Large deformation rheology of gelatin gels

Temperature effects on ribbon characteristics in soft gelatin capsule manufacture

In the manufacture of soft gelatin capsules using a rotary-die encapsulation machine, the formation of ribbons at the cooling drums and their subsequent mechanical performance are key attributes for a smooth machinability. In this paper we present the results of a comprehensive investigation of the intricate impact of the cooling drum temperature in the range between 5 and 25 °C on the mechanical and the microstructural properties of a highly concentrated gelatin formulation (40 % w/w) typically used in soft capsule manufacture. The study demonstrates that the temperature at the cooling drums strongly affects the gelation kinetics, the gel elasticity and the tensile strength of the ribbons. The temperature correlates linearly with the storage modulus G' under low shear deformation, i.e. the lower the temperature of the gel, the higher the gel elasticity. A reverse linear relationship was found for the temperature-dependent ultimate tensile strength (UTS) of the gelatin ribbons, i.e. a higher drum temperature leads to a higher UTS. This inverse effect of the ageing temperature on G' and UTS can be explained by temperature-induced microstructural differences within the gel network, as indicated by FTIR spectroscopy and Differential Scanning Calorimetry (DSC) measurements. Lower ageing temperatures result in a higher number of triple helical junction zones with fewer and/or weaker hydrogen bonds, which translate into a higher gel elasticity under low shear deformation, but a lower resilience of the ribbons against rupture in tensile testing. At higher temperatures, fewer but longer and/or more thermostable triple helical links in the gel network enhance the stability of the ribbons against tensile stress. In summary, the results clearly reveal that a detailed understanding of the complex relationship between the drum temperature, the gel network structure and the mechanical properties of gelatin ribbons is essential for process optimization.

Effects of acetic, malic, and citric acids on the large deformation behaviors of fish gelatin gels

This research was aimed to quantify the effects of acetic acid, malic acid, and citric acid (0, 0.5, 1.0, and 2.0 g/100 g H2 O) on the stress-strain responses of fish gelatin (FG) gels (2, 4, and 6.67 g/100 g H2 O) under uniaxial compression up to 68% of deformation. The first-order Ogden model fitted quite well for the compression responses of FG gels (R2  = 0.9909-0.9997). Protons from the acids played a key role on weakening the FG gel structures (gel rigidity, μ, decreased 11%-27%), as the μ values and pH values of FG gels were linearly correlated (R2  = 0.8240-0.9748), regardless of the acid type. The addition of an acid also resulted in a significant increase (p < .002) in the strain hardening capacity (α) of gels with 2 g FG/100 g H2 O. Both μ and α values of FG gels with higher gelatin concentrations were less affected by an acid partly due to their stronger buffering effects. The μ and α values of FG gels as affected by acids could not be fully explained based upon the pH changes, implying that the effects of acetate, malate, and citrate ions on the gel structure could not be ignored.

A Fully Biodegradable Ferroelectric Skin Sensor from Edible Porcine Skin Gelatine

High-performance biodegradable electronic devices are being investigated to address the global electronic waste problem. In this work, a fully biodegradable ferroelectric nanogenerator-driven skin sensor with ultrasensitive bimodal sensing capability based on edible porcine skin gelatine is demonstrated. The microstructure and molecular engineering of gelatine induces polarization confinement that gives rise the ferroelectric properties, resulting in a piezoelectric coefficient (d33) of ≈24 pC N-1 and pyroelectric coefficient of ≈13 µC m-2K-1, which are 6 and 11.8 times higher, respectively, than those of the conventional planar gelatine. The ferroelectric gelatine skin sensor has exceptionally high pressure sensitivity (≈41 mV Pa-1) and the lowest detection limit of pressure (≈0.005 Pa) and temperature (≈0.04 K) ever reported for ferroelectric sensors. In proof-of-concept tests, this device is able to sense the spatially resolved pressure, temperature, and surface texture of an unknown object, demonstrating potential for robotic skins and wearable electronics with zero waste footprint.

Probability of cavitation for single ultrasound pulses applied to tissues and tissue-mimicking materials

In this study, the negative pressure values at which inertial cavitation consistently occurs in response to a single, two-cycle, focused ultrasound pulse were measured in several media relevant to cavitation-based ultrasound therapy. The pulse was focused into a chamber containing one of the media, which included liquids, tissue-mimicking materials, and ex vivo canine tissue. Focal waveforms were measured by two separate techniques using a fiber-optic hydrophone. Inertial cavitation was identified by high-speed photography in optically transparent media and an acoustic passive cavitation detector. The probability of cavitation (P(cav)) for a single pulse as a function of peak negative pressure (p(-)) followed a sigmoid curve, with the probability approaching one when the pressure amplitude was sufficient. The statistical threshold (defined as P(cav) = 0.5) was between p(-) = 26 and 30 MPa in all samples with high water content but varied between p(-) = 13.7 and >36 MPa in other media. A model for radial cavitation bubble dynamics was employed to evaluate the behavior of cavitation nuclei at these pressure levels. A single bubble nucleus with an inertial cavitation threshold of p(-) = 28.2 megapascals was estimated to have a 2.5 nm radius in distilled water. These data may be valuable for cavitation-based ultrasound therapy to predict the likelihood of cavitation at various pressure levels and dimensions of cavitation-induced lesions in tissue.

Measurements of elastic moduli of silicone gel substrates with a microfluidic device

Thin layers of gels with mechanical properties mimicking animal tissues are widely used to study the rigidity sensing of adherent animal cells and to measure forces applied by cells to their substrate with traction force microscopy. The gels are usually based on polyacrylamide and their elastic modulus is measured with an atomic force microscope (AFM). Here we present a simple microfluidic device that generates high shear stresses in a laminar flow above a gel-coated substrate and apply the device to gels with elastic moduli in a range from 0.4 to 300 kPa that are all prepared by mixing two components of a transparent commercial silicone Sylgard 184. The elastic modulus is measured by tracking beads on the gel surface under a wide-field fluorescence microscope without any other specialized equipment. The measurements have small and simple to estimate errors and their results are confirmed by conventional tensile tests. A master curve is obtained relating the mixing ratios of the two components of Sylgard 184 with the resulting elastic moduli of the gels. The rigidity of the silicone gels is less susceptible to effects from drying, swelling, and aging than polyacrylamide gels and can be easily coated with fluorescent tracer particles and with molecules promoting cellular adhesion. This work can lead to broader use of silicone gels in the cell biology laboratory and to improved repeatability and accuracy of cell traction force microscopy and rigidity sensing experiments.

Microstructuration stages during gelation of gelatin under shear

We study gelation under shear of aqueous gelatin by measuring the evolution of the apparent viscosity, thus extending the previous study by de Carvalho and Djabourov (W. de Carvalho, M. Djabourov, Rheol. Acta 36, 591 (1997)). From a set of experiments under constant stress, we deduce that the microstructure evolves through the following succession of regimes: i) nucleation and growth until crowding of a microgel suspension; ii) coalescence into strata parallel to the flow; iii) gradual thickening of these strata via transverse cross-linking until the flow finally localizes into two interfacial sliding bands which close sequentially. The transition between these regimes occurs at characteristic viscosity values. This scenario is fully confirmed by experiments performed at constant shear rates. We expect it to be relevant for all materials forming thermoreversible gels.

Ballistic impact of single particles into gelatin: experiments and modeling with application to transdermal pharmaceutical delivery

The impact and penetration of high speed particles with the human skin is of interest for targeted drug delivery by transdermal powder injection. However, it is often difficult to perform penetration experiments on dermal tissue using micron scale particles. To address this, a finite element model of the impact and penetration of a 2 μm gold particle into the human dermis was developed and calibrated using experiments found in the literature. Using dimensional analysis, the model was linked to a larger scale steel ball-gelatin system in order to extract key material parameters for both systems and perform impact studies. In this manner, an elastic modulus of 2.25 MPa was found for skin, in good agreement with reported values from the literature. Further gelatin experiments were performed with steel, polymethyl methacrylate, titanium, and tungsten carbide balls in order to determine the effects of particle size and density on penetration depth. Both the finite element model and the steel-gelatin experiments were able to predict the penetration behavior that was found by other investigators in the study of the impact of typical particles used for vaccine delivery into the human dermis. It can therefore be concluded that scaled up systems utilizing ballistic gelatins can be used to investigate the performance of transdermal powder injection technology.

Electrowetting of complex fluids: perspectives for rheometry on chip

We explore the possibilities of electrowetting (EW) as a tool to assess the elastic properties of aqueous jellifying materials present in the form of a small droplet on a hydrophobic substrate. We monitored the EW response of aqueous solutions of gelatin (2-10 wt %) in ambient oil for various temperatures (8-40 degrees C) below and above the gel point. Whereas the drops remained approximately spherical cap-shaped under all conditions, the voltage-induced reduction of the contact angle became progressively less pronounced upon entering the gel state at lower temperatures. We modeled the decrease in contact angle by minimizing the total energy of the drops consisting of interfacial energies, electrostatic energy, and the elastic energy due to the deformation of the drop, which was taken into account in a modified Hertz model. This allowed fitting the data and extracting the elastic modulus G, which were found to agree well with macroscopic storage moduli G' obtained with oscillatory shear rheometry. These results show that EW can be used as a tool for characterizing soft materials with the elastic moduli ranging (at least) from 10 to 1000 Pa. Our observations also create interesting perspectives for performing in situ rheological measurement inside microfluidic chips.

Elasticity imaging of polymeric media

Viscoelastic properties of soft tissues and hydropolymers depend on the strength of molecular bonding forces connecting the polymer matrix and surrounding fluids. The basis for diagnostic imaging is that disease processes alter molecular-scale bonding in ways that vary the measurable stiffness and viscosity of the tissues. This paper reviews linear viscoelastic theory as applied to gelatin hydrogels for the purpose of formulating approaches to molecular-scale interpretation of elasticity imaging in soft biological tissues. Comparing measurements acquired under different geometries, we investigate the limitations of viscoelastic parameters acquired under various imaging conditions. Quasi-static (step-and-hold and low-frequency harmonic) stimuli applied to gels during creep and stress relaxation experiments in confined and unconfined geometries reveal continuous, bimodal distributions of respondance times. Within the linear range of responses, gelatin will behave more like a solid or fluid depending on the stimulus magnitude. Gelatin can be described statistically from a few parameters of low-order rheological models that form the basis of viscoelastic imaging. Unbiased estimates of imaging parameters are obtained only if creep data are acquired for greater than twice the highest retardance time constant and any steady-state viscous response has been eliminated. Elastic strain and retardance time images are found to provide the best combination of contrast and signal strength in gelatin. Retardance times indicate average behavior of fast (1-10 s) fluid flows and slow (50-400 s) matrix restructuring in response to the mechanical stimulus. Insofar as gelatin mimics other polymers, such as soft biological tissues, elasticity imaging can provide unique insights into complex structural and biochemical features of connectives tissues affected by disease.

Fracture of a biopolymer gel as a viscoplastic disentanglement process

We present an extensive experimental study of mode-I, steady, slow crack dynamics in gelatin gels. Taking advantage of the sensitivity of the elastic stiffness to gel composition and history we confirm and extend the model for fracture of physical hydrogels which we proposed in a previous paper (Nature Mater. 5, 552 (2006)), which attributes decohesion to the viscoplastic pull-out of the network-constituting chains. So, we propose that, in contrast with chemically cross-linked ones, reversible gels fracture without chain scission.

Observation of nonlinear shear wave propagation using magnetic resonance elastography

MR elastography (MRE) is an MRI modality that is increasingly being used to image tissue elasticity throughout the body. One MRE technique that has received a great deal of attention is based on visualizing shear waves, which reveal stiffness by virtue of their local wavelength. However, the shape of propagating shear waves can also provide valuable information about the nonlinear stress-strain behavior of tissue. Here an experiment is proposed that allows the observation of nonlinear wave propagation based on spatial-temporal phase contrast images. A theoretical description of the wave propagation was developed that reflects typical MRE excitation, which involves excitation modes both parallel and perpendicular to B0. Based on this model, it is shown that both odd and even higher harmonics are produced with their amplitudes dependent on the details of the actuator, imaging geometry, and the nonlinear tissue properties. With appropriate motion encoding, harmonic vibrations arising from nonlinear tissue response can be detected. The effect is demonstrated on an agarose gel phantom using a sinusoidal shear vibration of 150 Hz, and clearly shows the presence of harmonics at 600 and 750 Hz. Using an estimate of the strain energy of the phantom, we were able to determine the nonlinear tissue properties.

Influence of bulk elasticity and interfacial tension on the deformation of gelled water-in-oil emulsion droplets: an AFM study

We used atomic force microscopy (AFM) to study the deformation and wetting behavior of large (50-250 microm) emulsion droplets upon mechanical loading with a colloidal glass probe. Our droplets were obtained from water-in-oil emulsions. By adding gelatin to the water prior to emulsification, also droplets with a bulk elasticity were prepared. Systematic variations of surfactant and gelatin concentrations were made, to investigate their effect on the deformation and wetting behavior of the droplets and to identify the contributions of interfacial tension, bulk elasticity, and expelled water. The AFM experiments were performed in force--distance mode and showed on approach a repulsive regime which in many cases was terminated by a jump-in of the probe. In the case of pure water (i.e. gelatin-free) droplets, the repulsive part of the curve showed a good linearity, thus allowing the extraction of an effective droplet spring constant. This quantity was found to decrease on raising the surfactant concentration from below the critical micelle concentration (cmc) to well above the cmc, and its numerical values were found to correspond remarkably well to literature values for the interfacial tension. Our findings indicate that, on gelatin increase inside the droplets, the bulk elasticity gradually becomes dominant and the droplets' stiffness does not depend anymore on surfactant concentration. Also the stability of the droplet interface against wetting, as measured by the force at which the jump-in instability occurs, was enhanced by gelatin. For gelatin concentrations of > or =15 wt %, the droplets were found to behave like purely elastic bodies. Both gelatin and surfactant contribute positively to the stability against interface breakup.