Biomedical Engineering Reference
In-Depth Information
Subtractive processes
include the
particulate-leaching technique, controlled
rate freezing and lyophilization technique, and laser ablation techniques. In these
processes, materials are removed from the assembled matrices to obtain porous
structures.
Particulate leaching is a process whereby the internal architecture is determined
by embedding a high density of particles such as salt crystals into a dissolved poly-
mer or ceramic. The dissolved mixture is then poured into a mould and treated
under heat and pressure to form the external shape. The particles are subsequently
leached out to leave pores. Although pores of uniform size can be obtained by us-
ing particles of same size, nonporous layers are formed during different steps of
the generation process, which make controlling the pore size difficult. Due to the
transport limitations of the leaching technique, obtaining thick samples is cumber-
some in addition to the difficulty in uniformly distributing the particles within the
matrix. Scaffolds produced by solvent-casting particulate leaching cannot guaran-
tee the interconnection of pores because this is dependent on whether the adjacent
salt particles are in contact.
The controlled rate freezing and lyophilization technique involves introducing
phase changes in a homogeneous polymer solution by freezing solvent into crys-
tals followed by sublimation of solid crystals into vapors (Figure 6.13). This is the
method of choice while forming scaffolds from natural polymers such as collagen,
which typically dissolve in acidic water. One advantage of the process is that it is
carried out at low temperatures and avoids the loss of biological activity of proteins
or other conjugates due to thermal denaturation. Hence, bioactive molecules can
also be incorporated during the fabrication process. Also, custom shaped scaffolds
can be easily synthesized by freezing polymer solutions in appropriate molds. Al-
though pore sizes can be controlled to a certain degree by altering the initial freez-
ing temperature, the pore sizes show distribution, and controlling pore geometry
is difficult.
There are a number of other techniques based on the textile industry to form
porous structures. These include electrospinning, fiber spinning, and extrusion
Figure 6.13
Controlled rate freezing and drying. Microstructure of scaffolds formed using a mix-
ture of chitosan and gelatin.
