216
G. Keerthiga et al.
• Collagen is a triple helix protein found in the extracellular matrix (ECM) of
fibroblasts and osteoblasts. Generally utilized form type I collagen is isolated
from the body tissues, neutralized and broken down to single molecules called
‘gelatin.’ The final step determines the type of gelatin obtained. It is known to
undergo a ‘sol-gel’ transition in response to the surrounding temperature, making
it a suitable material for designing thermoresponsive hydrogel. Thus, collagen is
an impending component for a ‘smart drug delivery system’ [52].
2.1.2 Synthetic Polymers
Natural polymeric sources are being entirely replaced by synthetic polymers, such as
poly 2-hydroxylethyl methacrylate (pHEMA), polyvinyl alcohol (PVA), polyethylene glycol (PEG), etc. for increased of mechanical strength, acclimatized functionality, and degradability. The method of crosslinking effects hydrogel degradation
that can be controlled in response to external stimuli such as pH, temperature, light,
and electric field, resulting in making a ‘smart drug delivery system’ [51]. Some
of the polymers, such as PEG, are amphiphilic, that aid in more natural absorption
of hydrophobic drug molecules or intended cargos than conventional drug vehicles.
Polyethylene glycol (PEG) is a synthetic polymeric unit that is a polymer or oligomer
of ethylene oxide. Polyoxyethylene (POE), Polyethylene Oxide (PEO), polyvinyl
alcohol (PVA), polypropylene oxide (PPO) are chemically synonymous and differentiated by their molecular weights. Polymers with molecular weight <100,000 are
grouped under PEGs, and the higher molecular weight is arranged as PEOs. PEGs
of low molecular weight (<1000) are generally colorless, viscous, and increased
molecular weights are white color and waxy solids. All PEGs are soluble in both
aqueous and organic solvents, such as chloroform, ethanol, toluene, methyl chloride
[3]. It has unique properties such as biocompatibility, non-immunogenicity, and resistance to protein adsorption [135]. PEG is a Food and Drug Administration (FDA),
USA approved polymer that has found application in various frontiers of biomedical
applications, such as bone prostheses, wound healing and tissue engineering, and
drug delivery [165]. PEG cannot be a standalone polymer for polymeric network
formation. Additional functional groups such as acrylate, thiol, vinyl sulfone, amine,
and carboxyl, determine together of its mechanical strength and degradability (both
natural and stimuli-responsive) of the polymeric network. In situ gelling systems are
designed on the principles of photosensitive systems that can be cross-linked upon
ultraviolet (UV) or light irradiation and are self-assembled in vivo. The latter alters
its physical state according to the physiological environment inside the host system
[44–1].
Poloxamers are triblock polymers that contain a hydrophobic chain surrounded
by hydrophilic units on either side to facilitate hydrophobic drug delivery [192].
Hydrophobic polymers cross-linked in aqueous environment exhibit sol-gel transition by reversible thermal gelation as shown in the scheme below (Fig. 3).
G. Keerthiga et al.
• Collagen is a triple helix protein found in the extracellular matrix (ECM) of
fibroblasts and osteoblasts. Generally utilized form type I collagen is isolated
from the body tissues, neutralized and broken down to single molecules called
‘gelatin.’ The final step determines the type of gelatin obtained. It is known to
undergo a ‘sol-gel’ transition in response to the surrounding temperature, making
it a suitable material for designing thermoresponsive hydrogel. Thus, collagen is
an impending component for a ‘smart drug delivery system’ [52].
2.1.2 Synthetic Polymers
Natural polymeric sources are being entirely replaced by synthetic polymers, such as
poly 2-hydroxylethyl methacrylate (pHEMA), polyvinyl alcohol (PVA), polyethylene glycol (PEG), etc. for increased of mechanical strength, acclimatized functionality, and degradability. The method of crosslinking effects hydrogel degradation
that can be controlled in response to external stimuli such as pH, temperature, light,
and electric field, resulting in making a ‘smart drug delivery system’ [51]. Some
of the polymers, such as PEG, are amphiphilic, that aid in more natural absorption
of hydrophobic drug molecules or intended cargos than conventional drug vehicles.
Polyethylene glycol (PEG) is a synthetic polymeric unit that is a polymer or oligomer
of ethylene oxide. Polyoxyethylene (POE), Polyethylene Oxide (PEO), polyvinyl
alcohol (PVA), polypropylene oxide (PPO) are chemically synonymous and differentiated by their molecular weights. Polymers with molecular weight <100,000 are
grouped under PEGs, and the higher molecular weight is arranged as PEOs. PEGs
of low molecular weight (<1000) are generally colorless, viscous, and increased
molecular weights are white color and waxy solids. All PEGs are soluble in both
aqueous and organic solvents, such as chloroform, ethanol, toluene, methyl chloride
[3]. It has unique properties such as biocompatibility, non-immunogenicity, and resistance to protein adsorption [135]. PEG is a Food and Drug Administration (FDA),
USA approved polymer that has found application in various frontiers of biomedical
applications, such as bone prostheses, wound healing and tissue engineering, and
drug delivery [165]. PEG cannot be a standalone polymer for polymeric network
formation. Additional functional groups such as acrylate, thiol, vinyl sulfone, amine,
and carboxyl, determine together of its mechanical strength and degradability (both
natural and stimuli-responsive) of the polymeric network. In situ gelling systems are
designed on the principles of photosensitive systems that can be cross-linked upon
ultraviolet (UV) or light irradiation and are self-assembled in vivo. The latter alters
its physical state according to the physiological environment inside the host system
[44–1].
Poloxamers are triblock polymers that contain a hydrophobic chain surrounded
by hydrophilic units on either side to facilitate hydrophobic drug delivery [192].
Hydrophobic polymers cross-linked in aqueous environment exhibit sol-gel transition by reversible thermal gelation as shown in the scheme below (Fig. 3).