Polyvinyl Alcohol Cryogel Research Articles

Polyvinyl Alcohol Cryogels for Acoustic Characterization of Phase-Change Contrast Agents.

Phase-change contrast agents (PCCAs) consisting of lipid-encapsulated low-boiling-point perfluorocarbons can be used in conjunction with ultrasound for diagnostic and therapeutic applications. One benefit of PCCAs is site-specific activation, whereby the liquid core is acoustically vaporized into a bubble detectable via ultrasound imaging. For further evaluation of PCCAs in a variety of applications, it is useful to disperse these nanodroplets into an acoustically compatible stationary matrix. However, many traditional phantom preparations require heating, which causes premature thermal activation of low-boiling-point PCCAs. Polyvinyl alcohol (PVA) cryogels do not require heat to set. Here we propose a simple method for the incorporation of the low-boiling-point PCCAs using octafluoropropane (OFP) and decafluorobutane (DFB) into PVA cryogels for a variety of acoustic characterization applications. We determined the utility of the phantoms by activating droplets with a focused transducer, visualizing the lesions with ultrasound imaging. At 1 MHz, droplet activation was consistently observed at 2.0 and 4.0 MPa for OFP and DFB, respectively.

A hierarchical porous composite magnetic sorbent of reduced graphene oxide embedded in polyvinyl alcohol cryogel for solvent-assisted-solid phase extraction of polycyclic aromatic hydrocarbons.

A hierarchical porouscomposite magnetic sorbent was fabricated and applied to the dispersive solvent-assisted solid-phase extraction of five polycyclic aromatic hydrocarbons. A sorbent was first prepared by incorporating graphene oxide, calcium carbonate, and magnetite nanoparticles into a polyvinyl alcohol cryogel. The graphene oxide was converted to reduced graphene oxide using ascorbic acid and a hierarchical porous structure was produced by reacting hydrochloric acid with incorporated calcium carbonate to generate carbon dioxide bubbles which created a second network. Before extracting the target analytes, the extraction solvent was introduced into the hierarchical pore network of the sorbent. The extraction was based on the partition between the analytes and introduced extraction solvent and the adsorption of analytes on reduced graphene oxide.The extraction efficiency was enhanced through π-π and hydrophobic interactions between polycyclic aromatic hydrocarbons and reduced graphene oxide and extraction solvent. The extracted polycyclic aromatic hydrocarbons were determined by using high-performance liquid chromatography coupled with a fluorescence detector. The developed method was applied to extract polycyclic aromatic hydrocarbons in disposable diaper, coffee, and tea samples and recoveries from 84.5 to 99.4% were achieved with relative standard deviations below 7%. The developed sorbent exhibited good reproducibility and could be reused for 10 cycles.The developed sorbent exhibited good reproducibility and could be reused for 10 cycles.The developed sorbent exhibited good reproducibility and could be reused for 10 cycles.

A biomimetic skin phantom for characterizing wearable electrodes in the low-frequency regime

Advances in the integration of wearable devices in our daily life have led to the development of new electrode designs for biopotential monitoring. Historically, the development and testing of wearable electrodes for the acquisition of biopotential signals has been empirical, relying on experiments on human volunteers. However, the lack of explicit control on human variables, the intra-, and inter-subject variability complicates the understanding of the performance of these wearable electrodes. Herein, phantom mimicking the electrical properties of the skin in the low-frequency range (1–1000 Hz), which has the potential to be used as a platform for controlled benchtop experiments for testing electrode functionality, is demonstrated. The fabricated phantom comprises two layers representing the deeper tissues and stratum corneum. The lower layer of the phantom mimicking deeper tissues was realized using polyvinyl alcohol cryogel (PVA-c) prepared with 0.9% W/W saline solution by a freeze-thaw technique. The properties of the upper layer representing the stratum corneum were simulated using a 100 µm thick layer fabricated by spin-coating a mixture of polydimethylsiloxane (PDMS), 2.5% W/W carbon black (CB) for conductance, and 40% W/W barium titanate (BaTiO3) as a dielectric. The hydration of the stratum corneum was modeled in a controlled way by varying porosity of the phantom’s upper layer. Impedance spectroscopy measurements were carried out to investigate the electrical performance of the fabricated phantom and validated against the impedance response obtained across a physiological skin impedance range of five human subjects. The results indicated that the Bode plot depicting the impedance response obtained on the phantom was found to lie in the human skin range. Moreover, it was observed that the change of porosity provides control over the hydration and the phantom can be tuned as per the skin ranges among different individuals. Also, the phantom was able to mimic the impact of dry and hydrated skin on a simulated ECG signal in the time domain. The developed skin phantom is affordable, fairly easy to manufacture, stable over time, and can be used as a platform for benchtop testing of new electrode designs.

Proanthocyanidins Promote Endothelial Cell Viability and Angiogenesis.

Botanic drugs are reportedly effective in treating ischemic conditions by improving vascular circulation. However, it has been very rare for biomaterial researchers to look into the possibility of using such products in the context of tissue regeneration. This work studied 4 botanic drugs to explore their effects on vascular endothelial cell growth. Human umbilical endothelial cells were cultured in the presence of different doses of astragalus powder extract, astragalus injection, puerarin injection, and proanthocyanidin (PAC). Among the 4 drugs, PAC showed a potent effect on cell viability and stimulated cell growth in a dose-dependent manner. In particular, the PAC under test was able to maintain a high level of cell viability/proliferation comparable with the cells supplemented with the endothelial cell growth medium, at both low and normal serum conditions. Blocking either endothelial cell growth factor receptors or epithelial cell growth factor receptors was ineffective in reducing the stimulatory effect. The PAC released from polyvinyl alcohol cryogels stimulated HUVECs proliferation. The chick embryo chorioallantoic membrane model was further used to test the angiogenicity of PAC, showing that this botanic drug was potent in stimulating vasculature development. This work therefore demonstrates for the first time that PAC is capable of upregulating endothelial cell activity and growth in vitro in the absence of growth factors and that PAC can be loaded and released from drug carriers and can stimulate angiogenesis. These findings suggest the application of PAC in angiogenesis and tissue regeneration.

Design of heart phantoms for ultrasound imaging of ventricular septal defects.

Ventricular septal defects (VSDs) are common congenital heart malformations. Echocardiography used during VSD hybrid cardiac procedures requires extensive training for image acquisition and interpretation. Cardiac surgery simulators with heart phantoms have shown usefulness for such training, but they are limited in visualization and characterization of complex VSD. This study explores a new method to build patient-specific heart phantoms with VSD, with proper tissue echogenicity for ultrasound imaging. Heart phantoms were designed from preoperative imaging of three patients with complex VSDs. Each whole heart phantom, including atrial and ventricular septums, was obtained by manual segmentation and by surface reconstruction, then by molding and by casting in different materials. Heart phantoms in silicone and polyvinyl alcohol cryogel (PVA-C) were considered, and they were reconstructed in 3-D using 2-D freehand ultrasound imaging. An electromagnetic measurement system was used to measure the mean VSD diameters from the heart phantoms. Errors were evaluated below 1.0 mm for mean VSD diameters between 6.2 and 7.5 mm. Patient-specific heart phantoms promise for representing complex heart malformations such as VSDs. PVA-C showed better tissue echogenicity than silicone for VSDs visualization and characterization.

Helical toroid phantom for 3D flow imaging investigations

The medical physics community has hitherto lacked an effective calibration phantom to holistically evaluate the performance of three-dimensional (3D) flow imaging techniques. Here, we present the design of a new omnidirectional, three-component (3-C) flow phantom whose lumen is consisted of a helical toroid structure (4 mm lumen diameter; helically winded for 5 revolutions over a torus with 10 mm radius; 5 mm helix radius). This phantom’s intraluminal flow trajectory embraces all combinations of x, y, and z directional components, as confirmed using computational fluid dynamics (CFD) simulations. The phantom was physically fabricated via lost-core casting with polyvinyl alcohol cryogel (PVA) as the tissue mimic. 3D ultrasound confirmed that the phantom lumen expectedly resembled a helical toroid geometry. Pulsed Doppler measurements showed that the phantom, when operating under steady flow conditions (3 ml s−1 flow rate), yielded flow velocity magnitudes that agreed well with those derived from CFD at both the inner torus (−47.6 ± 5.7 versus −52.0 ± 2.2 cm s−1; mean ± 1 S.D.) and the outer torus (49.5 ± 4.2 versus 48.0 ± 1.7 cm s−1). Additionally, 3-C velocity vectors acquired from multi-angle pulsed Doppler showed good agreement with CFD-derived velocity vectors (<7% and 10° difference in magnitude and flow angle, respectively). Ultrasound color flow imaging further revealed that the phantom’s axial flow pattern was aligned with the CFD-derived flow profile. Overall, the helical toroid phantom has strong potential as an investigative tool in 3D flow imaging innovation endeavors, such as the development of flow vector estimators and visualization algorithms.

A 3.5-GHz pseudo-correlation type radiometer for biomedical applications

A pseudo-correlation type radiometer based on astrophysical instrumentation is proposed for biomedical applications. The working frequency band is centred at 3.5 GHz. The prototype performance and functionality are assessed. The theoretical analysis of the receiver topology is described, as well as the subsystems employed in its configuration, such as a customized on-body antenna and microwave circuitry based on off-the-shelf devices. Experimental characterization of the individual subsystems and preliminary measurements of the radiometer prototype are presented. Furthermore, customized phantoms, based on polyvinyl alcohol cryogel, are employed for the characterization of the on-body antenna, as well as the radiometer.

Bimodal microwave and ultrasound phantoms for non-invasive clinical imaging

A precise and thorough methodology is presented for the design and fabrication of bimodal phantoms to be used in medical microwave and ultrasound applications. Dielectric and acoustic properties of human soft tissues were simultaneously mimicked. The phantoms were fabricated using polyvinyl alcohol cryogel (PVA-C) as gelling agent at a 10% concentration. Sucrose was employed to control the dielectric properties in the microwave spectrum, whereas cellulose was used as acoustic scatterer for ultrasound. For the dielectric properties at microwaves, a mathematical model was extracted to calculate the complex permittivity of the desired mimicked tissues in the frequency range from 500 MHz to 20 GHz. This model, dependent on frequency and sucrose concentration, was in good agreement with the reference Cole–Cole model. Regarding the acoustic properties, the speed of sound and attenuation coefficient were employed for validation. In both cases, the experimental data were consistent with the corresponding theoretical values for soft tissues. The characterization of these PVA-C phantoms demonstrated a significant performance for simultaneous microwave and ultrasound operation. In conclusion, PVA-C has been validated as gelling agent for the fabrication of complex multimodal phantoms that mimic soft tissues providing a unique tool to be used in a range of clinical applications.

Light-Emitting Diode Based Photoacoustic Imaging System

Purpose: A Photoacoustic Imaging (PAI) as a non-invasive hybrid imaging modality has the potential to be used in a wide range of pre-clinical and clinical applications. There are different optical excitation sources that affect the performance of PAI systems. Our goal is proving the capability of the Light-Emitting Diode (LED) based PAI system for imaging of objects in different depths. Materials and Methods: In this study the Full Width of Half Maximum (FWHM) and Contrast to Noise Ratio (CNR) of LED-based PAI system is evaluated using agar, and Poly-Vinyl Alcohol Cryogel (PVA-C) phantoms. Results: The results show that axial and lateral FWHM of the photoacoustic image in agar phantom 1%, are 0.59 and 1.16 mm, respectively. It is capable of distinguishing objects about 250 µm. Furthermore, one of the main improvements of photoacoustic images is achieved by proposed LED-based system that is a 26% higher CNR versus the ultrasound images. Conclusion: Therefore, the provided technical characteristics in this study have made designed LED-based PAI system as a suitable tool for preclinical and clinical imaging.

Cryostructuring of Polymeric Systems †: Application of Deep Neural Networks for the Classification of Structural Features Peculiar to Macroporous Poly(vinyl alcohol) Cryogels Prepared without and with the Additives of Chaotropes or Kosmotropes.

Macroporous poly(vinyl alcohol) cryogels (PVACGs) are physical gels formed via cryogenic processing of polymer solutions. The properties of PVACGs depend on many factors: the characteristics and concentration of PVA, the absence or presence of foreign solutes, and the freezing-thawing conditions. These factors also affect the macroporous morphology of PVACGs, their total porosity, pore size and size distribution, etc. In this respect, there is the problem with developing a scientifically-grounded classification of the morphological features inherent in various PVACGs. In this study PVA cryogels have been prepared at different temperatures when the initial polymer solutions contained chaotropic or kosmotropic additives. After the completion of gelation, the rigidity and heat endurance of the resultant PVACGs were evaluated, and their macroporous structure was investigated using optical microscopy. The images obtained were treated mathematically, and deep neural networks were used for the classification of these images. Training and test sets were used for their classification. The results of this classification for the specific deep neural network architecture are presented, and the morphometric parameters of the macroporous structure are discussed. It was found that deep neural networks allow us to reliably classify the type of additive or its absence when using a combined dataset.

A Polyvinyl Alcohol-Based Thermochromic Material for Ultrasound Therapy Phantoms

Temperature estimation is a fundamental step in assessment of the efficacy of thermal therapy. A thermochromic material sensitive within the temperature range 52.5°C–75°C has been developed. The material is based on polyvinyl alcohol cryogel with the addition of a commercial thermochromic ink. It is simple to manufacture, low cost, non-toxic and versatile. The thermal response of the material was evaluated using multiple methods, including immersion in a temperature-controlled water bath, a temperature-controlled heated needle and high-intensity focused ultrasound (HIFU) sonication. Changes in colour were evaluated using both RGB (red, green, blue) maps and pixel intensities. Acoustic and thermal properties of the material were measured. Thermo-acoustic simulations were run with an open-source software, and results were compared with the HIFU experiments, showing good agreement. The material has good potential for the development of ultrasound therapy phantoms.

Dielectric properties of PVA cryogels prepared by freeze-thaw cycling.

Solutions of polyvinyl alcohol (PVA) in water can form gels upon repeated freezing and thawing. These PVA cryogels have applications as biomaterials, including artificial tissue and drug delivery systems. We have studied the dielectric properties of PVA cryogels within the freeze-thaw cycles as a function of both frequency and temperature in order to understand the physical changes that take place during the thermal cycling process. Our results indicate that most of the changes in dielectric properties occur during the cooling phase of the first cycle and suggest that the solution must be cooled below a critical temperature of about 263 K for the formation of a gel that persists after thawing. The material's dielectric spectrum shows the presence of several relaxation processes. We identify one of these with the dielectric relaxation of ice and two others with motions of the PVA polymer chains. The temperature dependence of the polymeric relaxation times suggests that they are both thermally activated, with an activation energy of roughly 300 kJ/mol.

Transparent polyvinyl-alcohol cryogel as immobilisation matrix for continuous biohydrogen production by phototrophic bacteria

BackgroundPhototrophic purple non-sulfur bacteria (PNSB) have gained attention for their ability to produce a valuable clean energy source in the form biohydrogen via photofermentation of a wide variety of organic wastes. For maturation of these phototrophic bioprocesses towards commercial feasibility, development of suitable immobilisation materials is required to allow continuous production from a stable pool of catalytic biomass in which energy is not diverted towards biomass accumulation, and optimal hydrogen production rates are realised. Here, the application of transparent polyvinyl-alcohol (PVA) cryogel beads to immobilisation of Rhodopseudomonas palustris for long-term hydrogen production is described. PVA cryogel properties are characterised and demonstrated to be well suited to the purpose of continuous photofermentation. Finally, analysis of the long-term biocompatibility of the material is illustrated.ResultsThe addition of glycerol co-solvent induces favourable light transmission properties in normally opaque PVA cryogels, especially well-suited to the near-infrared light requirements of PNSB. Material characterisation showed high mechanical resilience, low resistance to diffusion of substrates and high biocompatibility of the material and immobilisation process. The glycerol co-solvent in transparent cryogels offered additional benefit by reinforcing physical interactions to the extent that only a single freeze–thaw cycle was required to form durable cryogels, extending utility beyond only phototrophic bioprocesses. In contrast, conventional PVA cryogels require multiple cycles which compromise viability of entrapped organisms. Hydrogen production studies of immobilised Rhodopseudomonas palustris in batch photobioreactors showed higher specific hydrogen production rates which continued longer than planktonic cultures. Continuous cultivation yielded hydrogen production for at least 67 days from immobilised bacteria, demonstrating the suitability of PVA cryogel immobilisation for long-term phototrophic bioprocesses. Imaged organisms immobilised in cryogels showed a monolithic structure to PVA cryogels, and demonstrated a living, stable, photofermentative population after long-term immobilisation.ConclusionTransparent PVA cryogels offer ideal properties as an immobilisation matrix for phototrophic bacteria and present a low-cost photobioreactor technology for the further advancement of biohydrogen from waste as a sustainable energy source, as well as development of alternative photo-bioprocesses exploiting the unique capabilities of purple non-sulfur bacteria.

New synthetic mitral valve model for human prolapsed mitral valve reconstructive surgery for training

The training process of young surgeons is highly desirable in order for them to gain an understanding of the quality of care and patient safety required during cardiac surgeries, however, it may take a few years of practice in order for them to properly develop these skills. Artificial life-like platforms and models are extremely recommended for teaching hands-on and real-world practice in both junior and even experienced medical professionals and surgeons. Suitable and accessible training tools are of significant importance for simulating a particular surgery in order to provide practice opportunities for a specific surgical procedure. In this study, we focussed on the simulation of the human mitral valve prolapse reconstructive surgery. An innovative, artificial, biomimetic model was designed and fabricated made of Cryogel biomaterial developed in our lab that is suitable for the precise practice on the mitral valve prolapse model. The proposed model is mainly made up of polyvinyl alcohol (PVA) cryogel, which is designed to resemble the geometric and mechanical properties of a diseased (prolapse) mitral valve. We simulated the constructive prolapsed mitral valve surgery entirely on a synthetic platform. The platform was made available to four certified cardiac surgeon and there were unanimously very positive with no considerable differences in the procedural assessments between them. The proposed model has a similar appearance and texture to that of a diseased mitral valve and holds consistent mechanical properties to those of the real tissue. The offered technology may be used for other cardiothoracic reconstructive surgeries with high precision and consistency.

Treatment of low-strength ammonia wastewater by single-stage partial nitritation and anammox using upflow dual-bed gel-carrier reactor (UDGR)

This study investigated the single-stage partial nitritation and anammox (S-PNA) treatment of low-strength ammonia wastewater (≤140 mg NH4+-N/L). Upflow dual-bed gel-carrier reactor (UDGR) with polyvinyl alcohol cryogel biocarriers, developed in this study, was employed for the anammox biomass enrichment from conventional activated sludge and subsequent S-PNA experiments. Anammox biomass was successfully enriched from conventional activated sludge. The enriched anammox carriers were inoculated together with gel carriers containing nitrifying sludge into the S-PNA reactors. S-PNA activity developed rapidly, and the nitrogen removal efficiency and rate reached up to 90.1% (with complete ammonia removal) and 0.15 kg N/m3⋅d, respectively, under low nitrogen loading conditions (0.10–0.17 kg N/m3⋅d). The microbial community structure changed significantly while adapting to anammox and S-PNA conditions. Anammox was likely driven solely by a Candidatus Jettenia population accounting for ≤49.4% of bacterial 16S rRNA genes. The results demonstrate that the UDGR-based S-PNA is suitable for treating low-strength wastewater.

Ultrasound Assessment of the Relation Between Local Hemodynamic Parameters and Plaque Morphology

Mechanical factors, especially wall shear stress (WSS) and circumferential strain (CS), play an important role in the progression and rupture of atherosclerotic plaques. Previous studies have shown that the temporal phase angle between WSS and CS, referred to as the stress phase angle (SPA) may be a biomarker for plaque development and vulnerability. Since the SPA is influenced by the severity and length of the stenosis, a multifactorial relationship between local hemodynamic variables and plaque morphology can be hypothesized. However, due to ethical restrictions and the difficulty of developing animal models, there is little experimental data to support such a hypothesis about the biomarker function of SPA. In this study, a novel non-invasive ultrasound-based method for investigating the relationship between local hemodynamics and plaque morphology is developed and investigated in-vitro and in-vivo with an apolipoprotein deficient mouse model. In the in-vitro experiments, using polyvinyl alcohol cryogel (PVA-c) phantoms, we have observed that the SPA becomes more negative and the wall shear rate (WSR) rises, as the severity of the stenosis increases. Conversely, SPA becomes more positive and WSR falls with increasing plaque length. These changes in plaque morphology have little effect on CS. The in-vivo experiments show that, as the severity of the plaque increases, the SPA becomes more negative and WSR tends to more positive values; whereas, the CS drops. We conclude that these local hemodynamic parameters such as SPA, WSR and CS, as suggested by others, can be regarded as prognostic markers for the assessment of plaque vulnerability.

Polyvinyl alcohol cryogel based vessel mimicking material for modelling the progression of atherosclerosis

PurposeThe stiffness of Polyvinyl-alcohol cryogel can be adjusted through application of consecutive freeze-thaw cycles. This material has potential applications in the production of tissue mimicking phantoms in diagnostic ultrasound. The aim of this study was to use PVA-c to produce a range of geometrically and acoustically identical vessel phantoms modelling stages of atherosclerosis which could be verified through mechanical testing, thus allowing for more precision in quantitative in-vitro flow analysis of atherosclerosis. MethodsA series of anatomically realistic walled renal artery flow phantoms were constructed using PVA-c. In order to ensure precise modelling of atherosclerosis, a modified procedure of ISO27:2017 was used to compare the mechanical properties of PVA-c. Results were compared for the standard “dumbbell” test object and a modified vessel test object. The geometric accuracy and reproducibility of the vessel models were tested before and after implantation in flow phantoms. ResultsNo significant difference was found between the mechanical properties of the dumbbell test samples and the vessels for any number of freeze thaw cycles, with a correlation coefficient of R2 = 0.9767 across the dataset, indicating that a direct comparison between the mechanical properties of the dumbbell test samples and the phantom vessels was established. The geometric reproducibility showed that before and after implantation there was no significant difference between individual vessel geometries (p = 0.337 & p = 0.176 respectively). ConclusionsPolyvinyl-alcohol cryogel is a useful material for the production of arterial flow phantoms. Care should be taken when using this material to ensure its mechanical properties have been correctly characterised. The guidelines of ISO37:2017 potentially provide the best procedure to ensure this.

Enzymatically Functionalized Composite Materials Based on Nanocellulose and Poly(Vinyl Alcohol) Cryogel and Possessing Antimicrobial Activity.

In the present work, innovative composite biomaterials possessing bactericidal properties and based on the hexahistidine-tagged organophosphorus hydrolase (His6-OPH) entrapped in the poly(vinyl alcohol) cryogel (PVA-CG)/bacterial cellulose (BC) were developed. His6-OPH possesses lactonase activity, with a number of N-acyl homoserine lactones being the inducers of Gram-negative bacterial resistance. The enzyme can also be combined with various antimicrobial agents (antibiotics and antimicrobial peptides) to improve the efficiency of their action. In this study, such an effect was shown for composite biomaterials when His6-OPH was entrapped in PVA-CG/BC together with β-lactam antibiotic meropenem or antimicrobial peptides temporin A and indolicidin. The residual catalytic activity of immobilized His6-OPH was 60% or more in all the composite samples. In addition, the presence of BC filler in the PVA-CG composite resulted in a considerable increase in the mechanical strength and heat endurance of the polymeric carrier compared to the BC-free cryogel matrix. Such enzyme-containing composites could be interesting in the biomedical field to help overcome the problem of antibiotic resistance of pathogenic microorganisms.

An anthropomorphic prostatic urinary tract phantom for Doppler-based assessment of voiding dysfunction

Direct assessment of urinary flow dynamics during voiding is challenging to perform in vivo. New Doppler ultrasound techniques are being developed for this purpose, but their performance needs to be systematically evaluated using phantoms. Here, we present a new design protocol to fabricate a urinary tract phantom with realistic anatomical, mechanical and urodynamic properties. In this protocol, computer-aided design of the urethra geometry was first drafted and it was physically built using 3-D printing. Subsequently, the flexible urinary tract tube was fabricated from polyvinyl alcohol cryogel (Young’s modulus: 26.6 kPa), while an agar–gelatin mixture (Young's modulus: 17.4 kPa) forms the surrounding tissue mimic. Finally, the urethra phantom was connected to a flow circuit that simulates voiding. Deformable phantoms were devised for a normal urethra and a diseased urethra with benign prostatic hyperplasia (BPH) obstruction. The phantoms’ morphologies were measured with ultrasound imaging and their flow profiles were assessed using Doppler ultrasound. During voiding, short-axis lumen diameter at the obstruction of the BPH-featured phantom was significantly smaller (0.91 vs 2.49 mm) while the maximum flow velocity was higher (59.3 vs 22.7 cm/s) compared to those of the normal urethra. These fabricated phantoms were effective in simulating urethra deformation resulting from urine passage and may promote the development of new Doppler-based urodynamic measurement techniques.

Polyvinyl alcohol cryogel phantoms of biological tissues for wideband operation at microwave frequencies.

The aim of this work is to provide a methodology to model the dielectric properties of human tissues based on phantoms prepared with an aqueous solution, in a semi-solid form, by using off-the-shelf components. Polyvinyl alcohol cryogel (PVA-C) has been employed as a novel gelling agent in the fabrication of phantoms for microwave applications in a wide frequency range, from 500 MHz to 20 GHz. Agar-based and deionized water phantoms have also been manufactured for comparison purposes. Mathematical models dependent on frequency and sucrose concentration are proposed to obtain the complex permittivity of the desired mimicked tissues. These models have been validated in the referred bandwidth showing a good agreement to experimental data for different sucrose concentrations. The PVA-C model provides a great performance as compared to agar, increasing the shelf-life of the phantoms and improving their consistency for contact-required devices. In addition, the feasibility of fabricating a multilayer phantom has been demonstrated with a two-layer phantom that exhibits a clear interface between each layer and its properties. Thus, the use of PVA-C extends the option for producing complex multilayer and multimodal phantoms.