T. F. Shklyar

A correlation between mechanical and electrical properties of the synthetic hydrogel chosen as an experimental model of cytoskeleton

The correlation between the electrochemical (Donnan) potential and volume swelling was studied for synthetic polyelectrolyte hydrogels considered as models of cytoskeleton gel-forming biopolymers. Hydrogels involving polyacrylic and polymethacrylic acids with varying network density were synthesized by a radical polymerization in aqueous solution. Electrical charge was introduced into the gel network by partial neutralization of monomer acids with several alkali and alkali earth (hydr)oxides. The electrochemical (Donnan) potential of synthetic gels was determined using conventional microelectrode tools for cell potential determination. It was demonstrated that the negative electrical potential of many anionic gels with various charges and network densities decreased with the decrease of equilibrium swelling, i.e., with the decrease in water content in the gel. It was shown that a drastic phase transition in the gel structure from a swollen to a compressed state induced by K+/Ca2+ exchange is accompanied by an analogous decrease in the absolute Donnan potential of the gels. A kinetic study demonstrated that the gel volume changed ahead of its electrical potential. This suggests that the volume phase transition in gel is the main cause of the electrical response. A similarity between the swelling/compression transition in synthetic gels and the volume changes in the cytoskeleton in the vicinity of the cell membrane was demonstrated. Based on the universal analogy between the properties of synthetic and natural polymer gels, a possible involvement of swelling of the gel-like cytoskeleton structures in electrical regulation in the cell was postulated.

Effect of cytoskeletal elastic properties on the mechanoelectrical transduction in excitable cells.

This paper addresses the possible mechanism of stretch on cell electrochemical potential change, based on the physicochemical properties of cytoskeletal network. Synthetic polyelectrolyte gel was used as an experimental model of the cytoskeleton. Gel samples with different density of network cross linking were studied. Triangular axial deformations of samples were applied. Simultaneously, the electrochemical (Donnan) potential of the gel was measured between a micropipette electrode pinned into the swollen gel, and a reference electrode in the outer solution. We found that axial deformation shifts the gel potential toward depolarization. The extent of gel depolarization showed a close negative correlation with the Young modulus of the gel. We suggest that the underlying mechanism is likely to be a universal process of counterion adsorption on charged polymer filaments due to the decrease of distance between polymer filaments owing to gel elongation.

Mechanoelectric potentials in synthetic hydrogels: Possible relation to cytoskeleton

Mechanical and electrical properties of a synthetic polyelectrolyte hydrogel considered as a model of the cytoskeletal gel were studied. Hydrogels were synthesized from polymethacrylic acid by radical polymerization in aqueous solution. The electrical charge was introduced into the gel network by partial neutralization of monomer acids with magnesium hydroxide. Through the use of a motor, triangular longitudinal (axial) deformations were applied to gel samples. Simultaneously, the electrochemical (Donnan) potential of the gel was measured using conventional microelectrodes. We found that: (1) the Young modulus of the gel is 0.53 kPa; (2) at a given deformation velocity, the extent of gel deformation closely correlates with the gel potential; and (3) at the same level of gel deformation, the lower the deformation velocity, the higher the relative change of gel potential. These findings show a striking similarity to the data obtained in living cells, particularly in cardiac myocytes. A hypothesis involving the deformation-induced solvent migration from the gel to the surrounding solution is considered. It is concluded that the physicochemical features of the cytoskeletal gel may play a role in determining the mechanoelectric properties of excited cells.

Activity of counterions in hydrogels based on poly(acrylic acid) and poly(methacrylic acid): Potentiometric measurements

The behavior of partially ionized weakly crosslinked gels based on poly(acrylic acid) and poly(methacrylic acid) undergoing contraction in the presence of a low-molecular-mass salt is studied experimentally. The concentration dependences of the enthalpies of swelling of gels in water and aqueous solutions with potassium chloride concentrations of 1, 10, and 100 mmol/L are determined via the method of isothermal calorimetry. On the basis of the obtained data, the enthalpy parameter of interaction between a polymer network and a medium is estimated as a function of the amount of the salt. This parameter does not exceed 0.3 and monotonically decreases with an increase in the concentration of the salt. The Donnan potential of gels depending on the concentration of KCl in the external solution is measured via the method of potentiometry with the use of capillary electrodes, and the activity of potassium counterions in the medium of hydrogel is calculated. The main factor that causes contraction of polyelectrolyte gels in a solution of a low-molecular-mass salt is a decline in the activity of counterions that leads to a decrease in the osmotic pressure inside the gel. A decline in activity may be associated with both a reduction in the activity coefficient and ionic association processes in the hydrogel.

Mechanical characteristics of synthetic polyelectrolyte gel as a physical model of the cytoskeleton

A physical model of the cytoskeleton based on synthetic polyelectrolyte hydrogel of polymethacrylic acid has been proposed. From the physicochemical point of view, the structures of polyelectrolyte gel and the cytoskeleton show a high degree of similarity. It has been shown that polyelectrolyte gel can shorten and produce mechanical stress in response to changes in the composition of the surrounding solution. The mechanical properties of the model gel have been evaluated: Young modulus (2–6 kPa), stress relaxation time (0.1–1 s), and apparent viscosity (0.3–3 kPa s). The viscoelastic properties of the gel depend on the degree of its swelling. It has been demonstrated that the mechanical properties of gels of polymethacrylic acid are close to those of biological objects.

Does the right muscular atrioventricular valve in the avian heart perform two functions

The right atrioventricular valve of adult birds is a muscular unicuspid structure and unlike the right atrioventricular valve in the adult mammalian heart. The aim of this study is to test the hypothesis that the avian muscular valve (MV) is a part of the cardiac wall during systole and contributes to the right ventricle pump function. Six adult hens Gallus gallus domesticus were examined with a focus on MV structure and function. The thickness of the right ventricle (RV) wall and MV were examined post-mortem. RV wall and MV end-systolic thickness were estimated echocardiographically. The frame-by-frame processing of RV images was applied for the analysis of MV and RV free wall motion. According to the post-mortem measurements, no significant difference in the thickness between RV free wall and MV (1.8±0.3 and 1.6±0.4 mm, respectively) was found. In the course of the entire cardiac cycle, MV demonstrated the excursion of 10.3±0.9 mm. To the end of RV systole, MV thickness was increased roughly by a factor of two (2.9±0.57 mm), and reached almost the same value (3.0±0.25 mm) in RV free wall. Based on the findings obtained, we concluded that the MV may play specific and non-specific roles in the avian heart. First, MV determines the blood flow separation between the right heart chambers. Second, MV performs contractility to support for RV pump function.

Biomimetic Sensors of the Mechanoelectrical Transduction Based on the Polyelectrolyte Gels

The mechanoelectrical transduction is studied on the synthetic polyelectrolyte gel sensors prepared by the radical polymerization and the cross-linking of calcium methacrylate in aqueous solution. The electrochemical potential of the gel sensors was measured in the course of their stretching deformation in the stepwise and oscillatory regimes. As the network of the gel was negatively charged the potential was negative with the mean value –90 mV. Under the stretching load the negative potential of the sensor diminished proportionally to the deformation both in the stepwise and the oscillatory regimes. Upon the oscillatory triangular linear axial deformation the potential of the sensor closely followed the load/unload dynamics deformation with the small time shift. The sensitivity of the mechanoelectrical transduction was 1 – 2 mV per 1% of deformation depended on the network density and the regime of stretching.

Effect of the Polyelectrolyte Hydrogel Actuator Network Density on Its Mechanical Behavior in the DC Electric Field

This work addresses the mechanics of biocompatible actuators made of polyelectrolyte gels. Polyacrylic acid gels with sodium counterions (20% of ionization) were synthesized by free-radical polymerization in aqueous solution, and N,N’-methylene-diacrylamide was used as a cross-linker. In order to make gels with varying network density, cross-linker to monomer concentration was set at 1:25, 1:50, 1:100, and 1:200. The DC current initiated periodic bending of gel rectangular actuators (10 mm in length and 1x1 mm in crosssection) placed in the bath with 0.8 mM CaCl2. One end of the gel strip was attached to the wall of the bath while the other remained free. When the DC electric field with intensity 1.0 V/mm was applied across the actuator’s longer axis, the free end of gel first bent to cathode, then more substantially to anode, then to cathode again with damping amplitude. With the use of the optical system and the original software for the frame-by-frame actuator’s borders tracing, the angle of gel’s free end displacement (θ) from reference position was determined. We found that the increase of gel network density resulted in the decrease of θ and the decrease of actuator’s free end displacement velocity (θ/t) for all actuator’s oscillations. Close correlations between the extent of gel cross-linking and the θ and θ/t were determined for second and third oscillations of actuators. Thus, obtained results may help for the controlled optimization of actuators’ mechanics for the application in different areas of engineering.

Donnan Potential in Hydrogels of Poly(Methacrylic Acid) and its Potassium Salt

Donnan potentials have been measured in polyelectrolyte hydrogels gels of poly(methacr-ylic acid) and their potassium salts in water, using Ag/AgCl microelectrodes at 298 K. The Donnan potential varied from -80 to -40 mV as a function of gels’ cross-link density and the fraction of potassuim methacrylate monomer units. Negative values of the potential increase with the decrease in cross-link density of the gel. Gels with an increasing fraction of potassium methacrylate yield less negative values. The results are discussed from the viewpoint of the Donnan theory initially developed for membrane potential. The theory is qualitatively consistent with observed dependencies of the potential. However several quantitative differences are present, whose sources are analyzed

Regularities of Ultrasonography of Suspensions of Alumina Nanoparticles in Biological Media

This paper studies the echo-contrast properties of an alumina nanopowder suspension using ultrasonography (US) fully corresponding in its characteristics to the techniques of medical ultrasound diagnostics of organs and tissues. The purpose of this study was to search for the possible effect of the ionic and protein composition of the biological medium on the intensity of the reflected echo signal of the contrast material based on nanoparticles. It was found that the pH of the blood promotes the maximum use of echo contrast options of alumina nanopowder suspensions. Particle size measurements in the suspension using the dynamic light scattering technique showed the stabilizing effect of blood serum and plasma on the nanopowder suspension, resulting in the attenuation of the echo signal. The data offer a basis for the development of new contrast materials based on nanoparticles for the ultrasound imaging of the heart and blood vessels. The considered mechanisms of the established phenomena make it possible to elucidate the processes of interaction of metal oxide nanoparticles with biological molecules.