Experimental and Analytical Investigation of the Cavity Expansion Method for Mechanical Characterization of Soft Materials

In biomedical engineering, the mechanical properties of biological tissues are commonly determined by using conventional methods such as tensile stretching, confined and unconfined compression, indentation and elastography. With the exception of elastography, most techniques are implemented on ex-vivo soft tissue samples. This study evaluated a newly developed technique that has the potential to measure the mechanical properties of soft tissues in their in-vivo condition. This technique is based on the mechanics of internal spherical cavity expansion inside soft materials. Experimental, mathematical and numerical investigations were conducted. Experimentally, the pressure-cavity volume relationship was measured using two types of polyvinyl alcohol (PVA) hydrogels of different stiffnesses, namely Sample1 and Sample 2. In addition, unconfined compression tests were conducted to measure the stress-strain relationship of the two gels. Based on the cavity expansion test results, the measured pressure-volume data was translated into the stress-strain relationship using a mathematical model. The stiffness of the two gels was then compared to that determined by the unconfined compression technique. The resulting stiffness of the two techniques was then compared at overlapping range of strains, with the average percentage of difference being 8.46% for Sample1 and 5.36% for Sample 2. A numerical model was developed to investigate the analytical solution of the new technique. This investigation was based on verifying the displacement predicted by the analytical solution. The promising outcome of the technique encouraged extending this study to include bovine liver tissues. A tissue sample was extracted from a bovine liver and subjected to tensile loading to evaluate its stiffness. The result was a stiffness of 76.92 kPa. A second sample of the same bovine liver was evaluated using the spherical expansion technique which resulted in a stiffness of 87.94 kPa

Mechanical Characterization of Soft Materials Using Volume-Controlled Cavitation

The mechanical properties of soft materials are used in a wide range of fields and applications including biomedical engineering, sports, and automobile industry, in addition to medical applications. Therefore, several methods have been used to measure these properties including tension, compression, and indentation. This study focuses on the application of multiaxial loading using cavitation mechanics to measure nonlinear mechanical properties of soft materials. It was found that applying controlled cavitation within the internal structure of soft materials provided enough information to characterize their mechanical behavior. This is done by inserting a needle-balloon tool inside the tested material while being attached to a system that allows for injections of an incompressible fluid (water) into the balloon. To establish this methodology as a robust characterization technique of the mechanics of soft materials, it was used in a four-stages investigation: developing an analytical framework to characterize the non-linear elastic behavior of rubber-like materials (elastomeric gels), measuring the hyperelastic properties of soft biological tissues (porcine liver), comparing the cavity expansion test with a conventional uniaxial tensile testing, and establishing an analytical framework to characterize the time-dependent behavior of viscoelastic materials. In the first stage, a solution that relates the applied radial loads and tangential deformation is introduced. This solution allows the calibration of hyperelastic strain energy functions (SEF), which were Yeoh, Arruda-Boyce and Ogden (used in all stages). Finite element simulations were used to validate the material parameters of the three hyperelastic models. Computed tomography (CT) imaging was used to validate the spherical configuration assumption of the inflated balloon inside the sample. The validation process considered the two types of stresses generated during the test, radial and hoop stresses. It was observed that the radial stresses were insignificant compared to the hoop stresses. In the second stage, a smaller balloon was used to test porcine liver tissues; however, the protocol of this stage was similar to the first stage. Few changes were introduced to the definition of the deformation term, as a result, the measured deformations in the cavity test coincided with the deformation levels reported in literature. In addition, the three hyperelastic models predicted initial shear moduli that agreed with their counterparts reported in literature using conventional testing techniques. To understand the similarities and differences between the cavity expansion test and conventional axial loading, the third stage addressed the comparison between the cavitation and uniaxial tension characterization. The comparison focused on the stress levels, range of strains as well as the initial shear moduli. It was found that the strain levels in the hydrogels were similar up to the failure point. In addition, the hoop stresses generated due to cavity loads were similar to the tensile stresses generated in uniaxial tension up to a strain level of 45%. Afterward, hoop stresses increased exponentially reaching a peak magnitude that was twice that observed in the uniaxial tension. Since the radial stresses were insignificant, the previous two observations provided an indication to the equi-biaxial nature of the cavity expansion test. The final stage of this study addressed the characterization of the viscoelastic properties of rubber-like materials. In this stage, linear viscoelastic theory was used. The cavitation rheology is used to measure the non-linear elastic response of the hydrogels at three different strain rates. The simple shear relaxation test was used to measure the viscous response of the hydrogels. While the elastic material parameters were calibrated using the same method used in previous stages, the viscous coefficients of the Prony series were determined using Abaqus’ calibration tool. Afterward, the elastic parameters and viscous coefficients were used to reproduce the experimental data numerically using FE simulations, and analytically using Matlab code. The agreement between experimental data, FE simulations and the analytical code showed that the cavity expansion test was capable of measuring the time-dependent response of rubber-like materials

Hydrogel Biomaterials for Drug Delivery: Mechanisms, Design, and Drugs

Due to their unique physical and chemical properties, hydrogels have attracted significant attention in several medical fields, specifically, drug delivery applications in which gel-based nanocarriers deliver drug molecules to the region of interest in biological organs. For different drug delivery applications, hydrogel systems can be manipulated to provide passive and/or active delivery. Thus, several drug targeting, loading, and releasing mechanisms have been devised and reported in the literature. This chapter discusses these mechanisms and their efficacy with respect to different drug delivery applications. Furthermore, the drug dosage is dependent on the design and shape of the hydrogel systems, which in turn depend on the route of the drug administration. This chapter covers the types of hydrogel-based products applied via different routes of drug administration. Lastly, this chapter addresses different classifications of delivered drugs including small molecular weight drugs; therapeutic proteins and peptides; and vaccines

Targeted nanoparticle binding & detection in petroleum hydrocarbon impacted porous media

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.chemosphere.2018.10.046 © 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/Targeted nanoparticle binding has become a core feature of experimental pharmaceutical product design which enables more efficient payload delivery and enhances medical imaging by accumulating nanoparticles in specific tissues. Environmental remediation and geophysical monitoring encounter similar challenges which may be addressed in part by the adoption of targeted nanoparticle binding strategies. This study illustrates that engineered nanoparticles can bind to crude oil-impacted silica sand, a selective adsorption driven by active targeting based on an amphiphilic polymer coating. This coating strategy resulted in 2 mg/kg attachment to clean silica sand compared to 8 mg/kg attachment to oil-impacted silica sand. It was also shown that modifying the surface coating influenced the binding behaviour of the engineered nanoparticles – more hydrophobic polymers resulted in increased binding. Successful targeting of Pluronic-coated iron oxide nanoparticles to a crude oil and silica sand mixture was demonstrated through a combined quantitative Orbital Emission Spectroscopy mass analysis supported by Vibrating Scanning Magnetometer magnetometry, and a qualitative X-ray micro-computed tomography (CT) visualization approach. These non-destructive characterization techniques facilitated efficient analysis of nanoparticles in porous medium samples with minimal sample preparation, and in the case of X-Ray CT, illustrated how targeted nanoparticle binding may be used to produce 3-D images of contaminated porous media. This work demonstrated successful implementation of nanoparticle targeted binding toward viscous LNAPL such as crude oil in the presence of a porous medium, a step which opens the door to successful application of targeted delivery technology in environmental remediation and monitoring.Natural Sciences and Engineering Research Council of Canad

Mechanical characterization of PVA hydrogels' rate-dependent response using multi-axial loading.

The time-dependent properties of rubber-like synthesized and biological materials are crucial for their applications. Currently, this behavior is mainly measured using axial tensile test, compression test, or indentation. Limited studies performed on using multi-axial loading measurements of time-dependent material behavior exist in the literature. Therefore, the aim of this study is to investigate the viscoelastic response of rubber-like materials under multi-axial loading using cavity expansion and relaxation tests. The tests were performed on PVA hydrogel specimens. Three hyperelasitc models and one term Prony series were used to characterize the viscoelastic response of the hydrogels. Finite element (FE) simulations were performed to verify the validity of the calibrated material coefficients by reproducing the experimental results. The excellent agreement between the experimental, analytical and numerical data proves the capability of the cavity expansion technique to measure the time-dependent behavior of viscoelastic materials