We monitored the viability and morphology of mouse fibroblast cells cultured on PDMS substrates with different degrees of polymer stiffness. The stiffness was controlled by varying the ratio between base and crosslinker agent during mixing. Although the standard PDMS mixing ratio is 10 : 1 (base to crosslinker; Young's modulus, E= =580 kPa), we found that a PDMS substrate with a high stiffness (mixing ratio of 5 : 1, E= =1,000 kPa) was more favorable as a substrate for fibroblast cell growth. It is important to note that an extracellular matrix coating was not applied to the PDMS so that the effect of stiffness on cell growth could be studied in isolation. A stiffness reduction of 40% (from a mixing ratio of 5 : 1 to 10 : 1) produced a significant reduction in survival rate (viability was reduced by 15%), and viability worsened (was reduced by 45%) for a substrate stiffness of 280 kPa (a mixing ratio of 20 : 1). The rate of spreading for the cells was measured to show that stiffer materials promoted more prolific fibroblast growth. These results provide PDMS stiffness guidelines for cell culture substrates.

Cells are very sensitive to various microenvironmental cues, including mechanical stress and chemical gradients. Therefore, physiologically relevant models of cells should consider how cells sense and respond to microenvironmental cues. This can be accomplished by using microfluidic systems, in which fluid physics can be realized at a nanoliter scale. Here we describe a simple and versatile method to study the generation of chemical concentration and mechanical shear stress gradients in a single microfluidic chip. Our system uses an osmotic pump that produces very slow (

Urinary hippuric acid (HA), of molecular weight 180 Da, is one of the major metabolites in toluene-exposed humans and is a major biological indicator. Simple and ubiquitous monitoring of exposure to toluene is very important in occupational health care, and a microfluidic chip-based electrochemical immunoassay for rapid and quantitative detection of HA in human urine is proposed in this paper. The system employs a conjugate of ferrocene (Fc) and hippuric acid (HA). The competition between hippuric acid (HA) and the ferrocene-hippuric acid complex (Fc-Lys-HA) to bind with a HA antibody coated onto polybeads generated electrical signals proportional to the HA concentration in the range of 0-40 mg mL(-1). All the complicated HA detection processes were integrated on the single microfluidic platform. The quantitative advantages of our HA detection chip are as follows: (1) the total chip size was reduced to 3.0 x 2.0 x 0.5 cm and is small enough to be portable, (2) the assay time took 1 min, and is shorter than that of conventional electrochemical HA immunoassay systems (about 20 min) and (3) 40 microL of the sample solution was enough to detect HA in the range of 0-40 mg mL(-1), which is enough range to be used for the point-of-care system. In addition, we suggest the improved chip-based HA assay method by the combination of electrochemical and enzymatic amplification processes for the detection of greater electrical signals. The sensitivity of the combined method was increased about three times compared to that of the non-enzymatic process.