Biomedical Engineering Reference
In-Depth Information
hydroxyproline. Then polypeptides arrange, via hydrogen bonding,
into left-handed triple helix microfibrils which organize together in a
number of different architectures to create collagen fibers. Therefore,
the morphology of collagen is a 300 nm long rod with a molecular
weight of 300,000 g.mol
− 1
[NAI 07]. Its structural integrity and ability
to withstand high tensile loads (92.5 MPa) make collagen a good
candidate for tissue engineering in load-bearing applications, as well
as for skin regeneration. Collagen is soluble in acidic aqueous
solutions and can be processed into different forms. Moreover, due to
its high reactivity, it can be cross-linked by a variety of cross-linking
agents [ULE 11]. There are 28 types of collagen molecules with the
most common being types I, II, III and IV. Type I represents 90% of
all collagen and is found in skin, tendon and bone. Commercial
sources of collagen type I are generally derived from rat tail, bovine
dermis and human placenta. However, the risk of infectious diseases
transmission from allogenic or xenogenic materials, the potential for
immunogenicity, the high cost of purification, as well as the concern
of quality and product homogeneity for mass production lead to the
development of recombinant systems [MON 11].
Collagen can be converted into gelatin by denaturation and/or
physical-chemical degradation. Therefore, gelatin consists of 19
amino acids and is arranged in single-stranded molecules. The process
has a huge impact on the properties of the final gelatin products.
Gelatin is water soluble, possesses good film forming abilities and is
enzymatically degraded by collagenases since it is a derivative of
collagen [VRO 09]. It has similar hemostatic properties as the
collagen precursor but lacks antigenicity, and degrades much more
rapidly. Gelatin is used in tissue engineering and drug delivery
applications where mechanical strength is not of much importance.
However, its mechanical stability may be enhanced by chemical cross-
linking with various agents [MON 11].
Silk is a natural protein mainly produced by the domestic
silkworm, but other natural sources may be used, such as honeybees,
wasps, ants and spiders. Silk fiber consists of two types of self-
assembled proteins: fibroin and sericin, containing the same 18 amino
