Variety of methods are employed to manufacture biomedical composite components. Most of the methods have been adapted from those available for manufacturing composites for non-biomedical applications (e.g., aerospace applications)., and tweaked as required to meet design, regulatory and manufacturing requirements unique to biomedical applications. Method selection for a particular component depends on the biomaterials, design requirement and application.
The first step is to get the components of composites – the discontinuous phase and continuous phase together. In most cases, the continuous phase is a polymer. It can be a thermoset or thermoplastic polymer. Fiberglass, carbon fibers or these fibers woven, knitted, stitched or bonded together to form a fabric or cloth, are some examples of the discontinuous phase. Layup is a term that most often describes this process. Hand layup and spray layup are the two most common types of layups.
Hand layup is further classified into dry and wet layup. During dry layup, dry fabric is placed over each other to form a layered stack. The fabric layers are also known as “plies,” or prepreg plies. The continuous phase, a resin, is infused through this stack.
A variation of this method is known as wet layup. Each ply layer is coated with the resin using rollers or brushes. Then the next ply layer is placed over this coated layer and resin is coated on this layer and so on. The resin applied during hand layup are infused into the pry by application of pressure. This video succinctly explains this process.
Spray layup involves passing Fibers through a hand-held gun to cut them into shorter lengths. These shortened fibered are passed into a resin steam spraying them together onto a surface.
Hand or spray laid composites are laid out or filled into molds (also called tools) that hollow shaped designed according to the implant part specifications. In other cases, these semifinished composites do not take the shape of molds but are available in standard sheet or bulk shapes. If they are present as sheets, they are known as sheet molding compounds, and if present in any other bulk shape, they are known as bulk molding compounds.
All composite materials are cured. Curing is a chemical reaction that crosslinks the polymer resins. The most common method of initiating crosslinking is through the application of heat. Other methods include exposure to ultraviolet light, microwaves, or high-energy electrons (e-beam curing). The viscosity of the resin which is originally has a paste-like consistency increases with increase in crosslinking between the polymer strands. The resin first forms a gel which indicates the formation of a three-dimensional network and then further tightens into a stiff structure. The network at this stage is said to be vitrified. No more reaction occurs at this stage at the particular temperature.
Curing in the simplest case can happen under standard atmospheric conditions. However, in a large number of cases pressure is applied to improve or accelerate the curing process. Depending on the type of pressure applied and additional processing of the semifinished composite, you would come across terminologies that represent more sophisticated composite manufacturing methods such as vacuum bag autoclaving, compression molding and injection molding.