Transparent Polyacrylamide Gel Research Articles

Influence of High-Intensity Focused Ultrasound (HIFU) Ablation on Arteries: Ex Vivo Studies.

High-intensity focused ultrasound (HIFU) has been used to ablate solid tumors and cancers. Because of the hypervascular structure of the tumor and circulating blood inside it, the interaction between the HIFU burst and vessel is a critical issue in the clinical environment. Influences on lesion production and the potential of vessel rupture were investigated in this study for the efficiency and safety of clinical ablation. An extracted porcine artery was embedded in a transparent polyacrylamide gel phantom, with bovine serum albumin (BSA) as an indicator of the thermal lesion, and degassed water was driven through the artery sample. The HIFU focus was aligned to the anterior wall, middle of the artery, and posterior wall. After HIFU ablation, the produced lesion was photographically recorded, and then its size was quantified and compared with that in the gel phantom without artery. In addition, the bubble dynamics (i.e., generation, expansion, motion, and shrinkage of bubbles and their interaction with the artery) were captured using high-speed imaging. It was found that the presence of the artery resulted in a decrease in lesion size in both the axial and lateral directions. The characteristics of the lesion are dependent on the focus alignment. Acoustic and hydrodynamic cavitation play important roles in lesion production and interaction with the artery. Both thermal and mechanical effects were found on the surface of the artery wall after HIFU ablation. However, no vessel rupture was found in this ex vivo study.

Design and Characterization of an Acoustically and Structurally Matched 3-D-Printed Model for Transcranial Ultrasound Imaging.

For investigating human transcranial ultrasound imaging (TUI) through the temporal bone, an intact human skull is needed. Since it is complex and expensive to obtain one, it requires that experiments are performed without excision or abrasion of the skull. Besides, to mimic blood circulation for the vessel target, cellulose tubes generally fit the vessel simulation with straight linear features. These issues, which limit experimental studies, can be overcome by designing a 3-D-printed skull model with acoustic and dimensional properties that match a real skull and a vessel model with curve and bifurcation. First, the optimal printing material which matched a real skull in terms of the acoustic attenuation coefficient and sound propagation velocity was identified at 2-MHz frequency, i.e., 7.06 dB/mm and 2168.71 m/s for the skull while 6.98 dB/mm and 2114.72 m/s for the printed material, respectively. After modeling, the average thickness of the temporal bone in the printed skull was about 1.8 mm, while it was to 1.7 mm in the real skull. Then, a vascular phantom was designed with 3-D-printed vessels of low acoustic attenuation (0.6 dB/mm). It was covered with a porcine brain tissue contained within a transparent polyacrylamide gel. After characterizing the acoustic consistency, based on the designed skull model and vascular phantom, vessels with inner diameters of 1 and 0.7 mm were distinguished by resolution enhanced imaging with low frequency. Measurements and imaging results proved that the model and phantom are authentic and viable alternatives, and will be of interest for TUI, high intensity focused ultrasound, or other therapy studies.

Micro-cavitation on demand via the nanoparticle mediated interaction of light and sound

The safe utilization of controlled cavitation for HIFU therapy and ultrasound assisted drug delivery requires nucleation sites for bubble formation. We consider the potential for nucleating transient vapor cavities using laser-illuminated gold nanoparticles combined with high-intensity focused ultrasound. An transparent polyacrylamide gel phantom was seeded with 82-nm diameter gold particles and exposed to 20 ns pulses from a 532 nm Nd:Yag laser. Laser firing was synchronized with the arrival of a burst of 1.1 MHz focused ultrasound. Acoustic emissions from ensuing inertial cavitation were detected passively using a 15 MHz focused transducer. At a laser energy of 0.10 mJ/pulse, the resulting inertial cavitation nucleation threshold pressure (peak-negative focal pressure) was as low as 0.92 MPa. In comparison, a peak-negative focal pressure of 4.50 MPa was required to nucleate detectable cavitation without laser illumination (nano-particles were present in both cases). Experimental results agree well with a simple model for transient heating and cavity formation. Since the particles are durable, one can re-activate them as needed, essentially yielding cavitation nuclei “on demand.” [Work supported by the Dept. of the Army (Award No. DAMD17-02-2-0014) and the Center for Subsurface Sensing and Imaging Systems (NSF ERC Award No. EEC-9986821).]

Dependence of Boiling Histotripsy Treatment Efficiency on HIFU Frequency and Focal Pressure Levels

Boiling histotripsy (BH) is a high-intensity focused ultrasound (HIFU)–based method of mechanical tissue fractionation that utilizes millisecond-long bursts of HIFU shock waves to cause boiling at the focus in milliseconds. The subsequent interaction of the incoming shocks with the vapor bubble mechanically lyses surrounding tissue and cells. The acoustic parameter space for BH has been investigated previously and an inverse dependence between the HIFU frequency and the dimensions of a BH lesion has been observed. The primary goal of the present study was to investigate in more detail the ablation rate and reliability of BH in the frequency range relevant to treatment of deep abdominal tissue targets (1–2 MHz). The second goal was to investigate the effect of focal peak pressure levels and shock amplitude on BH lesion formation, given a constant duty factor, a constant ratio of the pulse duration to the time to reach boiling and a constant number of BH pulses. A custom-built 12-element sector array HIFU transducer with F-number = 1.05 was used in all experiments. BH pulses at 5 different frequencies (1, 1.2, 1.5, 1.7 and 1.9 MHz) were delivered to optically transparent polyacrylamide gel phantoms and ex vivo bovine liver and myocardium tissue to observe cavitation and boiling bubble activity with high-speed photography and B-mode ultrasound imaging, correspondingly. In gel phantoms, a cavitation bubble cloud was shown to form prefocally and to shield the focus in all exposures at 1 and 1.2 MHz and in the highest amplitude exposures at 1.5–1.7 MHz; shielding was not observed at 1.9 MHz. In ex vivo tissue, this shielding effect was observed in 25% of exposures when peak negative in situ pressure exceeded 10.2 MPa at 1 MHz and 14.5 MPa at 1.5 MHz. When shielding occurred, the exposures resulted in mild tissue disruption in the prefocal region, but not liquefaction. The dimensions of liquefied lesions followed the inverse proportionality trend with frequency; consequently, the frequency range of 1.2–1.5 MHz appeared to be preferable for BH exposures in terms of the compromise between the ablation rate and reliability. The lesion size was independent of the duration of the BH pulses (or the total “HIFU on” time), provided that the number of pulses was constant and boiling was induced within each pulse. Thus, the use of shorter (1 ms vs. 10 ms), higher amplitude BH pulses allowed up to 10-fold reduction in treatment time for a given duty factor.

Thermal Energy during Irreversible Electroporation and the Influence of Different Ablation Parameters

PurposeIrreversible electroporation (IRE) uses high-voltage electric fields to achieve cell death. Although the mechanism of IRE is mainly designated as nonthermal, development of secondary Joule heating is inevitable. The study purpose was to gain understanding of temperature development and distribution during IRE. Materials and MethodsIRE was performed in a transparent polyacrylamide gel resembling soft tissue. Mechanical effects, changes in temperature gradient, and absolute temperature changes were measured with three different optical techniques (high-speed, color Schlieren, and infrared imaging) to investigate the effect on temperature of variations in voltage, pulse length, active tip length (ATL), interelectrode distance, electrode configuration (parallel, convergent, and divergent), and sequential pulsing (pulse delivery interrupted by breaks). The total delivered energy was calculated. ResultsA temperature gradient, starting at the tips of both electrodes and expanding toward each other, developed immediately with pulse delivery. Temperatures increased with increasing voltage (by 2.5°C–40.4°C), pulse length (by 5.3°C–9.8°C), ATL (by 5.9°C–17.6°C), and interelectrode distance (by 7.6°C–21.5°C), in accordance with higher energy delivery. Nonparallel electrode placement resulted in heterogeneous temperature distribution with the peak temperature focused in the area with the shortest interelectrode distance. Sequential pulse delivery significantly reduced the temperature increase compared with continuous pulsing (4.3°C vs 11.7°C). ConclusionsVoltage, pulse length, interelectrode distance, ATL, and electrode configuration each have a strong effect on temperature development and distribution during IRE. Sequential pulsing reduces the extent and volume of thermal distribution and may prove beneficial with respect to procedural safety.

A simple and cost-effective solid-phase protein nano-assay using polyacrylamide-coated glass plates

A new solid-phase protein nano-assay is suggested for simple and sensitive estimation of protein content in sample buffers (a 1-μl sample is sufficient for analysis). The assay is different from conventional “on-filter” assays in that it uses inexpensive fully transparent polyacrylamide gel (PAAG)-coated glass plates as solid support and, thus, combines the convenience of “on-membrane” staining with the sensitivity and ease of documentation of “in-gel” staining (and, therefore, is especially suited for standard lab gel documentation systems). The PAAG plates assay is compatible with all dyes for in-gel protein staining. Depending on the sensitivity of the staining protocol, the assay can be used in macro-, micro-, and nano-assay formats. We also describe a low-cost two-component colloidal Coomassie brilliant blue G-250 (CBB G-250) staining protocol for fast quantitative visualization of proteins spotted on a PAAG plate (the detection limit is up to 2ng of proteins even when using a Nikon CoolPix digital camera and white light transilluminator instead of a gel scanner). The suggested colloidal CBB G-250 protocol could also be used for visualizing nano-amounts of proteins in polyacrylamide gels. The PAAG plate assay could be useful for proteomic applications and, in general, for all cases where a fast, sensitive, and easily documentable cost-effective solid-phase protein assay is required.

Separating nonlinear propagation and cavitation effects in HIFU

HIFU waves can effectively destroy tumors or stop internal bleeding. The primary physical mechanism in HIFU is the conversion of acoustic energy into heat, which is often enhanced by nonlinear acoustic propagation and nonlinear scattering from bubbles. The goal of this work is to study and separate the effects of nonlinear propagation and cavitation on HIFU heating of tissue. Transparent polyacrylamide gel was used as a tissue-mimicking phantom. The phantom contained protein that turned opaque at high temperature. The lesions were produced for the same time-averaged intensities, but with variation of acoustic pressure amplitude combined with different duty cycles. A computer program was written to quantify the size and shape of lesions captured on CCD. Substantial increase of the lesion size was observed with high-amplitude waves. In order to separate the two effects, the experiments were repeated under static pressure greater than the pressure amplitude of the sound waves, which suppressed cavitation. It was found that with the same time-averaged power, a shorter, higher amplitude pulse creates a lesion faster and grows the lesion larger, than a longer, lower amplitude pulse. Cavitation was found to play a subtle role in the initial formation of the lesion. [Work supported by NIH, NSF, NSBRI.]

Enzyme cytochemical staining of individual cells with the use of a polyacrylamide carrier: Studies on the synthetizing reaction technique, the indigogenic method, the metal salt method, the post-azo-coupling technique, and the tetrazolium salt technique

A recently developed method for the (quantitative) demonstration of glucose-6-phosphate dehydrogenase activity in individual cells with the use of a polyacrylamide carrier has been extended for other enzyme cytochemical techniques. Isolated hepatocytes have been incorporated in the matrix of a thin transparent polyacrylamide gel prior to incubation in a cytochemical medium. The techniques which have been applied are the synthetizing reaction technique for glycogen phosphorylase, the indigogenic method for nonspecific esterase, the metal salt method for glucose-6-phosphatase, the post-azo-coupling technique for acid phosphatase, and the tetrazolium salt technique for succinate and lactate dehydrogenase activities. In all cases a few major problems which occur in the cytochemistry on single cells seem to be solved. The morphology is very well preserved, the final reaction product seems to be precipitated at the expected site of enzyme activity and the coloured end-product is highly specific for the enzyme activity to be studied, as has been demonstrated well with control experiments. The conclusion is reached, therefore, that this relatively simple device can be used routinely for the optimalization of enzyme cytochemistry of single cells.

A highly crosslinked, transparent polyacrylamide gel with improved mechanical stability for use in isoelectric focusing and isotachophoresis

Polyacrylamide gels highly (10–50%) crosslinked with N,N′-methylenebisacrylamide (BIS) provide large pore sizes but are mechanically labile. By substituting N,N′-diallyltartardiamide (DATD) for BIS, mechanical stability and firm adherence to glass walls is conferred to these gels, with only a minor decrease in pore size compared to highly BIS-crosslinked gels. The highly DATD-crosslinked gels have the further advantage of being transparent. It appears that highly DATD-crosslinked gels combining large pore size with mechanical stability are superior to presently used gel types in isoelectric focusing on polyacrylamide gel and isotachophoresis on polyacrylamide gel.