Kuo-Kang Liu

Optical tweezers for single cells

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.

An Indentation Technique to Characterize the Mechanical and Viscoelastic Properties of Human and Porcine Corneas

Cornea is a load-bearing tissue whose mechanical and viscoelastic characteristics are not well understood, due to the challenge associated with most of the measurements. A novel indentation technique has been developed for mechanical characterization of human and porcine corneal tissue, using a tailored depth-sensing microindentation instrument. During indentation, the corneas were suspended by clamping the edges of the cornea, thus allowing depth-sensing measurement free from the complication of the backing substrate. The deformation displacement and the amount of force applied by the indenter were used to obtain hysteresis and stress relaxation data for both human and porcine corneas. Optical coherence tomography was used to measure the thickness of the cornea. Simple theoretical analyses have been undertaken to explain the loading–unloading and the stress relaxation data. The effect of swelling on the mechanical properties of the cornea was also examined. Porcine corneas appeared to be less stiff and to demonstrate more linear response than human corneas under loading. More importantly, it is shown that swelling reduced the strength of the corneas. Our results demonstrate that this new indentation system can be used to characterize the mechanical and viscoelastic properties of corneas.

Characterizing the viscoelastic properties of thin hydrogel-based constructs for tissue engineering applications

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 Youngs 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 real-time fashion. Thus this novel technique can become a valuable tool for soft tissue engineering.

A novel technique for mechanical characterization of thin elastomeric membrane

This paper reports a new method to characterize the mechanical properties, such as the elasticity, of a thin elastomeric film. A sensitive microscope visualization instrument is developed for measuring the deformation of a circular membrane of thickness less than 200 µm, under a constant load. The elastic deformation of the thin membrane was measured laterally. A theoretical model is constructed to quantitatively correlate the elasticity to the deformation profile of the membrane. The good agreement between the experimental and theoretical results facilitates the determination of the elastic modulus of a thin elastomeric membrane.

A novel technique for characterizing elastic properties of thin biological membrane

Abstract We present a new technique to characterize the elastic properties of a thin biological membrane. An effective instrument that embodies video enhanced microscope was developed primarily to provide the capability of simultaneously measuring both the applied force and the resultant displacement of the biological membrane under a central point force. A theoretical linear elastic solution was applied to quantitatively interpret the measured central deflection of the membrane under a central point load. The Young’s modulus of raw and boiled Leghorn egg inner tissue can be easily determined once the applied point load and the central deflection, together with the essential dimensions are known. The viscoelasticity property of natural tissue was manifest in the experiment. The experimental results have verified that the novel technique is applicable to determine the Young’s modulus and other viscoelatic properties of thin biological tissue.

Microfluidic Systems for Biosensing

In the past two decades, Micro Fluidic Systems (MFS) have emerged as a powerful tool for biosensing, particularly in enriching and purifying molecules and cells in biological samples. Compared with conventional sensing techniques, distinctive advantages of using MFS for biomedicine include ultra-high sensitivity, higher throughput, in-situ monitoring and lower cost. This review aims to summarize the recent advancements in two major types of micro fluidic systems, continuous and discrete MFS, as well as their biomedical applications. The state-of-the-art of active and passive mechanisms of fluid manipulation for mixing, separation, purification and concentration will also be elaborated. Future trends of using MFS in detection at molecular or cellular level, especially in stem cell therapy, tissue engineering and regenerative medicine, are also prospected.

Non-destructive mechanical characterisation of UVA/riboflavin crosslinked collagen hydrogels

Aims: To establish a non-destructive method of characterising the mechanical properties of collagen hydrogels to model corneal tissue and to examine the effect of photochemical crosslinking on their mechanical properties. Methods: Collagen hydrogels were manufactured, submerged in 0.1% riboflavin solution and crosslinked using two UVA tube bulbs with an intensity of between 2.8 and 3.2 mW/cm2. The hydrogels were clamped around their outer edge and deformed using a sphere. The deformation was measured in situ using a long-working-distance microscope connected to a CCD camera, and the deformation displacement was used with a theoretical model to calculate the Young modulus of the hydrogels. Collagen hydrogels seeded with human corneal fibroblasts were used to examine cell viability after UVA irradiation. Results: There was an increase in Young modulus of the collagen hydrogels after UVA/riboflavin treatment that was dependent on the exposure time. UVA irradiation without riboflavin showed decreased mechanical integrity and strength. Cell viability was reduced with increased UVA exposure time. Conclusion: The non-destructive technique demonstrated a new methodology comparable with strip extensiometry for cornea or corneal model specimens but with more convenient features. This approach could be used as an initial step in developing new crosslinking treatments for patients with keratoconus.

Contact mechanics of a thin-walled capsule adhered onto a rigid planar substrate.

A thin-walled capsule, modelled as an incompressible liquid droplet contained in a thin flexible membrane, was allowed to adhere onto a rigid substrate. The contact mechanics were formulated, based on linear elasticity, to portray quantitatively the relationships between osmotic inflation, contact area and angle, membrane stretching and adhesion strength. The predicted results shed light on fundamental adhesive contact mechanics in a cell-substrate system.

Deformation behaviour of soft particles: a review

The study of soft particle deformation is of paramount importance for the advancement of fundamental colloidal science as well as its biomedical applications, particularly in drug delivery and cell mechanics/adhesion. Recent developments of both theoretical modelling and experimental techniques have made it possible to measure the deformation behaviour of a single micro-/nano-particle under both adhesive and non-adhesive deformation and, therefore, to facilitate the determination of its mechanical and interfacial properties. This review aims to introduce several modern experimental techniques, such as atomic force microscopy, the micro-compression method and reflectance interference contrast microscopy, and a number of theoretical models, which have been applied to characterize the mechanical and interfacial properties of the soft particles in a quantitative manner. More specifically, their recent applications to biomimetic/biological particles or vesicles, which normally inherit non-linear elasticity and inhomogeneous structure, will also be reviewed.

Biomimetic three-dimensional microenvironment for controlling stem cell fate

Stem cell therapy is an emerging technique which is being translated into treatment of degenerated tissues. However, the success of translation relies on the stem cell lineage commitment in the degenerated regions of interest. This commitment is precisely controlled by the stem cell microenvironment. Engineering a biomimetic three-dimensional microenvironment enables a thorough understanding of the mechanisms of governing stem cell fate. We review the individual microenvironment components, including soluble factors, extracellular matrix, cell–cell interaction and mechanical stimulation. The perspectives in creating the biomimetic microenvironments are discussed with emerging techniques.