Biomedical Engineering Reference
In-Depth Information
stretched into liquid jets, which are finally deposited on the collector as ultrafine fibrous
membranes after the evaporation of solvent. The obtained membranes are further cross-
linked, usually in a glutaraldehyde vapor. Since the electrospinning of chitosan itself
sometimes proved to be difficult, chitosan was mixed with other synthetic or natural poly-
mers, such as PEO, PVA, PLGA, collagen, and so on [100]. Jiang et al. [101] prepared ibupro-
fen-loaded chitosan-based membranes composed of PLGA and PEG-g-chitosan by
electrospinning. Ibuprofen conjugated to the side chains of PEG-g-chitosan was released
sustainably for more than 2 weeks from the membrane.
6.4.4 liquids
When we say that a hydrogel is in the form of liquid, it means that the hydrogels can be
formed by heating or cooling their solutions [6]. Hydrogels that undergo a sol-gel transition
upon heating are always called injectable or
in situ
thermosensitive hydrogels. Today,
in situ
thermosensitive hydrogels have attracted wide attention as DDSs because of several advan-
tages, such as the facile procedure and the minimally invasive surgical procedure [102]. Such
a system enables therapeutic drugs such as cells, genes, peptides, and proteins to be simply
mixed with the aqueous polymer solution at low temperatures and then form a semisolid
hydrogel after injection into the body at a target site. It may also be possible to fill up a defect
or target site completely with these injectable hydrogels, including bioactive molecules.
Chitosan can only be dissolved in an acid aqueous solution. However, by simply adding
GP into its solution, the pH reached about 7. Thus this solution showed thermosensitive
property. It remained in liquid state at room temperature, whereas it formed a hydrogel at
body temperature. The preparation procedure is very simple, but one thing that needs to
be paid attention to is that GP solution should be added carefully drop by drop to chitosan
solution, obtaining a clear and homogeneous liquid solution [53,54]. Certain copolymers of
chitosan and other synthetic polymers, for example, chitosan-g-PEG and chitosan-g-PNI-
PAM, exhibit thermosensitive property too. The preparation process is so simple that only
dissolution of them into water is needed.
6.5 Drug Loading
Delivering a drug to the right part of the body in the correct dose and for long enough for
it to have an effect can be tricky. Some drugs are highly toxic and need to be targeted spe-
cifically to avoid damaging healthy cells; others must bypass the liver or else they will be
degraded; while some need to be released slowly over a long period if they are to be effec-
tive. Because of the diversity in the chemistry and size of the delivered molecules, the drug
loading for controlled release in any particular hydrogel can differ widely from one appli-
cation to another. The method by which the drugs are loaded directly impacts the avail-
ability of the drugs during release [4]. There are typically three methods to load drugs into
the hydrogel.
6.5.1 Physical entrapment
Loading drugs by diffusion or simple mixing is known as physical entrapment. The easiest
loading method involves the diffusion process of the drugs. When a hydrogel is formed, it
is allowed to immerse into a suitable drug solution [102]. The drugs slowly diffuse into the
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