We present a novel technology for microfluidic elastometry and demonstrate its ability to measure stiffness of blood clots as they form. A disposable micro-capillary strip draws small volumes (20 μL) of whole blood into a chamber containing a surface-mounted micropost array. The posts are magnetically actuated, thereby applying a shear stress to the blood clot. The posts’ response to magnetic field changes as the blood clot forms; this response is measured by optical transmission. We show that a quasi-static model correctly predicts the torque applied to the microposts. We experimentally validate the ability of the system to measure clot stiffness by correlating our system with a commercial thromboelastograph. We conclude that actuated surface-attached post (ASAP) technology addresses a clinical need for point-of-care and small-volume elastic haemostatic assays.

A clot's function is to achieve hemostasis by resisting fluid flow. Permeability is the measurement of a clot's hemostatic potential. It is sensitive to a wide range of biochemical parameters and pathologies. In this work, we consider the hydrodynamic phenomenon that reduces the mobility of fluid near the fiber surfaces. This no-slip boundary condition both defines the gel's permeability and suppresses nanoparticle diffusion in gel interstices. Here we report that, unlike previous work where steric effects also hindered diffusion, our system-nanoparticles in fibrin gel-was subject exclusively to hydrodynamic diffusion suppression. This result enabled an automated, high-throughput permeability assay that used small clot volumes. Permeability was derived from nanoparticle diffusion using the effective medium theory, and showed one-to-one correlation with measured permeability. This technique measured permeability without quantifying gel structure, and may therefore prove useful for characterizing similar materials (e.g., extracellular matrix) where structure is uncontrolled during polymerization and difficult to measure subsequently. We also report that PEGylation reduced, but did not eliminate, the population of immobile particles. We studied the forces required to pull stuck PEG particles free to confirm that the attachment is a result of neither covalent nor strong electrostatic binding, and discuss the relevance of this force scale to particle transport through physiological clots.

In the past decade, high throughput screening ͑HTS͒ has changed the way biochemical assays are performed, but manipulation and mechanical measurement of micro-and nanoscale systems have not benefited from this trend. Techniques using microbeads ͑particles ϳ0.1-10 m͒ show promise for enabling high throughput mechanical measurements of microscopic systems. We demonstrate instrumentation to magnetically drive microbeads in a biocompatible, multiwell magnetic force system. It is based on commercial HTS standards and is scalable to 96 wells. Cells can be cultured in this magnetic high throughput system ͑MHTS͒. The MHTS can apply independently controlled forces to 16 specimen wells. Force calibrations demonstrate forces in excess of 1 nN, predicted force saturation as a function of pole material, and powerlaw dependence of F ϳ r −2.7Ϯ0.1. We employ this system to measure the stiffness of SR2+ Drosophila cells. MHTS technology is a key step toward a high throughput screening system for micro-and nanoscale biophysical experiments.

Retinyl ester concentration is regulated by retinoic acid (RA) through an autoregulatory loop, which acts on lecithin:retinol acyltransferase (LRAT). We tested whether retinol esterification activity is downregulated in human mammary carcinoma cells and whether LRAT expression is RARregulated. Normal human mammary epithelial (HMEC) cells expressed a retinoid-upregulated 5-kb LRAT transcript and synthesized retinyl esters from 3 H-retinol. Human carcinoma MCF-7 cells failed to express the 5-kb LRAT transcript and to synthesize retinyl esters. Instead, they expressed a 2.7-kb LRAT transcript. Both transcripts were upregulated by RA. Stable expression of the dominant-negative RAR␣403 blunted the up-regulation of LRAT mRNA by RA. We conclude that retinol esterification is decreased in MCF-7 vs normal mammary cells; that these cancer cells express a shorter (2.7 kb) LRAT transcript, and that retinoid receptors are involved in the regulation of LRAT-mediated retinyl ester synthesis by RA.