in the polymer.
PLA undergoes hydrolytic scission to its monomeric form, lactic acid, which is eliminated from the body by respiration via the tricarboxylic acid cycle, and its carbon is primarily excreted in the lungs as CO2. In melt processing of PLA, care must be taken to avoid excessive heating of the polymer to minimize the extent of monomer formation during the moulding or the extrusion process.
PGA, on the other hand, can be broken down in two ways: by hydrolysis and by non-specific esterases and carboxypeptidases. The glycolic acid monomer is either excreted in the urine or enters the tricarboxlic acid cycle. Owing to its Hydrophilic?nature, PGA tends to lose its mechanical strength rapidly over a period of two weeks and is absorbed in about four weeks after implantation. It can be completely absorbed in about four to six months.
Information on PLA and PGA fibrous scaffolds can be found at:
Polyanhydrides
These polymers degrade by surface erosion without the need for the incorporation of catalysts. They are highly sensitive to hydrolysis and degrade rapidly and predictably.
An abstract on a polyanhydride scaffold can be found at:
Polycaprolactones
These are polyesters related to PLA and PGA with similar Biocompatibility?, although they exhibit much slower degradation rates.
More information on PCL scaffolds can be found at:
Poly(hydroxbutyrate) (PHB), poly(hydroxyvalerate) (PHV), and their copolymers
This could be considered a natural polymer as it is derived from microorganisms, but it is listed here as it has many similar properties and applications to PLA. These polymers take several years to degrade in vivo. PHA degrades in to D-3-hyrodybutric acid, which is a normal constitute of human blood, which may in part be a factor in its low toxicity. This family of polymers and copolymers promise a range of valuable scaffold properties. PHB typically retains 80% of its original stiffness over 500 days on In vivo ?degradation. One major advantage of the materials is that it is thermoplastic and so can be shaped by injection moulding and extrusion blow moulding technologies.
An abstract on a PHB fibrous scaffold can be found at:
Electrospinning of fibrous scaffolds
Techniques used for creating porous scaffolds include solvent casting and particle leaching; phase separation; gas formation; melt moulding; and membrane lamination; compression moulding; freeze-drying and solid freeform fabrication (all non-textiles techniques).One of the benefits of using textiles is that there are several different options for imparting the desired properties into the final construct. These properties can either be imparted at a polymer level (natural or synthetic materials), at a fibre level or in the processing of the final scaffold. There has been a considerable amount of research into the development of nonwoven fibrous scaffolds but currently the majority of it is focused on the technique of electrospinning (Fig.3).
Electrospinning is a method of creating microfibres and nanofibre webs. The fineness of the fibres that are produced depends upon the chemistry of the polymer, its solution or melt viscosity, the strength and uniformity of the applied field, and the geometry and operating conditions of the spinning system. Fibre diameters in the range of 1µm down to 100nm are used. There have already been many studies on the benefits of using nanofibre scaffold, as they are considered to more closely replicate the natural ECM. Nanofibre scaffolds have been manufactured with pore sizes up to 200µm, and porosities in excess of 80%.
Figure 3Electrospinning
Agrawal, C.M. and R.B. Ray, Biodegradable? polymeric scaffolds for musculoskeletal tissue engineering. Journal Of Biomedical Materials Research, 2001. 55(2): p. 141-150.
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Martins-Green, M., Dynamics of Cell-ECM Interactions. 2nd ed. Principles of Tissue Engineering, ed. P.L. Lanza, R. Langer, and J. Vacanti. 2000, San Diego, California: Academic Press.
Oxford Reference Online: www.oxfordreference.com
