Microfabricated polymeric array detectors (mPADs) for measuring cell forces (D.A. Hammer, C. Chen)

Purpose: A variety of techniques are available to measure the forces that cells exert as they are adhering or crawling. These techniques are known as traction force microscopies, and they include bead based traction microscopy (where beads are embedded in hydrogels) and microfabricated polymeric array detectors (or mPADs). The Chen laboratory has been a pioneer in the development of mPADs. The post arrays can be made in a variety of different architectures (stiffnesses, lengths, pitch spacing, and sizes). These materials can measure forces during cell spreading and migration, and provide some of the most sensitive cell force measurement techniques available.

Fabrication of elastomeric post arrays. a) Schematic of the microfabrication process used to generate PDMS post arrays. b-d) SEM images of PDMS post arrays with target post diameters of 1.5, 1 and 0.75 m, and center-to-center spacings of 2.5, 2 and 1.5 m, respectively. Scale bars are 3 m. Figure taken from  (Yang, 2007).

Fabrication of elastomeric post arrays. a) Schematic of the microfabrication process used to generate PDMS post arrays. b-d) SEM images of PDMS post arrays with target post diameters of 1.5, 1 and 0.75 m, and center-to-center spacings of 2.5, 2 and 1.5 m, respectively. Scale bars are 3 m. Figure taken from (Yang, 2007).

Method of Fabrication: mPADs are micromolded from masters made up of arrays of posts with different diameters and a center-to-center spacing’s. A schematic of the method of fabrication is shown on the right (Yang, 2007). The fabrication of higher density arrays of smaller posts is made possible by incorporating a contrast enhancement agent into SU-8 photolithography. Achieving even higher density arrays with SU-8-based micromolding methods is difficult due to diffraction effects and other process factors in patterning tall SU-8 structures. Furthermore, to achieve even higher densities of mPADs, conventional photolithography and deep reactive ion etching have been used to fabricate arrays of holes in silicon, from which posts with diameters as small as 1 mm and center-to-center distances of 3 mm.

Use/Results: The Chen laboratory has used the mPADs extensively to measure forces during cell adhesion and spreading. Yang and coworkers determined how spacing of the post arrays affects the measurement of energy exerted during adhesion, and showed that below a certain spacing, the measured energy is invariant. Hammer and Chen, with collaborators at Penn, have been using array detectors to measure the forces of cell adhesion and spreading of amoeboid cells, recently measuring the forces exerted by dendritic cells in gradients of chemokine on mPADs stamped with fibronectin (Jannat, 2011). It was shown that dendritic cells exert forces from the front (through the lamellipod) and pull themselves forward. mPADs were necessary for these experiments because they have better sensitivity than bead based TFM; forces down to half a nanoNewton were measured (Jannat, 2011).  The Chen laboratory recently moved to Boston University, but Chris Chen will be actively involved in the Center and there remains significant expertise in the fabrication and use of mPADs at Penn (e.g., in the Hammer Group).

References

Yang MT, Sniadecki NJ, Chen CS. Geometric considerations of micro- to nanoscale elastomeric post arrays to study cellular traction forces. Advanced Materials Oct 19 2007; 19 (20): 3119-+.

Jannat RA, Dembo M, Hammer DA. Traction forces of neutrophils migrating on compliant substrates. Biophys J Aug 3 2011; 101 (3): 575-584.

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