2 MECHANISM AND
APPLICATION OF
BIOMIMETIC MATERIALS
2.1 Biomimetic Materials of Silk
Protein
Silk protein can be used to prepare excellent
biomaterials due to its good biocompatibility, low
immunogenicity, processability, and degradability.
In recent years, based on the basic principles of
tissue engineering, through material processing
technology, various forms of silk protein scaffolds
(including three-dimensional porous scaffolds,
nanofibers, hydrogels, two-dimensional membranes,
etc.) (Applegate, 2016) and silk protein drug carriers
(Microspheres, nanoparticles, etc.) have been used in
the repair of hard bones, ligaments, skin,
cardiovascular, cartilage, cornea, nerves and other
organs and tissues and anti-cancer therapy (Melke,
2016).
According to different technologies for obtaining
silk protein, the source of silk protein can be divided
into natural silk protein, recombinant silk protein,
and genetically modified silk protein. At present,
natural silks that are used and studied more include
spider silk protein and fibroin. In spider silk protein,
major ampullate silk has the advantages of high
strength, good tensile strength, and large breaking
energy, so it is currently the most studied kind of
spider silk (Hinman, 2000). The silk fibroin in the
silkworm silk fiber is coated inside the sericin, with
a diameter of about 10 μm. Studies have found that
the secondary structure of silk protein includes four
types: β-sheet, random coil, α-helix, and β-turn
(Gray, 2016).
Silk protein biomaterials prepared by different
processing technologies have different application
forms in the field of biomedicine. Silk fibers were
originally used as surgical sutures due to their
biodegradability and good biocompatibility.
Effective weaving of silk fibers can produce
"artificial ligaments" that can be used to repair or
replace broken ligaments (Altman, 2002). XU et al.
combined silk fibroin and chitosan to prepare a
composite membrane and obtained an excellent
healing effect after covering the membrane on the
wounded part of the rat (Xu,2015). YODMUANG et
al. found that silk protein gel can effectively
promote the growth of chondrocytes and the
secretion of related collagen. The formation of the
internal network of silk protein gel is conducive to
the loading of drugs or foreign genes (Yodmuang,
2015). In addition, the most studied silk protein
scaffold is used for bone tissue repair. A study
showed that adding growth factors to the scaffold
can better repair defective tissues (Meinel,2005).
Recently, Shi et al. 3D printed silk protein and
gelatin to obtain a composite scaffold that can repair
damaged cartilage in rabbit knee joints (Shi, 2017).
2.2 Biomimetic Materials of Nacre
In order to protect molluscs from water-carried
debris and predators, the shells of organisms like
abalone have evolved into a stiff and impact-
resistant material named shell. Shells are composite
materials made by mollusks that combine inorganic
minerals (CaCO3) in the surrounding environment
with organic matter generated by themselves under
ambient temperature and pressure. The formation
process of shells is a biological mineralization
process.
The shell structure is divided into 3 layers, from
the outside to inside are stratum corneum, prismatic
layer, and nacre layer. The organic layer and the
mineral layer in the nacre are arranged alternately
under appropriate magnification. The so-called
"brick and mortar" structure can be easily observed.
Its comprehensive mechanical properties, especially
fracture toughness can be rasied by 2 to 3 orders of
magnitude than that of single-phase calcium
carbonate ceramics. The alternate laminated
arrangement of nacre aragonite crystals and an
organic matrix is the key to its high toughness.
Based on this principle, material scientists have
developed the development of imitation pearl
laminated composite materials. Based on this
principle of biomineralization, Yoo et al. prepared a
boron nitride nanosheet (BNNS)/gelatin
nanocomposite. In this study, hyperbranched
polyglycerol was used to functionalize BNNS to
enhance the bonding strength between entities. By
changing the composition and arrangement degree
of BNNSs and gelatin in the nanocomposite, the
mechanical properties of the nanocomposite can be
controlled. This adjustment of mechanical properties
can produce a material with properties similar to
human cortical bone. In vitro cell experiments show
that this artificial nacre can support the adhesion and
proliferation of adipose-derived stem cells,
indicating that it can be used in biomedicine (Yoo,
2018).
Fabrication techniques such as hot-press assisted
slip casting, freeze casting, extrusion and roll
compaction, paper-making method, and layer-by-
layer self-assembly have successfully yielded