Microfluidic Devices for Angiogenesis - Mechanical and Chemical Signaling in Angiogenesis - page 100

Herpes simplex virus type-1) techniques to deliver pro-angiogenic factors, tissue-
engineered products, hyperbaric oxygen and negative pressure wound therapy are
current avenues for stimulating angiogenesis [ 1 , 57 ]. Recombinant growth factors
including VEGF, bFGF and PDGF-BB have been used in animal models of
chronic limb ischemia [ 58 , 59 ]. More recently, using plasmid-based gene delivery
systems, local intramuscular administration of FGF-1 was shown to be safe and
improved amputation-free survival in patients with critical limb ischemia [ 60 ].
Hyperbaric oxygen therapy (HBOT), the intermittent exposure of patients to
100 % oxygen at pressures above 1 atmosphere, has been known to promote
angiogenesis and collagen synthesis but the underlying mechanisms remain
unclear [ 61 ]. Another strategy, negative pressure therapy has been used exten-
sively in clinical management of wounds [ 62 ]. The underlying mechanism by
which topical negative pressure stimulates angiogenesis has been investigated
using in vitro methods and linked to increased endothelial cell migration and
proliferation [ 63 , 64 ].
Angiogenesis is clearly a complex process requiring the coordination of mul-
tiple cell types and integration of a host of chemical and mechanical microenvi-
ronmental signals. Despite these enormous challenges small successes provide a
map for future directions and motivation for further pursuits in the application of
angiogenesis principles for tissue repair and regeneration.
3 Endothelial Cell Culture in Microfluidics
3.1 Pre-formed Microvascular Systems
Microfluidic cell culture began approximately 30 years ago with studies in which
the cells were plated on channels on a 2D rigid surface [ 65 ]. This was a natural
extension of studies in traditional culture dishes, adding some advantages in terms of
improved imaging capabilities and a more natural means of introducing fluid shear
stress, a factor that had been recognized to play a significant role in numerous
endothelial cell processes since the early experiments by Gimbrone, Davies and
Dewey [ 66 , 67 ] beginning in the 1980s. These early flow devices were not ''micro''
in scale, however, and were typically machined out of hard plastic, limiting their use
and their ability to mimic the actual in vivo microenvironment. In particular, the
plastic surfaces were orders of magnitude higher in stiffness than the basement
membrane these cells adhere to in blood vessels, and were incapable of being
stretched by 5-10 % as would normally occur in vivo as a consequence of normal
blood pressure pulsatility. Despite these limitations, these studies greatly advanced
our understanding of endothelial biology, for example, leading to a progressively
deeper appreciation of the importance of fluid shear stress and of the wide range of
biological processes influenced by it (see e.g., reviews by Davies [ 68 ]).