Biomedical Engineering Reference
In-Depth Information
Engineered ECMs
Natural ECM provides cells with specific biochemical and topographical cues [23-25]. Though
the use of stem cells alone when applied directly to wound beds has shown increased healing
time, the use of an ECM substitute or scaffold has been shown to enhance tissue regeneration.
Application of a scaffold provides cells an initial attachment site, as well as biological and
topographical cues for differentiation [26]. Requirements for tissue-engineering scaffolds
include adequate porosity for cellular migration, infiltration, and waste removal, as well as a
high surface area for cell adhesion, growth, and differentiation [25, 27]. In addition, scaffolds
can serve a dual purpose as a wound dressing for the prevention of infections and loss of fluid
and proteins [25]. Lastly, scaffolds must have a degradation rate that mimics the rate of native
tissue regeneration [27] and improve the esthetic appearance of the wound site [25].
Nanotopography
Nanotechnology remains a pervasive tool in regenerative engineering due to the nanoscale
level of biological systems. The use of nanotechnology to create biomimetic scaffolds with
nanotopographic features such as ridges, groves, nodules, and fibers can lead to changes
to cell behavior. Changes to cell attachment, spreading, contact guidance, cytoskeletal
architecture, nuclear shape, nuclear orientation, programmed cell death, macrophage
activation, transcript levels, and protein abundance [28] have been reported. The resem-
blance of nanofibers to collagen fibers found in the dermal layer has popularized the use
of nanofibrous scaffolds for skin tissue engineering applications.
In general, nanofibers provide a connection between the nano- and macroscopic objects
[29]. Due to their high surface to mass ratio, the fibers possess properties such as: low
density, high pore volume, variable pore size, and exceptional mechanical properties. Such
properties provide more surface area for cell attachment, and can protect wound areas from
the loss of fluid and proteins, making nanofiber scaffolds well suited for skin regeneration
[25]. Though several methods exists for the fabrication of nanofibers, which include drawing,
freeze drying, and self-assembly, electrospinning remains the most widely used fabrication
technique due to its ease of fabrication and reproducibility [30].
Electrospinning
The basic equipment needed for electrospinning is quite minimal: a high voltage power
supply, polymer solution, and a collection plate (Figure 18.2). The electrospinning process
begins with a polymer melt solution at a concentration high enough for polymer chain
entanglement to occur. As solution viscosity increases with polymer concentration, the
resulting fiber diameter increases linearly [31]. The polymer solution is fed through a needle
or some type of capillary tool at constant flow. During the electrospinning process, a high
electric potential is applied to the polymer solution. When the electrostatic forces in the
polymer overcome the surface tension forces, a jet is ejected and is attracted to a grounded
or oppositely charged collector. The polymer jet undergoes a series of bending and stretch-
ing instabilities that causes large amounts of plastic stretching before hitting the target or
collector, resulting in ultrathin fibers [26]. Electrospinning can fabricate fibers in the dia-
meter range of 3 nm to 6 μm.
