Collagen scaffolds are very common within the field of tissue engineering. They are varied in composition as different types of collagen can be used, and virtually anything added s a supplement. In this post I’m going to discuss the simplest method of creating a collagen scaffold, a method that I employed for two out of the three materials I made for my PhD project. The first material was a plain fibril bovine collagen type I scaffold, and the second was the same except with an added hydroxyapatite component. I actually made the hydroxyapatite myself, through the standard precipitation method (topic for a future technique post) in order to eliminate any byproducts present in commercially available product.
Basically, all materials in the final scaffold are mixed together to form a homogenous aqueous solution, which is then centrifuged and vacuumed in order to remove any air bubbles. The solution is then placed in a mold (we used PVC) and frozen at the desired temperature (in my experiments I used -20 degrees Celsius, though in past experiments I have also used -80 degrees Celsius freezers). The samples are then lyophilized (a.k.a. freeze-dried) for a few hours, pushed out of their molds, and then lyophilized some more. Note: beware of removing the samples too early; they dry from the outside, so if moisture is still present in the center they can collapse later. Finally, before use, samples are trimmed and cut as desired. I found that some trimming was necessary as sem micrographs showed walls of collagen on the outer edges, hindering cell and nutrient penetration.
Like any other polymer scaffold, the size and shape of the pores can be easily modified for different desired pore sizes etc. Given a set amount of collagen in an area, pore size is directly controllable by modifying the freezing temperature. At lower temperatures, ice crystals nucleate faster, and therefore the crystals formed will be more numerous and smaller. Since final pore size is directly related to aqueous crystal size, this means that the lower the freezing temperature, the smaller the pores. If the pores are more numerous, this means that the number of walls between pores also rises; but, since the total amount of collagen remains the same, this means each individual wall is thinner. One of the advantages to collagen and this fabrication method is the variation of the scaffold in its micro-porosity under 100 micrometers. It has actually been shown that variation on this scale is more conducive to biological activity, a long as the larger pore structures average the same as an otherwise similar scaffold.