Synthetic Morphogens and Pro-morphogens for Aided Tissue Regeneration - Biologically Responsive Biomaterials for Tissue Engineering - page 56

be oriented towards molecular and biomaterial designs specific for their integration
into specific clinical applications and their relative procedures.

3.8.2.1 Solid-Phase Peptide Synthesis

Peptide synthesis methods have provided an opportunity to synthesise highly reproduc-
ible and scalable batches of peptides with analogue bioactivity of morphogens, growth
factors and bioligands. In particular, the solid-phase synthesis approach appears par-
ticularly attractive as it allows a fine control of the product quality and the simplification
of puri fi cation procedures [ 64 ]. Indeed, the scaled-up synthesis of therapeutically active
peptides has already been applied to the pharmaceutical field and its application to the
production of medical device and regenerative medicine products is therefore realistic.
There are various strategies to synthesise peptides by solid-phase synthesis. The com-
mon denominator of these strategies is a process of gradual addition of amino acid units
to a resin previously functionalised with a linker. Resins are available on the market that
expose functional groups such as -NH 2 and -COOH that are able to anchor molecules
through peptidic bonds. These resins are usually functionalised with linker molecules
that are susceptible to cleavage by specific organic solvents. The presence of this cleav-
able linker allows the liberation of the final peptide product from the resin. The peptide
product is obtained through the addition of amino acids according to the sequence
identified as bioactive by phage display technique. To ensure control over this process,
amino acid with protecting groups has been made available on the market. Different
types of protecting groups (e.g. Fmoc or Bmoc groups) have been identified to facilitate
various conditions of synthesis. These functional groups can block either the -NH 2 or
-COOH of the amino acid. Once grafted to the linker or to the previously attached
amino acid, the protecting group is cleaved off from the amino acid to expose the func-
tional group (i.e. -NH 2 or -COOH) necessary for the formation of the following pep-
tidic bond. Specific reagents activate the exposed functional groups to allow this
reaction. Hence, the synthesis of the peptide can proceed towards it carboxy-terminal
(C-terminal) or amino-terminal (N-terminal) end. Table 3.3 summarises the various
steps of a typical protocol of solid-phase synthesis as applied to the synthesis of a
BMP-2 analogue.

3.8.2.2 Hyperbranched Molecular Scaffolds

The concept of protein scaffolds is distinct from biomaterial scaffolds. Protein scaf-
folds are generally understood to encompass a range of protein domain-based
frameworks that are neither conventional antibodies nor peptides [ 65 ] . These molec-
ular scaffolds are used to stabilise the conformation of bioactive peptides and to
maximise their exposure to the biochemical or cellular component with which they
are destined to interact. In other words, protein scaffolds aim at replacing or
mimicking the tertiary structure of proteins that determine the molecular folding