Optical tweezers (OT) have emerged as an essential tool for manipulating single biological cells and performing sophisticated biophysical/biomechanical characterizations. Distinct advantages of using tweezers for these characterizations include non-contact force for cell manipulation, force resolution as accurate as 100 aN and amiability to liquid medium environments. Their wide range of applications, such as transporting foreign materials into single cells, delivering cells to specific locations and sorting cells in microfluidic systems, are reviewed in this article. Recent developments of OT for nanomechanical characterization of various biological cells are discussed in terms of both their theoretical and experimental advancements. The future trends of employing OT in single cells, especially in stem cell delivery, tissue engineering and regenerative medicine, are prospected. More importantly, current limitations and future challenges of OT for these new paradigms are also highlighted in this review.

We present a novel indentation method for characterizing the viscoelastic properties of alginate and agarose hydrogel based constructs, which are often used as a model system of soft biological tissues. A sensitive long working distance microscope was used for measuring the time-dependent deformation of the thin circular hydrogel membranes under a constant load. The deformation of the constructs was measured laterally. The elastic modulus as a function of time can be determined by a large deformation theory based on Mooney-Rivlin elasticity. A viscoelastic theory, Zener model, was applied to correlate the time-dependent deformation of the constructs with various gel concentrations, and the creep parameters can therefore be quantitatively estimated. The value of Young's modulus was shown to increase in proportion with gel concentration. This finding is consistent with other publications. Our results also showed the great capability of using the technique to measure gels with incorporated corneal stromal cells. This study demonstrates a novel and convenient technique to measure mechanical properties of hydrogel in a non-destructive, online and realtime fashion. Thus this novel technique can become a valuable tool for soft tissue engineering.

Inflation of membrane tubes and the associated problem of bifurcation and instability is a classical subject that has been studied in many books and papers. This problem shares the same features as a family of other problems such as propagating buckles in long metal tubes under external pressure, propagating necks in some polymeric materials when pulled in tension, phase transition, and kink band formation and propagation in layered structures and fibre-reinforced composites. Whereas the determination of the propagation pressure through the equal-area rule is now universally accepted, there is still some uncertainty concerning the determination of the pressure (the initiation pressure) at which bulging first occurs. Kyriakides and Chang (1990) took this pressure to be the limiting pressure corresponding to the first turning point in the pressure-volume diagram for uniform inflation, but this value was found to be slightly higher than what had been observed in the experiments. Existing instability and bifurcation results were discussed by Chang (1990, 1991) but were found not to be relevant to the determination of the initiation pressure. This is probably because neither the bifurcation nor the stability analysis that has been conducted so far is able to describe correctly the bifurcation mode having zero wave number and it is in fact this mode that is relevant to the determination of the initiation pressure. This special mode is not sinusoidal -its correct variation can only be found from a near-critical nonlinear analysis.In my talk, I shall present a simple procedure that can be used to derive the bifurcation condition and to determine the near-critical behaviour analytically. It is shown that the axial variation of near-critical bifurcated configurations associated with the bifurcation at zero mode number is s governed by a first-order differential equation that admits a locally bulging or necking solution. This result suggests that the corresponding bifurcation pressure can be identified with the initiation pressure. This is supported by good agreement with one set of experimental results. It is also shown that the Gent material model can support both bulging and necking solutions whereas the Varga and Ogden material models can only support bulging solutions. Relevance of the present method to the study of nonlinear wave propagation in a fluid-filled distensible tube is also discussed.

This paper reports an experimental and theoretical study of the compressive behavior of single microcapsules; that is, liquid-filled cellular entities ͑approximately 65 m in diameter͒ with a thin polymeric membrane wall. An experimental technique which allows the simultaneous measurement of both the compressive displacement and the reaction forces of individual microcapsules deformed between two parallel plates up to a dimensionless approach ͓͑compressive displacement͒/͑initial particle diameter͔͒ of 60% is described. The corresponding major geometric parameters of the deformed microcapsule, such as central lateral extension as well as the failure phenomena, are reported and recorded through a microscopic visualization system. The elastic modulus, the bursting strength of the membrane, and the pressure difference across the membrane are computed by using a theoretical analysis which is also presented in this paper. This theoretical model, which was developed by Feng and Yang ͓J. Appl. Mech. 40, 209 ͑1973͔͒ and then modified by Lardner and Pujara ͓in Mechanics Today, edited by S. Nemat-Nasser ͑Pergamon, New York, 1980͒, Vol. 5͔, considers the deformation of a nonlinear elastic spherical membrane which is filled with an incompressible fluid. The predictions of the theory are consistent with the experimental observations. ͓S1063-651X͑96͒05011-8͔

Aims/hypothesis A key pathology in diabetic nephropathy is tubulointerstitial fibrosis. The condition is characterised by increased deposition of the extracellular matrix, fibrotic scar formation and declining renal function, with the prosclerotic cytokine TGF-β1 mediating many of these catastrophic changes. Here we investigated whether TGF-β1-induced epithelial-to-mesenchymal transition (EMT) plays a role in alterations in cell adhesion, cell coupling and cell communication in the human renal proximal tubule. Methods Whole-cell and cell compartment abundance of E-cadherin, N-cadherin, snail, vimentin, β-catenin and connexin-43 was determined in human kidney cell line (HK)2 and human proximal tubule cells with or without TGF-β1, using western blotting and immunocytochemistry, followed by quantification by densitometry. The contribution of connexin-43 in proximal tubule cell communication was quantified using small interfering RNA knockdown, while dye-transfer was used to assess gap junctional intercellular communication (GJIC). Functional tethering was assessed by single-cell force spectroscopy with or without TGF-β1, or by immunoneutralisation of cadherin ligation. Results High glucose (25 mmol/l) increased the secretion of TGF-β1 from HK2 cells. Analysis confirmed early TGF-β1-induced morphological and phenotypical changes of EMT, with altered levels of adhesion and adherens junction proteins. These changes correlated with impaired cell adhesion and decreased tethering between coupled cells. Impaired E-cadherin-mediated adhesion reduced connexin-43 production and GJIC, these effects being mimicked by neutralisation of Ecadherin ligation. Upregulation of N-cadherin failed to restore adhesion or connexin-43-mediated GJIC. Conclusions/interpretation We provide compelling evidence that TGF-β1-induced EMT instigates a loss of Ecadherin, cell adhesion and ultimately of connexinmediated cell communication in the proximal tubule under diabetic conditions; these changes occur ahead of overt signs of renal damage.