Microfluidics for the study of mechanotransduction

Abstract

Mechanical forces regulate a diverse set of biological processes at cellular, tissue, and organismal length scales. Investigating the cellular and molecular mechanisms that underlie the conversion of mechanical forces to biological responses is challenged by limitations of traditional animal models and in vitro cell culture, including poor control over applied force and highly artificial cell culture environments. Recent advances in fabrication methods and material processing have enabled the development of microfluidic platforms that provide precise control over the mechanical microenvironment of cultured cells. These devices and systems have proven to be powerful for uncovering and defining mechanisms of mechanotransduction. In this review, we first give an overview of the main mechanotransduction pathways that function at sites of cell adhesion, many of which have been investigated with microfluidics. We then discuss how distinct microfluidic fabrication methods can be harnessed to gain biological insight, with description of both monolithic and replica molding approaches. Finally, we present examples of how microfluidics can be used to apply both solid forces (substrate mechanics, strain, and compression) and fluid forces (luminal, interstitial) to cells. Throughout the review, we emphasize the advantages and disadvantages of different fabrication methods and applications of force in order to provide perspective to investigators looking to apply forces to cells in their own research.

Figure 1.

The mechanical cellular microenvironment. (a) Forces that modulate biological processes via mechanotransduction are indicated with bold text and key mechanical elements of the cell are labeled. Red arrows indicate solid forces while blue arrows indicate fluid forces and flows that activate mechanotransduction pathways. (b) The adherens junction complex modulates mechanotransduction at cell-cell junctions and shares constuients of the adaptor protein plaque with focal adhesions. (c) The focal adhesion complex transmits mechanical signals across the cell membrane and is a key site of mechanotransduction.

Figure 2.

Fabrication methods for microfluidic devices. (a) Photolithography involves using a transparency mask to pattern UV light and resultant crosslinking of photoresist, which is a photosensitive polymer (sample image of a silicon master fabricated with photolithography modified from [45]). (b) Soft lithography involves casting an elastomer on a master mold to create a negative copies of the master for use as a final microfluidic device (example of PDMS device modified from [168]). (c) Embossing is an alternative replica molding approach in which thermoplastic materials are heated and pressed against a master mold to form negative copies (example of thermoplastic device modified from [169]). (d) Laser cutting, milling, and cutting are methods for removing material from bulk plastic to directly fabricate channels or to create a master mold (example of micromilled channel modified from [170]). (e) Lamination involves bonding individual layers with heat and pressure to create multilayer devices (example of multichannel device modified from [68]). (f) 3D printing involves fabricating devices or molds through additive processes. Fused deposition modeling (FDM) involves directly patterning material ejected from a nozzle, and stereolithography (SLA) uses a laser or focused light source and movable stage to crosslink resin and build structures with 3D resolution (sample multicomponent device modified from [171]).

Figure 3.

Advances in fabrication methods will allow the integration of many mechanical stimuli within a single device to more accurately recapitulate the native mechanical microenvironment.

Figure 4.

Microfluidic devices for applying solid and fluid forces to cells. (a) Patterning fluidic channels with dimensions similar to or smaller than a single cell enables the investigation of how ECM architecture influences cell migration. For example, patterning vertical pillars with gap sizes smaller than the nucleus enabled the observation that tumor cell migration through confined environments can lead to nuclear rupture and DNA damage (modified from [92]. (b) Introducing biomaterials with controlled mechanical properties into microfluidic platforms allows investigation into the interplay of matrix architecture and mechanics. Using hydrogels with controlled degradability in a microfluidic device with elucidated the role of matrix degradation in angiogenic sprouting (modified from [70]). (c) Platforms for applying strain to cell culture substrates have been developed using soft lithography to pattern compliant devices. In a PDMS-based microfluidic model of the lung, endothelial cells were found to align orthogonal to the direction of cyclic strain [100]. (d) By incorporating flexible substrates and applying pneumatic pressure in microfluidic platforms, the effects of compressive forces on cells can be investigated. For example, a multichannel microfabricated device was used to generate pressures up to 15 psi to investigate the effects of compressive forces on the cytoskeleton and nucleus (modified from [172]). (e) Fluid flow through microfabricated channels imparts fluid shear stress on cells cultured on the channel walls. Such platforms have been used to investigate the effects of fluid shear stress on endothelial cytoskeletal dynamics and barrier function (modified from [17,45]). (f) Using microfluidics to apply pressure gradients across hydrogels enables investigation of how fluid forces from interstitial flow impacts cells cultured within the hydrogel. By applying interstitial flow to endothelial cells in a microfluidic device, it was found that flow and VEGF signaling can drive angiogenic sprouting (modified from [162]).