Microfluidic Devices for Quantifying the Role of Soluble Gradients in Early Angiogenesis - Mechanical and Chemical Signaling in Angiogenesis - page 48

Table 1 Comparative advantages of published microfluidic devices for studying the role of
soluble gradients in angiogenesis

No-flow Cell-chamber-flow Restricted-flow

Steady-state gradient No Yes Yes

Commercially available Yes Yes By custom order

Minimal shear in culture chamber Yes No Yes

Quantitative specification of a gradient No Yes Yes

Fully substrate-independent No No Yes

Compatible with 3D matrices Yes No Yes

Representative examples Refs. [ 16 - 20 ] Refs. [ 23 , 24 ] Refs. [ 27 - 30 ]

transient gradient, though, is still insufficient for studying relevant processes in
angiogenesis. The initial concentration in the source chamber also determines both
the concentration magnitude and the gradient steepness as detected by cells at a
given time during the evolution of the concentration profile. Growth factor decay,
moreover, is a problem for static devices, i.e., those without input of fresh reagent;
for example, vascular endothelial growth factor's (VEGF) active half-life is 50 min
under standard culture conditions [ 21 ]. Due to these limitations, novel microfluidic
devices are needed to create controlled gradients of active factors that operate at
steady-state and, therefore, are suited to long-term culture.
Limitations of traditional assays have led to the development of several
microfluidic devices that use continuous flow to generate stable gradients. By
employing steady-state flows to maintain a gradient, these devices enable long-
term culture and the study of emergent properties of angiogenic sprouts. These
microfluidic devices can be separated into two broad categories based on whether
the fluid stream flows directly over the cells, i.e., ''cell-chamber-flow devices'', or
through channels adjacent to the cells, i.e., ''restricted-flow devices'', (Table 1 ).
As a consequence of direct contact with fluid flow, cells in cell-chamber-flow
devices experience significant shear stresses. For example, early microfluidic
devices, such as the 'Y' chamber [ 22 ], create a gradient at steady-state by merging
a source and sink flow over the cell culture chamber. Other devices, which use a
serpentine network to merge flows [ 23 , 24 ], have the additional advantage of being
able to create arbitrary concentration profiles (Fig. 3 b). These devices mimic key
aspects of in vivo vasculature, where blood flow and interstitial flow are important
anisotropic signals for sprouting angiogenesis [ 25 ]. However, in these devices, the
direction of shear with respect to the gradient cannot be varied: they are always
perpendicular. This coupling of shear and gradient limits these devices as plat-
forms for developing a systematic understanding of the role of soluble gradients.
Furthermore, studies have demonstrated that cell migration can be significantly
altered by the presence of shear alone [ 26 ]. Reliance on flow through the chamber
also precludes the use of 3D matrices as culture scaffolds, which greatly restricts
their use in the evaluation of biomaterials and cell-extracellular matrix (ECM)
interactions.