Micropipette aspiration

Houchmandzadeh, B., Marko, J.F., Chatenay, D., and Libchaber, A. (1997) Elasticity and structure of eukaryote chromosomes studied by micromanipulation and micropipette aspiration. J. Cell. Biol. 139, 1-12. 

Micropipette aspiration has been used widely to measure the mechanical properties of many types of cells including red blood cells (RBCs), leukocytes and chondrocytes (Lim et al., 2006). One important... 

Figure 1 Diagram of the micropipette aspiration technique (modified from Lim et al., 2006). L is the length of extension into the pipette, Rc the inner radius of the pipette and AP the suction pressure.

Chondrocytes respond biologically to mechanical stimuli, and this is called mechanotransduction. Understanding cellular responses to mechanical stimuli and how they can be related to tissue level characteristics has many applications in biology, tissue engineering and medical science. Information about the mechanical properties of chondrocytes is fundamental to such research. Micropipette aspiration has been applied to investigate the deformation of single chondrocytes (Jones et al., 1999). It was found that human chondrocytes behave like viscoelastic solids. The Young s modulus of normal chondrocytes appeared to be very variable, but was of the order of 0.65 + 0.63 kPa. However, the viscoelastic behaviour was not quantified. 

Passive deformability. Two methods were devised to measure cell properties that are related to their deformability. The micropipette aspiration method (Sato and Suzuki, 1976) consists in measuring the negative pressure required for a cell or a portion thereof to enter a pipet the opening of which is smaller than the diameter of the cell. This method is laborious and requires a skilled operator. Cells have to be measured one at a time, and since cell and capillary diameters must have a constant ratio, the sampling of a population is time consuming and requires a set of different pipettes. As a consequence, this method is not widely used, and will not be further described. 

The critical transmembrane potential for the latter was found to be in the range 8-lOV [111]. This value is significantly higher than the critical potenhal of IV reported for fluid membranes [59, 98, 99]. Thus, membranes in the gel phase can sustain higher tensile stresses (see Eqs. (7.3) and (7.4)). This is also confirmed by micropipette aspiration experiments showing that fluid-phase dimyristoylphos-phahdylcholine membranes undergo lysis at tensions around but... 

The membrane elastic behavior of PEO-PEE giant polymersomes has been studied by a micropipette aspiration method [5], The results showed that the polymer membrane elasticity is comparable to fluid-state lipid membranes however the vesicles could sustain a greater strain before rupture, proving an enhanced polymersome toughness, which originates from membrane thickness. 

Recently, Nam and Santore examined adhesive plaque formation dynamics of unilamellar PEG-PDMS copolymer membranes, driven by avidin-biotin binding [150], With the help of dual micropipette aspiration the authors controlled the... 

Subsequent to these original formulations, a number of refinements to these relationships have been proposed. Observations of persistent deformations after micropipette aspiration for extended periods of time formed the basis for the development of a model for long-term stress relaxation [Markle et al., 1983]. The characteristic times for these relaxations were on the order of 1 to 2 h, and they were thought to correlate with permanent rearrangements of the membrane elastic network. 

Values for the coefficients determined from fluorescence measurements of skeletal density distributions during micropipette aspiration studies are /xn 0.01 mN/m and Kn 0.02 mN/m. The value for n is estimated to be 2 [Discher et al, 1994]. 

There are several major techniques that are used to extract the mechanical properties of ceUs. The models and experiments are interconnected the experiments provide parameters for the models and, in turn, the models are the basis for the interpretation of the experiments. One common technique is micropipette aspiration, where a pipette is sealed on the surface of a cell, negative pressure is appUed inside the pipette, and a portion of the ceU is aspirated into the pipette. The height of the aspirated portion is considered as an inverse measure of the ceU stiffness. The same technique is used to observe the time response of the ceU to the appUcation of pressure, and in this case, the corresponding relaxation time is a measure of the cell s viscoelastic properties. The experiment with the micropipette aspiration of a red blood ceU was interpreted by considering the ceU membrane (including the cytoskeleton) as a nonlinear elastic half-space... 

FIGURE 62.1 Simulation of the micropipette aspiration of the red blood cell cytoskeletal network for different ratios, (a) and (b), of the aspiration length I to the radius of the pipette Rp , (c) shows the profiles of the network element density p along the z-axis of the pipette. (From Discher, D.E., Boal, D.H., and Boey, S.K. Biophys. /., 75,1584, 1998. 

Hochmuth, R.M. Micropipette aspiration of living cells,/. Biomech., 33,15,2000. 

Micropipette Aspiration Centrifugation Laminar Flow Chambers Rotating Disc Interpretation of... 

To predict the shape of an adherent tissue cell and quantify the stress distribution inside it, the fibrous actin cytoskeleton or the ECM can be modeled as a two-dimensional network of elastic cables. Previously, elastic cable network provided remarkable quantitative predictions of erythrocyte elastic properties and micropipette aspiration experiments. The cable networks have the additional feature that filaments buckle under compressive load. This model has already been tested successfully to model cell poking, magnetic twisting cytometry, magnetic bead microrheometry experiments. Although the cable network is far from representing the complexity of the actin network mechanics, it incorporates some of its essential features. This model is extended to include the effect of spatial distribution of adhesion points along the periphery of the cell. 

There are a number of force measuring techniques from which the interactions between particles can be measured Surface Force Apparatus (SFA), Atomic Force Microscopy (AFM), Total Internal Reflection Microscopy (TIRM), Optical Tweezers (OT), Micropipette Aspiration (MPA). The SFA and AFM are currently the most versatile in that the surface forces and separations can be accurately controlled and measured over a large range. 

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