Formation Of Large Pores Research Articles

Piezoelectric properties of Mn-doped 0.75BiFeO3–0.25BaTiO3 ceramics

The effects of Mn doping were investigated in 0.75BiFeO3–0.25BaTiO3 ceramics sintered at 940–1020°C in air. 0.75BF–0.25BT ceramics doped with 0–0.7wt% MnO2 were prepared by a conventional ceramic process. The effects of the doping are strongly dependent on the sintering temperature and doping concentration. MnO2 doping at less than the solubility limit of 0.5wt% suppressed the formation of large pores during the sintering, and kept the sample dense at high sintering temperatures. The onset temperature of abrupt grain growth also increased. The piezoelectric properties were poor when the grain size was about 1µm or less, but showed a significant increase in samples with grain size greater than a few micrometers, independent of the Mn doping concentration. The piezoelectric properties are more strongly influenced by the grain size than the Mn doping.

Dual mechanism of action of the atypical tetracycline chelocardin

Classical tetracyclines targeting the protein biosynthesis machinery are commonly applied in human and veterinary medicine. The development and spread of resistance seriously compromise the successful treatment of bacterial infections. The atypical tetracycline chelocardin holds promise as it retains activity against tetracycline-resistant strains. It has been suggested that chelocardin targets the bacterial membrane, thus differing in mode of action from that of classical tetracyclines. We investigated the mechanism of action of chelocardin using global proteome analysis. The proteome profiles after sublethal chelocardin stress were compared to a reference compendium containing antibiotic response profiles of Bacillus subtilis. This approach revealed a concentration-dependent dual mechanism of action. At low concentrations, like classical tetracyclines, chelocardin induces the proteomic signature for peptidyl transferase inhibition demonstrating that protein biosynthesis inhibition is the dominant physiological challenge. At higher concentrations B. subtilis mainly responds to membrane stress indicating that at clinically relevant concentrations the membrane is the main antibiotic target of chelocardin. Studying the effects on the membrane in more detail, we found that chelocardin causes membrane depolarization but does not lead to formation of large pores. We conclude that at growth inhibiting doses chelocardin not only targets protein biosynthesis but also corrupts the integrity of the bacterial membrane. This dual mechanism of action might prove beneficial in slowing the development of new resistance mechanisms against this atypical tetracycline.

Effect of alkaline ultrasonic pretreatment on crystalline morphology and enzymatic hydrolysis of cellulose

In this study, ultrasound-assisted alkaline pretreatment is developed to evaluate the morphological and structural changes that occur during pretreatment of cellulose, and its effect on glucose production via enzymatic hydrolysis. The pretreated samples were characterized using scanning electron microscopy, infrared spectroscopy, and X-ray diffraction to understand the change in surface morphology, crystallinity and the fraction of cellulose Iβ and cellulose II. The combined pretreatment led to a great disruption of cellulose particles along with the formation of large pores and partial fibrillation. The effects of ultrasound irradiation time (2, 4 h), NaOH concentration (1–10 wt%), initial particle size (20–180 μm) and initial degree of polymerization (DP) of cellulose on structural changes and glucose yields were evaluated. The alkaline ultrasonic pretreatment resulted in a significant decrease in particle size of cellulose, besides significantly reducing the treatment time and NaOH concentration required to achieve a low crystallinity of cellulose. More than 2.5 times improvement in glucose yield was observed with 10 wt% NaOH and 4 h of sonication, compared to untreated samples. The glucose yields increased with increase in initial particle size of cellulose, while DP had no effect on glucose yields. The glucose yields exhibited an increasing tendency with increase in cellulose II fraction as a result of combined pretreatment.

Modeling a failure criterion for U–Mo/Al dispersion fuel

The breakaway swelling in U–Mo/Al dispersion fuel is known to be caused by large pore formation enhanced by interaction layer (IL) growth between fuel particles and Al matrix. In this study, a critical IL thickness was defined as a criterion for the formation of a large pore in U–Mo/Al dispersion fuel. Specifically, the critical IL thickness is given when two neighboring fuel particles come into contact with each other in the developed IL. The model was verified using the irradiation data from the RERTR tests and KOMO-4 test. The model application to full-sized sample irradiations such as IRISs, FUTURE, E-FUTURE, and AFIP-1 tests resulted in conservative predictions. The parametric study revealed that the fuel particle size and the homogeneity of the fuel particle distribution are influential for fuel performance.

Aging of atmospherically plasma sprayed chromium evaporation barriers

Abstract Chromium evaporation barriers are frequently used in solid oxide fuel cells to protect the porous cathode from chromium poisoning. Volatile chromium species are generated at the operation temperature of about 600–900 °C in a humid atmosphere for chromia scale forming steels as interconnect materials. In order to reduce this effect, barrier coatings are applied, often by atmospheric plasma spraying. However, also in these coatings microstructural changes as densification and in parallel formation of large pores have been observed. In order to clarify these mechanisms plasma sprayed Mn1.0 Co1.9Fe0.1O4 (“MCF”) are deposited on ferritic steels and furthermore coated with a perovskite based contact layer as used in stack build-up. These coatings are annealed in air up to 1000 h and the microstructural changes and bending of the samples are studied. The results show increasing bending with increasing aging time. High temperature curvature measurements indicate that the amount of bending is not significantly dependent on temperature. As an explanation the creep deformation of the substrate/coating system at high temperatures due to compressive stress levels in the coating is given. The origin of the stress is related to phase changes in combination with the oxidation of the coatings. In addition, interdiffusion and densification processes are discussed.

ATP-induced P2X Receptor-Dependent Large Pore Formation: How Much Do We Know?

P2X receptors are a group of homo/hetero-trimeric membrane protein complexes with an integral ion channel that opens upon extracellular adenosine triphosphate (ATP) binding (North, 2002; Khakh and North, 2006). There are seven P2X subunits (P2X1-P2X7), all having a membrane topology of cytosolic N- and C-termini, and two transmembrane segments (TM1 and TM2) connected by a large extracellular domain (Figure ​(Figure1A;1A; Jiang L.-H. et al., 2013). During application of ATP for a few seconds, P2X receptors function as classical ligand-gated ion channels selectively permeable to small physiological cations such as Ca2+, Na+, and K+, with the exception of the human P2X5 receptor which exhibits significant Cl− permeability (Bo et al., 2003). Site-directed mutagenesis and functional studies of mammalian P2X receptors, in addition to the determination of the crystal structures of zebrafish P2X4 receptors in the apo, closed state and ATP-bound, open state, have defined the structural basis for ATP binding, ion permeation and channel gating (Kawate et al., 2009; Browne et al., 2010; Hattori and Gouaux, 2012; Jiang L.-H. et al., 2013; Jiang R. et al., 2013). Three ATP-binding pockets are located at the subunit interfaces (Figure ​(Figure1B),1B), each consisting of highly conserved residues from two adjacent subunits. Occupation of these sites by ATP or its synthetic analog agonists induces conformational changes of the extracellular domain which open the ion-permeating pathway formed by three TM2s (Figures 1C,D). The narrowest part of the ion-permeating pathway or the physical gate is provided by A347 and L351 in the crystal structures of zebrafish P2X4 receptor (Hattori and Gouaux, 2012) or the corresponding residues S342 and L346 in the structural models of rat and human P2X7 receptors (Figure ​(Figure1D)1D) (Bradley et al., 2011; Browne et al., 2013; Jiang L.-H. et al., 2013).

Silver Sintering Paste Rendering Low Porosity Joint for High Power Die Attach Application

Abstract Silver joints with ~10% porosity for die-attach have been achieved with specially engineered Ag sintering pastes, through combined pressureless sintering process plus thermal aging. The pastes developed in this work have the following advantages: 1) they can be used under pressure-less processing condition; 2) they are compatible with conventional reflow oven, resulting in a higher through-put as compared to that of stepwise heating oven; 3) they can be used under air reflow condition; 4) a highly reliable joint has been obtained as judged from the shear strength results from thermal aging and temperature cycling tests. The silver sintering pastes are versatile in bonding different metallization surface including Au, Ag and Cu. It can also be used in bonding large area (10mm × 10mm) dies. Shear test results under varied temperatures implied that the maximum service temperature of Ag sintered joints can be as high as 470 – 530 °C, depending on the shear strength pass criteria, and this is more than 250°C higher than that of high-Pb joints. Thermal aging test at 250°C for the joints generated on Ag-die/Au-DBC combination revealed that Ag continuously consolidates in the bulk phase resulting in the formation of larger pores with reduced numbers as compared to that of untreated samples. At the same time, it diffuses to sintered Ag/Au-DBC interface to form a dense Ag layer induced by alloying with Ni(Au) and Cu, which strengthened the bonding. A large bondline thickness is critical for obtaining highly reliable joints. The total porosity of the joint is found slightly decreased during the course of 3200h aging test. Temperature cycling at −55 °C to 200°C shows that the silver joints are stable for at least 1000 cycles.

Inorganic particle enhanced polymer hollow fiber membranes with high mechanical properties

Inorganic particle/polymer hollow fiber membranes with different inorganic particle sizes and particle loadings were synthesized and characterized in terms of morphology and mechanical properties. The incorporation of inorganic particles induces the formation of large surface pores in the skin layer yet decreases the pore size of large voids in the support layer. The particle size and particle loading are determining factors in such morphology changes. Moreover, due to the different out-diffusion ability of inorganic particles with different sizes, smaller particles migrated and diffused out from polymer much more easily; therefore nano-sized inorganic particles were usually observed on the top surface of polymer whereas micron-sized inorganic particles were usually partially embedded inside the polymer. As a result of such morphology changes and the intrinsic mechanical properties of inorganic particles, both tensile and compressive properties were improved in such inorganic-polymer composite membranes compared to the pristine membranes. Specifically, the tensile strength can be increased by 47.2% with 1.96 wt% nano-sized alumina whereas the yield compressive strength was enhanced by 55.4% with 76.9 wt% micron-sized alumina in comparison with the pristine polymeric membranes. Besides the improvement of strength, the tensile and compressive Young's modulus was increased and the micron-sized particles are more effective to enhance the Young's modulus than the nano-sized particles. Besides the improvement in mechanical properties, the permeability of polymeric membranes was also enhanced via the inorganic particle incorporation. Therefore, such inorganic-polymer hollow fiber membranes are more promising to be used for practical industrial filtration applications regarding their higher mechanical properties to resist breakage and deformation and their higher permeability in comparison with the polymeric hollow fiber membranes.

Current Understanding of the Mechanisms by which Membrane-Active Peptides Permeate and Disrupt Model Lipid Membranes.

Three classes of membrane active peptides (MAPs) are considered in this review: cell penetrating peptides (CPPs); anti-microbial peptides (AMPs), and amyloidal peptides. We summarize both experimental and theoretical results for several representative peptides in these different classes, which highlight commonalities in their interactions with model lipid membranes. While it is clear that no fixed set of mechanisms completely characterize any particular class of MAPs, there is certainly evidence that common mechanisms can be found within and between classes. For example, CPPs appear to undergo rapid translocation across lipid bilayers through small transient pores, which nevertheless appear not to cause persistent damage to membranes. On the other hand, AMPs also show evidence of rapid translocation, but associated with this, is membrane rupture to form large pores, which are subsequently stabilized by peptide adsorption to the pore edges. This disruption to the membrane is presumably responsible for cell death. Amyloidal peptides also show evidence of stable large pore formation, however, the mechanism for pore stabilization appears linked with their ability to form fibrils and prefibrillar aggregates and oligomers. There is some evidence that pores and membrane defects in fact act as nucleation sites for these structures. Where possible we have related the experimental and theoretical work to our own simulation findings in an effort to produce a comprehensive, albeit speculative picture for the mechanisms of action for this important group of peptides.

Effects of 1-methyl-2-pyrrolidinone (NMP) on polyamide-polysulfone TFC membrane pore morphology and ICP and membrane performance in forward osmosis

In forward osmosis (FO), the influence of the pore morphology in the membrane support layer on membrane performance is still unclear. As a result, investigation into the relationship between membrane pore morphology and membrane performance in FO is meaningful. During the fabrication of a thin-film composite (TFC) FO membrane, the membrane pore morphology is related to the composition of the coagulant bath. In this study, varied concentrations (from 0 to 9 wt.%) of 1-methyl-2-pyrrolidinone (NMP) in a coagulant bath were applied to fabricate TFC FO membranes of different pore morphologies. The morphologies of the fabricated membranes were then characterized using FESEM, and the performance of each of the membranes was evaluated using a lab-scale FO setup. The FESEM images indicated that the increase in the NMP concentration elongated finger-like pores and suppressed the formation of large pores in the profile. When the NMP concentration was 9 wt.%, the finger-like pores nearly spanned the entire SL of the fabricated membrane. In addition, the open pores at the membrane bottom surface became smaller and fewer in number with the increase in the NMP concentration. The experimental FO results of the membranes indicated that internal concentration polarization reduction could be achieved by creating more long finger-like pores in the membrane SL as well as more and larger open pores at the membrane bottom surface. All of the fabricated membranes had a low rate of water flux (Jw) to reverse salt flux (Js). In addition, the membrane fabricated with 3 wt.% NMP had the best performance in minimizing the draw solute loss, with a high Jw/Js of 4.2 L/g.

Rapidly reversible adsorption of methane with a high storage capacity on the zeolite templated carbons with glucose as carbon precursors

Abstract Microporous carbon material with a high surface area of 1562 m 2 /g is prepared via an impregnation method with NaY zeolite as hard template. The results indicate that glucose, an environmental friendly material, could be used as the carbon precursor of zeolite templated carbons, in comparison with the toxicity of furfuryl alcohol and lower surface area of carbons generated from sucrose. Effects of reactant molar ratios and carbonization conditions on the textural properties are addressed. Although higher carbonization temperature favors more complete carbonization reactions, it also causes the collapse of small pores and the formation of large pores at the same time. Further characterizations of the obtained carbon materials with XRD, SEM and N 2 adsorption–desorption measurements are presented. Methane adsorption tests on carbon materials are investigated with the static volumetric method at 300 K. The higher adsorption amount of 9.9 wt.% was achieved at 3.5 MPa on sample with a higher micropore volume, indicating that micropores were playing a crucial role for the methane storage process.

Carbon Nanotube/Alumina/Polyethersulfone Hybrid Hollow Fiber Membranes with Enhanced Mechanical and Anti-Fouling Properties.

Carbon nanotubes (CNTs) were incorporated into alumina/polyethersulfone hollow fibre membranes to enhance the mechanical property and the efficiency of water treatment. Results show that the incorporation of CNTs can greatly limit the formation of large surface pores, decrease the void size in support layers and improve the porosity and pore connectivity of alumina/polyethersulfone membranes. As a result of such morphology change and pore size change, both improved flux and rejection were achieved in such CNTs/alumina/polyethersulfone membranes. Moreover, the CNTs/alumina/PES membranes show higher antifouling ability and the flux recoveries after being fouled by bovine serum albumin (BSA) and humic acid were improved by 84.1% and 53.2% compared to the samples without CNT incorporation. Besides the improvement in water treatment performance, the incorporation of CNTs enhanced the tensile properties of inorganic/polymer membranes. Therefore, such CNTs/alumina/PES hollow fiber membranes are very promising candidates for good filter media in industry, considering their high efficiency and high mechanical properties.

Pore morphology of low-k SiCxNy films prepared with a cyclic silazane precursor using plasma-enhanced chemical vapor deposition

Low-k SiCxNy films were prepared using radio-frequency plasma-enhanced chemical vapor deposition (PECVD), with only 1,3,5-trimethyl-1,3,5-trivinylcyclotrisilazane (VSZ) as the precursor; VSZ has cyclic Si–N–Si linkages and three pendent vinyl groups. At lower PECVD temperatures, SiCxNy films possess relatively low film densities, indicating the existence of a loose structure or voids within the cross-linked matrix structure. The pore morphology of SiCxNy films deposited at distinct temperatures were examined using grazing-incidence small-angle X-ray scattering, while the chemical bondings and structural information were analyzed using Fourier-transform infrared spectroscopy. At 100°C, SiCxNy films without porogen displaying 4.9nm pores and a mean pore spacing of 30.1nm generated low-density films because Si-(CH2)n-Si and/or Si-(CH2)n-CH3 could be incorporated free volume into the N–Si–C cross-linked structure under plasma deposition. At 300°C, the N–Si–C cross-linked structure and the some organic phase were disrupted and transformed into a denser structure, reducing pore size (3.5nm) and losing pore correlation. Thus, low deposition temperatures facilitate the formation of large pores and the ordering of the pores. Post annealing converted the 100 and 300°C as-deposited SiCxNy films into loose and dense structures, respectively, and maintained slightly reduced pore size and pore correlation in the annealed films.

Investigation of Durability of MEAs at Higher Temperature

PEFCs recently attract much more attention owing to the commercialization of fuel cell vehicles (FCVs). We are interested in the operation of PEFC at high temperature to improve overvoltage loss of reactions occurring in PEFC. In this study, extracting problems on catalytic degradation at higher temperature is particularly focused on, and then durability of MEAs at 105 °C is considered. MEAs were prepared using standard catalyst and electrolyte, Pt/Ketjen black catalyst (TEC10E50E, TKK Corp.) and Nafion. For obtained MEAs, current-voltage (IV) performance and durability were studied. Regarding to durability evaluation, two protocols[1] recommended by Fuel Cell Commercialization of Japan (FCCJ), “load cycles” and “start-stop cycles”, were in use. Three different conditions of operation temperature and humidity (80 °C/RH100%, 105 °C/RH57%, and 80 °C/RH57%) were applied. Regarding to durability analyses using “start-stop cycles”,[2] the increased loss of IV performance was clearly observed at 105 °C/RH57%. To understand degradation mechanism, the structure change of cathode layers was evaluated using FIB-SEM. The cathode layers became thinner in all the cases, but the condition of 105 °C/RH57% led to the thinnest layer. By further three-dimensional evaluation of cathode layers, increased size of pores constructing the catalyst layer was observed. The condition of 105 °C/RH57% resulted in formation of large pores especially near the Nafion membrane. Therefore, we believe such large pore formation derived by carbon corrosion led to collapsing and finally thinning the cathode layer. On evaluation of “load cycles” durability tests, the operation at 105 °C/RH57% resulted in lower durability than that at 80 °C/RH100%. However, the difference was much smaller comparing to the case in “stop-start cycles”. Regarding to Pt size evaluated by SEM (Figure 1), the size increase of Pt particles was more pronounced in the case of 80 °C/RH100%. The reason most likely comes from enhanced Pt growth under the higher water content. Thus, the decreased IV performance at 105 °C/RH57% involves additional factors such as Nafion degradation at the low humidity condition. Possible degradation mechanism using two different durability protocols is discussed with macrostructure change in cathode layers. Reference [1] A. Ohma et al., ECS Trans, 41(2011) 755. [2] M. Kitamura et al., ECS Trans, 64 (2014) 755. Figure 1

Comparative Thermal and Mechanical Performance of Geopolymers derived from Metakaolin and Fly Ash

AbstractThis paper presents comparative thermal and mechanical performance of geopolymers derived from metakaolin (MK), fly ash (FA), and their combinations. Bending and compression tests, thermogravimetric analysis, scanning electron microscopy and energy dispersive spectrum, mercury intrusion porosimetry, and dilatometric tests were conducted on three types of geopolymers (MK, MK-FA, and FA based) to evaluate comparative mechanical properties, thermal behavior, and microstructure. Results from these tests show that MK-based geopolymers experience significant thermal shrinkage, high mass loss, and large strength degradation after exposure to high temperatures up to 800°C. FA-based geopolymers exhibit low bending and compressive strength at ambient temperature due to incomplete reaction of fly ash particles and high retention in compressive strength at high temperature, and this is facilitated from the formation of large size pores as channels to release vapor pressure and sinter reaction of fly ash parti...

Contribution of the Juxtatransmembrane Intracellular Regions to the Time Course and Permeation of ATP-gated P2X7 Receptor Ion Channels

P2X7 receptors are ATP-gated ion channels that contribute to inflammation and cell death. They have the novel property of showing marked facilitation to repeated applications of agonist, and the intrinsic channel pore dilates to allow the passage of fluorescent dyes. A 60-s application of ATP to hP2X7 receptors expressed in Xenopus oocytes gave rise to a current that had a biphasic time course with initial and secondary slowly developing components. A second application of ATP evoked a response with a more rapid time to peak. This facilitation was reversed to initial levels following a 10-min agonist-free interval. A chimeric approach showed that replacement of the pre-TM1 amino-terminal region with the corresponding P2X2 receptor section (P2X7-2Nβ) gave responses that quickly reached a steady state and did not show facilitation. Subsequent point mutations of variant residues identified Asn-16 and Ser-23 as important contributors to the time course/facilitation. The P2X7 receptor is unique in having an intracellular carboxyl-terminal cysteine-rich region (Ccys). Deletion of this region removed the secondary slowly developing current, and, when expressed in HEK293 cells, ethidium bromide uptake was only ∼5% that of WT levels, indicating reduced large pore formation. Dye uptake was also reduced for the P2X7-2Nβ chimera. Surprisingly, combination of the chimera and the Ccys deletion (P2X7-2NβdelCcys) restored the current rise time and ethidium uptake to WT levels. These findings suggest that there is a coevolved interaction between the juxtatransmembrane amino and carboxyl termini in the regulation of P2X7 receptor gating.

Hybrid silica membranes with enhanced hydrogen and CO2 separation properties

Hybrid silica membranes are of great interest for molecular separation owing to their outstanding hydrothermal stability. Despite good separation properties in liquid applications, the selectivity for gas separations has yet been too low. Here, we report membranes from 1,2-bis(triethoxysilyl)ethane (BTESE) with H2/N2 permselectivity between 50 and over 400. The membranes are fabricated from a dip-sol with a H+:Si ratio of 0.01 that is applied onto a support system with a controlled low water content (pre-treated at RH<0.5%). For support systems pre-treated at 90% RH, H2/N2 permselectivities≤10 are obtained, indicating larger pores. The pore formation process is studied in situ by Small-Angle X-ray Scattering in a dedicated setup. The formation of larger pores can be understood by a higher condensation rate and longer drying times when more water is present. This results in a stronger network that better withstands the compressive forces during drying. By limiting both the water and acid contents in the dipped sol, a dense pore structure is obtained that gives the highest H2/N2 and CO2/CH4 permselectivities found to date for hybrid silica membranes. Further variation of the water and acid concentration will allow for additional tuning of the separation properties for both gas and liquid separation.

Hierarchical nanoporous metal/metal-oxide composite by dealloying metallic glass for high-performance energy storage

A free-standing nanoporous YNiCo metal/metal-oxide composite with hierarchical porosity is fabricated by chemically dealloying Al85Y6Ni6Co3 metallic glass in alkaline solutions. The mixed core–shell-like metal/metal-oxide structure formed during dealloying due to the active properties of these metals. Time-dependent etching experiments suggest that the formation of large and small pores occur simultaneously, which may be related to the different dissolution rate of Al at different sites. The nanoporous composite with a highly conductive metal core exhibits a high areal capacitance. Moreover, this strategy can be extended to fabricate other nanoporous composites considering that the composition of metallic glass can be easily tuned.

The natural, peptaibolic peptide SPF-5506-A4 adopts a β-bend spiral structure, shows low hemolytic activity and targets membranes through formation of large pores

The medium-length fungal peptaibol SPF-5506-A4 has been shown to inhibit formation of the Aβ peptide involved in Alzheimer'’s disease. As Aβ is a cleavage-product from the membrane-bound APP protein, we hypothesized that SPF-5506-A4's activity might be linked to membrane interactions in general. Here we describe the synthesis, structure and membrane interactions of SPF-5506-A4. The challenging synthesis was carried out on solid phase and a detailed conformational analysis in solution revealed a β-bend ribbon spiral core structure with flexible termini. Investigations of its membrane activity revealed low hemolytic activity, limited inhibition of both Gram-positive and Gram-negative cell growth and a preference for an overall negatively charged membrane surface mimicking the bacterial cell surface. SPF-5506-A4 is the first peptaibol to be shown to facilitate leakage of large (4.6nm diameter) fluorescence-labeled dextran from vesicles while leaving the vesicles intact. We conclude that SPF-5506-A4 follows the toroidal pore model in its mode of action.

HIV drug nucleoside reverse transcriptase inhibitors as promising anti-inflammation therapeutics by targeting P2X7-dependent large pore formation: one stone for two birds?

Nucleoside reverse transcriptase inhibitors (NRTIs) have been the backbones of HIV therapy since introduced in the late 1980s. These HIV drugs act by blocking the viral reverse transcriptase activity that is crucial in the HIV life cycle. In a recent study published by Science, Fowler et al. (2014) have shown that such antiretroviral agents can inhibit P2X7 receptor (P2X7R)-mediated Alu RNA-induced activation of caspase-1 in retinal pigment epithelial (RPE) cells and ATP-induced activation of caspase-1 in macrophage cells, independently of reverse transcriptase inhibition.