Viscoelastic Properties of Low-Viscosity Liquids Studied with Thickness-Shear Mode Resonators

Mesoscopic Monitoring of Human Skin Explants Viscoelastic Properties

The investigation of the mechanical properties of skin is of great interest for monitoring physiological and pathological changes in the cutaneous barrier function for dermatological and cosmetic issues. Skin constitutes a complex tissue because of its multi-layered organisation. From a rheological point of view, it can be considered to be a soft tissue with viscoelastic properties. In order to characterise ex vivo mechanical properties of skin on the mesoscopic scale, a biosensor including a thickness shear mode transducer (TSM) in contact with a skin explant was used. A specific experimental set-up was developed to monitor continuously and in real-time human skin explants, including the dermis and the epidermis. These were kept alive for up to 8 days. Skin viscoelastic evolutions can be quantified with a multi-frequency impedance measurement (from 5 MHz to 45 MHz) combined with a dedicated fractional calculus model. Two relevant parameters for the non-destructive mesoscopic characterisation of skin explants were extracted: the structural parameter αapp and the apparent viscosity ηapp. In this study, the validity of the biosensor, including repeatability and viability, was controlled. A typical signature of the viscoelastic evolutions of the different cutaneous layers was identified. Finally, monitoring was carried out on stripped explants mimicking a weakened barrier function.

Capillary-size flow of human blood plasma: Revealing hidden elasticity and scale dependence

The dynamical mechanical analysis of blood generally uses models inspired by conventional flows, assuming scale-independent homogeneous flows and without considering fluid-surface boundary interactions. The present experimental study highlights the relevance of using an approach in line with physiological reality providing a strong interaction between the fluid and the boundary interface. New dynamic properties of human blood plasma are found: a finite shear elastic response (solid-like property) is identified in nearly static conditions, which also depends on the scale (being reinforced at small scales). The elastic behavior is confirmed by the induction, without heat transfer, of local hot and cold thermodynamic states evidencing a thermo-mechanical coupling in blood plasma so far known only in elastic materials. This finding opens new routes for medical diagnosis and device fabrication.

Establishing a link between amino acid sequences and self-associating and viscoelastic behavior of two closely related monoclonal antibodies

PURPOSE

To investigate the underlying cause for the observed differences in self-associating and viscoelastic behavior between two monoclonal antibodies, MAb1, and MAb2.

METHODS

Several mutants were designed by swapping charged residues in MAb1 with those present in MAb2 at their respective positions and vice versa. Rheological analysis was done at low and high shear rates. Dynamic light scattering quantified intermolecular interactions in dilute solutions; sedimentation equilibrium analysis determined the corrected weight average molecular weight (M (wc)) to assess the self-associating behavior in high concentration. The molecular charge was estimated from electrophoretic mobility measurements.

RESULTS

Replacing the charged residues in the CDR of MAb1 resulted in a lower M (wc) and solution viscosity. The corresponding changes in either just the variable light (VL) or variable heavy (VH) chain showed only a partial decrease in viscosity, whereas changes in both VL and VH chains resulted in a dramatic reduction in viscosity. The converse case where the VL and VH chains of MAb2 were made to look like MAb1 did not self-associate or show increased viscosity.

CONCLUSIONS

Exposed charged residues in the CDR of MAb1 are critical in determining the self-associating and highly viscous behavior observed at high concentrations.

Factors affecting the viscosity in high concentration solutions of different monoclonal antibodies

The viscosity profiles of four different IgG(1) molecules were studied as a function of concentration at pH 6.0. At high concentrations, MAb-H and -A showed significantly higher viscosities as compared to MAb-G and -E. Zeta Potential (ξ) measurements showed that all the IgG(1) molecules carried a net positive charge at this pH. MAb-G showed the highest positive zeta potential followed by MAb-E, -H, and -A. A consistent interpretation of the impact of net charge on viscosity for these MAbs is not possible, suggesting that electroviscous effects cannot explain the differences in viscosity. Values of k(D) (dynamic light scattering) indicated that the intermolecular interactions were repulsive for MAb-E and -G; and attractive for MAb-H and -A. Solution storage modulus (G') in high concentration solutions was consistent with attractive intermolecular interactions for MAb-H and -A, and repulsive interactions for MAb-G and -E. Effect of salt addition on solution G' and k(D) indicated that the interactions were primarily electrostatic in nature. The concentration dependent viscosity data were analyzed using a modified Ross and Minton equation. The analysis explicitly differentiates between the effect of molecular shape, size, self-crowding, and electrostatic intermolecular interactions in governing high concentration viscosity behavior.

Method for measuring the losses and loading of a quartz crystal microbalance

A novel, experimentally simple, and highly sensitive method for measuring the loading of a quartz crystal resonator was developed. The method is based on the use of double-sideband suppressed-carrier modulated high-frequency signal, which is swept through the resonance range of the resonator. Induced current in the resonator is passed through a capacitor, and the voltage over the capacitor is demodulated on an analog multiplier. The phase and amplitude information is carried to the frequency-doubled modulation signal and measured on a conventional low-frequency two-phase lock-in amplifier. A complex dimensionless loading parameter is obtained from the experimental data by nonlinear model fitting. The real and complex parts of this loading parameter have a simple relationship with other parameters commonly used for characterizing the resonator loading. The performance of the method was demonstrated by measuring a series of different glycerol-water mixtures ranging from 0 up to 100% glycerol. The results were close to the shear acoustic impedance of these mixtures measured and calculated from their viscosities and densities.

Kinetic study of silica gels by a new rheological ultrasonic investigation

The last decades have seen the development of sol-gel (SG) process currently used to develop new materials in a wide range of scientific applications. The SG process leads to an oxide macromolecular network through a sol (liquid phase) to gel transition. To optimize this process, the control of the kinetic of the chemical reaction is required. This kinetic can be deduced from the temporal evolution of the viscoelastic parameters. Upto date no complete investigation during the SG formation can be achieved by a unique non-destructive technique. In this paper, we present an ultrasonic technique to measure the viscoelastic parameters (storage G' and loss G'' shear moduli) of the gel material during its formation. By using a suitable model which takes into account the mass loading on the surface, the viscoelastic parameters of these materials are accurately deduced. In order to study the efficiency of this technique, silica gels transition is monitored at various formation temperatures and for different initial hydrolysis molar ratio (h). In addition, the monitoring is performed at different oscillatory shear measurements in the 6-54 MHz frequency range to determine a new characteristic time t(vs) corresponding to the moment when the material is no more a newtonian liquid. This characteristic time is then compared to the gelation time t(g) determined by rheological or acoustic audible range methods. Thus the new characteristic time is also a good criterion to characterize earlier the SG matrix transition. Our AT-cut quartz technique using our model can also be used as a high frequency rheometer for the sol-gel materials.

Acoustic microsensors—the challenge behind microgravimetry

Acoustic microsensors are commonly known as high-resolution mass-sensitive devices. This is a restricted view in many chemical and biosensor applications, especially in liquids. Sensitivity to non-gravimetric effects is a challenging feature of acoustic sensors. In this review we give an overview of recent developments in resonant sensors including micromachined devices and also list recent activity relating to the (bio)chemical interface of acoustic sensors. Major results from theoretical analysis of quartz crystal resonators, descriptive for all acoustic microsensors are summarized, and non-gravimetric contributions to the sensor signal from viscoelasticity and interfacial effects are discussed. We finally conclude with some future perspectives.

Application of Ultrasonic Shear Rheometer to Characterize Rheological Properties of High Protein Concentration Solutions at Microliter Volume

The purpose of this work was to conduct preliminary rheological analysis on high protein concentration solutions by using the technique of ultrasonic shear rheometry at megahertz frequencies. The work was aimed at establishing the viability of the technique for analyzing protein solution rheology as well as obtaining an initial understanding of the effect of solution conditions on solution rheology of a model protein. Bovine serum albumin (BSA) was used for this study, and rheological analysis was conducted at 20 microL sample volume between pH 2.0 and 9.0 at different ionic strengths at 25 degrees C using 5 and 10 MHz quartz crystals. Significant differences in storage modulus among solutions at pH 5.0, 7.0, and 9.0 could only be detected at 10 MHz, and the errors associated with measurements were smaller as compared to those at 5 MHz for all the solutions studied. Solutions at pH 2.0 and 3.0 showed a time-dependent change in solution rheology. For solutions at pH 5.0, 7.0, and 9.0, which did not show time dependence in solution rheology, loss modulus data at lower concentrations correlated well with the dilute solution data in the literature. At higher concentrations, pH 5.0 solutions exhibited a higher loss modulus than pH 7.0 and pH 9.0 solutions. Storage modulus decreased with increasing ionic strength, unlike loss modulus, which did not show any change, except at pI of protein when no effect was observed. The results show the potential of high frequency rheometry for analyzing subtle differences in rheology of pharmaceutically relevant protein solutions at microliter volume.

An EQCM Study of the Electropolymerization of Benzene in an Ionic Liquid and Ion Exchange Characteristics of the Resulting Polymer Film

The direct electropolymerization of benzene dissolved in the ionic liquid 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate was studied at room temperature applying the electrochemical quartz-crystal microbalance technique. Analysis of the damping changes showed that the Sauerbrey equation could be applied for data evaluation. In the polymer, every third to fourth benzene ring carried a positive charge in the oxidized state. During electropolymerization, some ionic liquid was absorbed in the growing polymer. The redox behavior was characterized by wide peaks typical for conducting polymers. Charge neutrality of the polymer during redox cycling was maintained by anion and cation exchange with the ionic liquid. With increasing scan rate, cation exchange became more and more important.

Measurement of fluid viscosity at microliter volumes using quartz impedance analysis

The purpose of this work was to measure viscosity of fluids at low microliter volumes by means of quartz crystal impedance analysis. To achieve this, a novel setup was designed that allowed for measurement of viscosity at volumes of 8 to 10 microL. The technique was based on the principle of electromechanical coupling of piezoelectric quartz crystals. The arrangement was simple with measurement times ranging from 2 to 3 minutes. The crystal setup assembly did not impose any unwanted initial stress on the unloaded quartz crystal. Quartz crystals of 5- and 10-MHz fundamental frequency were calibrated with glycerol-water mixtures of known density and viscosity prior to viscosity measurements. True frequency shifts, for the purpose of this work, were determined followed by viscosity measurement of aqueous solutions of sucrose, urea, PEG-400, glucose, and ethylene glycol at 25 degrees C +/- 0.5 degrees C. The measured viscosities were found to be reproducible and consistent with the values reported in the literature. Minor inconsistencies in the measured resistance and frequency shifts did not affect the results significantly, and were found to be experimental in origin rather than due to electrode surface roughness. Besides, as expected for a viscoelastic fluid, PEG 8000 solutions, the calculated viscosities were found to be less than the reported values due to frequency dependence of storage and loss modulus components of complex viscosity. From the results, it can be concluded that the present setup can provide accurate assessment of viscosity of Newtonian fluids and also shows potential for analyzing non-Newtonian fluids at low microliter volumes.

Ionic liquids: a new class of sensing materials for detection of organic vapors based on the use of a quartz crystal microbalance

A QCM device employing ionic liquids as the sensing materials for organic vapors has been developed and evaluated. The sensing mechanism is based on the fact that the viscosity of the ionic liquid membrane decreases rapidly due to solubilization of analytes in the ionic liquids. This change in viscosity, which varies with the chemical species of the vapors and the types of ionic liquids, results in a frequency shift of the corresponding quartz crystal. The QCM sensor demonstrated a rapid response (average response time of less than 2 s) to organic vapors with an excellent reversibility because of the fast diffusion of analytes in ionic liquids. Furthermore, the ionic liquids, with zero vapor pressure and stable chemical properties, ensure a long-term shelf life for the sensor.

An extended Butterworth-Van Dyke model for quartz crystal microbalance applications in viscoelastic fluid media

An extended Butterworth-Van Dyke (EBVD) model with frequency-independent parameters for the characterization of a resonant compound formed by a quartz crystal in contact with a finite viscoelastic layer contacting a semi-infinite viscoelastic medium is extracted by analysis of the lumped element model. The formulation of the EBVD model is compared with the complete expression of the electrical admittance of the loaded quartz derived from the transmission line model (TLM). Relative deviations between them do not exceed 3% around 1% bandwidth near resonance. An extended Martin & Granstaff's model and an explicit expression for the frequency shift that supposes an extension of Kanazawa's model for viscoelastic media are obtained. An analysis of the errors associated with the extraction of shear parameters of the coating for different materials prove that, to obtain an error less than 5% in the shear parameters determination, the viscoelastic contribution, defined as the relative error in the thickness computed from the frequency shift by Sauerbrey equation, must be greater than a limit that depends on Q, which is defined as the ratio of the shear storage modulus (G') to shear loss modulus (G"). In the materials studied, polymers in the transition range or in the rubbery state with Q = 1 and 10, the viscoelastic contribution must be higher than 15% and 50%, respectively, for a 5% limit error in the shear parameters extraction. A criterion for a practical determination of the appropriate viscoelastic regimes is indicated.