Cultured skin substitutes - Biomaterials for Treating Skin Loss - page 196

grounded collection plate. When the electric field reaches a threshold, the repul-
sive electric force within the liquid overcomes the surface tension of the solution
causing a charged jet of solution to be ejected from the droplet of polymer
solution. 96 The ejected polymer jet is accelerated towards the target, which is
oppositely charged or grounded. This process generates nonwoven meshes com-
posed of micrometer to nanometric sized fibers. The fiber diameter and morphology
of the electrospun scaffold are largely controlled by the concentration and molecu-
lar weight of the polymer. 100-103 Many other factors including flow rate, quality of
the solvent and surface tension result in variation in fiber diameter and morphol-
ogy. 96 By altering these factors, a nonwoven mesh with a fibrous structure
analogous to native ECM can be generated with narrow tolerances.
Electrospun polymers, such as polystyrene, have been used to generate skin
substitutes in vitro. 23 However, the scaffold is not degradable and will remain within
the wound. Thus electrospinning of degradable biopolymers such as gelatin or
collagen is preferred. Electrospun gelatin can be easily spun into a wide range of fiber
diameters ( Fig. 12.5 ) and has been evaluated for its utility in dermal and epidermal
tissue regeneration. Electrospun gelatin scaffolds with interfiber distances between
5 and 10 µm appear to yield the most favorable skin substitute in vitro , demonstrating
high cell viability and optimal cell organization ( Fig. 12.6A). 100 Electrospun
collagen has also been evaluated as a scaffolding material for skin substitutes in
comparison with the standard cultured skin substitute model which utilizes freeze-
dried collagen. CSS made with electrospun collagen exhibit the same excellent
stratification and a continuous layer of basal keratinocytes seen in the freeze-dried
sponge model (Fig. 12.6B and C). 104 When grafted to full thickness wounds in
athymic mice, electrospun collagen skin substitutes had high rates of engraftment
and appeared to inhibit wound contraction compared to that of wounds grafted with
skin substitutes made with freeze-dried collagen sponges. 104 Electrospun biopolymers
and polymer blends show great promise for future models of skin substitutes which
hope to reconstruct the structure of native human skin more faithfully.

12.7.3 Pigmentation by addition of melanocytes

In the current cultured skin substitute model, pigmentation is irregular or absent ( Fig.
12.7 ). In intact skin, pigmentation results from the proper distribution and function
of epidermal melanocytes. Pigmentation protects skin from ultraviolet irradia-
tion 105,106 but also shapes a patients body image and personal identity. Melanocytes
can sometimes unintentionally exist in cultures of epidermal keratinocytes; referred
to as passenger melanocytes, these can result in foci of pigmentation after grafting
(Fig. 12.7). 39,58,107,108 In preclinical studies, selective culture of human melanocytes
and deliberate addition to CSS showed that uniform pigmentation can be achieved 24
although the intensity of pigment was not regulated. Future models of pigmented
skin will benefit from a more thorough understanding of melanocyte function and
factors that regulate skin pigmentation. 109,110