Freeze-thaw Technique Research Articles

Experimental study on the seismic behavior of RC shear walls after freeze-thaw damage

Previous research shows that frost action represents one of the most dangerous threats to reinforced concrete (RC) structures. However, very few experimental studies have been performed on the seismic behavior of frost-damaged RC shear walls. Thus, this paper used the artificial climate rapid freeze-thaw technique to simulate the freeze-thaw environment. Successive freeze-thaw cycle tests and quasi-static tests were performed on eight RC shear wall specimens to investigate the seismic performance of frost-damaged RC shear walls. The key variables include the number of freeze-thaw cycles (FTCs), concrete strength and axial load ratio. The seismic behavior of test specimens was evaluated in terms of damage process, hysteretic behavior, load carrying capacity, deformation capacity, energy dissipation capacity, and shear deformation. The experimental results indicated that as the number of FTCs increased, the flexural-shear failure of the RC shear walls shifts from a flexure-dominated mode to a shear-dominated one. Meanwhile, the load carrying capacity, deformation capacity, and energy dissipation capacity gradually decreased. Furthermore, the freeze-thaw cycle can weaken the shear resistance of RC shear walls and can increase the average shear distortion and the ratio of shear deformation to the total deformation under different loading states. Based on the test results, a calculation equation was proposed for RC shear wall skeleton curves considering the influence of freeze-thaw damage and axial load ratio.

Alginate Oligosaccharides Affect Mechanical Properties and Antifungal Activity of Alginate Buccal Films with Posaconazole.

Sodium alginate and its oligosaccharides through potential antifungal properties might improve the activity of antifungal drugs enhancing their efficacy and potentially reducing the frequency of application. Mucoadhesive buccal films are oral dosage forms designed for maintaining both local or systemic drug effects and seem to be a very promising alternative to conventional oral formulations. Hence, in this study, mucoadhesive buccal films based on the alginate and its oligosaccharide oligomer composed predominantly of mannuronic acid for the administration of posaconazole-antifungal drug from the azole group were developed. As the polymer gelation method, a relatively new freeze-thaw technique was chosen. All prepared formulations were examined for pharmaceutical tests, swelling, mechanical, and mucoadhesive properties. In addition, the influence of sodium alginate (ALG) and alginate oligosaccharides (OLG) on POS antifungal activity on Candida species was performed. It was observed that film formulation containing 1% ALG and 1% OLG (F2) was characterized by optimal mucoadhesive and swelling properties and prolonged drug release up to 5 h. Additionally, it was shown that OLG affected the growth reduction of all tested Candida spp. The obtained data has opened the way for future research for developing OLG-based dosage forms, which might increase the activity of antifungal drugs.

Nanocrystalline cellulose prepared by double oxidation as reinforcement in polyvinyl alcohol hydrogels

Abstract As a biopolymer with high mechanical strength, nanocellulose was increasingly studied to improve polymer properties. In this study, nanocrystalline cellulose (NCC) was efficiently isolated from eucalyptus pulp by double oxidation (ammonium persulfate oxidation and ultrasonic oxidation). The total yield of NCC (405.1 ± 180.5 nm long and 31.7 ± 9.5 nm wide) was 38.3%. A novel hybrid hydrogel was produced from polyvinyl alcohol (PVA) and NCC using the freeze-thaw technique. In this hybrid architecture, hydrogen bonds were formed between PVA and NCC. With the increasing proportion of NCC, the pore size of hydrogels shank gradually and the structure of the hybrid hydrogels became denser. The tensile strength of PVA/NCC hybrid hydrogels increased by 42.4% compared to the neat PVA hydrogel. The results showed that NCC can improve the swelling, thermal properties, and water evaporation rate of PVA hydrogels due to the hydrophilic hydroxyl groups of NCC and hydrogen bonds between PVA and NCC, indicating that PVA hydrogels would have a wider range of application due to the existence of NCC, a green hybrid filler. Most importantly, this novel double oxidation method for preparing nanocellulose will promote an efficient production of nanocellulose.

Fabrication and evaluation of a medicated hydrogel film with embelin from Embelia ribes for wound healing activity

BackgroundThere has been huge interest among the researchers to incorporate a medicinally active compounds in hydrogel sheets for effective treatment of wound healing. This research work involves development and evaluation of medicated hydrogel sheet incorporated with embelin which has both antimicrobial and wound healing activity. Embelin was isolated from the fruits of Embelia ribes and characterized by various physical and analytical methods like melting point, UV/VIS spectroscopy, and HPTLC. The hydrogel sheets containing polyvinyl alcohol and polyethylene glycol was prepared by freeze-thaw technique, where isolated embelin was successfully incorporated within the sheet. The prepared hydrogel sheets were further characterized by in vitro drug release study, swelling capacities, gel fraction, water vapor transmission rate (WVTR), mechanical strength, and scanning electron microscopy (SEM) study. Finally, the optimized hydrogel with embelin was evaluated for its wound healing efficacy in vivo using excision wound model on Sprague–Dawley rats.ResultsThe optimized hydrogel sheet had a composition of 5% PEG 400 and 10% PVA. It had acceptable in physico-chemical properties with respect to swelling capacities, gel fraction, water vapor transmission rate (WVTR), and mechanical strength The release of the drug from hydrogel followed zero order kinetics with more than 80% drug release within 12 h. The in vivo studies on the Sprague–Dawley showed faster healing process with embelin loaded hydrogels as compared to the control and market formulation.ConclusionsSheet hydrogel with 0.2% embelin was found to have huge potential for moist wound healing activity.

Developmental potential of surplus morulas with delayed and/or incomplete compaction after freezing-thawing procedures

BackgroundMorulas with delayed growth sometimes coexist with blastocysts. There is still limited evidence regarding the optimal disposal of surplus morulas. With the advancement of vitrification, the freezing-thawing technique has been widely applied to zygotes with 2 pronuclei, as well as embryos at the cleavage and blastocyst stages. The freezing of morulas, however, has rarely been discussed. The purpose of this study was to investigate whether these poor-quality and slow-growing morulas are worthy of cryopreservation.MethodsThis is a retrospective, observational, proof-of-concept study. A total of 1033 day 5/6 surplus morulas were cryopreserved from January 2015 to December 2018. The study included 167 women undergoing 180 frozen embryo transfer cycles. After the morulas underwent freezing-thawing procedures, their development was monitored for an additional day. The primary outcome was the blastocyst formation rate. Secondary outcomes were clinical pregnancy rate, live birth rate and abortion rate.ResultsA total of 347 surplus morulas were thawed. All studied morulas showed delayed compaction (day 5, n = 329; day 6, n = 18) and were graded as having low (M1, n = 54), medium (M2, n = 138) or high (M3, n = 155) fragmentation. The post-thaw survival rate was 79.3%. After 1 day in extended culture, the blastocyst formation rate was 66.6%, and the top-quality blastocyst formation rate was 23.6%. The day 5 morulas graded as M1, M2, and M3 had blastocyst formation rates of 88.9, 74.0, and 52.8% (p < 0.001), respectively, and the top-quality blastocyst formation rates were 64.8, 25.2, and 9.0% (p < 0.001), respectively. The clinical pregnancy rate was 33.6%.ConclusionsThe post-thaw blastocyst formation rate was satisfactory, with approximately one-half of heavily fragmented morulas (M3) developing into blastocysts. Most of the poor-quality morulas were worth to freeze, with the reasonable goal of obtaining pregnancy and live birth. This alternative strategy may be a feasible approach for coping with poor-quality surplus morulas in non-PGS (preimplantation genetic screening) cycles.

High yield and concentration exfoliation of defect-free 2D nanosheets via gentle water freezing-thawing approach and stabilization with PVP

The scalable exfoliation of bulk hexagonal boron nitride (h-BN) into high quality mono- or few-layer nanosheets still remains an enormous challenge and becomes the bottleneck to essential research and applications of h-BN. Here, a gentle water freeze-thaw technique, with a particular focus on the use and efficacy of polyvinylpyrrolidone (PVP) as a dispersant, is put forth as a novel and efficient approach for exfoliation of BNNSs in a large yield (32%) and high concentration (1.64 mg ml−1). Through the investigation into the exfoliation mechanism, it’s found that the essence of peeling driving forces comes from the high adsorption ability of PVP, which presents strong π-π interaction with few-layered h-BN nanosheets (BNNS) and the expansion of the volume of water upon freezing. The results of systematic characterization for thin flakes show that dispersions with PVP to initial h-BN concentration ratio as high as 27%, should be favorable for the preparation of samples and devices with minimal interference from PVP. Furthermore, the synthesized BNNSs after 30 repetitive freeze−thaw cycles (BNNSs-30) with original lateral dimensions consists primarily of fewer than five atomic layers, as evidenced by electron microscopy. XRD, XPS and Raman results confirm that as prepared materials have pure phase, and no defects are observed. This simple and novel strategy can synthesize large quantities of defect-free BNNSs. Above all, the PVP assisted freeze-thaw method not only paves the way for wide applications, but also can be versatile, having been used to produce dozens of different 2D nanosheets, such as MoS2 and MoO3 nanoflakes, in large yields.

Pullulan/Poly(Vinyl Alcohol) Composite Hydrogels for Adipose Tissue Engineering.

Composite hydrogels based on pullulan (HP) and poly(vinyl alcohol) (PVA) were both prepared by simple chemical crosslinking with sodium trimethaphosphate (STMP) or by dual crosslinking (simultaneously chemical crosslinking with STMP and physical crosslinking by freeze-thaw technique). The resulting hydrogels and cryogels were designed for tissue engineering applications. PVA, with two different molecular weights (47,000 and 125,000 g/mol; PVA47 and PVA125, respectively), as well as different P/PVA weight ratios were tested. The physico-chemical characterization of the hydrogels was performed by FTIR spectroscopy and scanning electron microscopy (SEM). The swelling kinetics, dissolution behavior, and degradation profiles in simulated physiological conditions (phosphate buffer at pH 7.4) were investigated. Pullulan concentration and the crosslinking method had significant effects on the pore size, swelling ratio, and degradation profiles. Cryogels exhibit lower swelling capacities than the conventional hydrogels but have better stability against hydrolitic degradation. Biocompatibility of the hydrogels was also investigated by both MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) and LDH (lactaten dehydrogenase) assay. The MTT and LDH assays proved that dual crosslinked HP/PVA125 (75:25, w/w) scaffolds are more biocompatible and promote to a greater extent the adhesion and proliferation of L929 murine fibroblast cells than chemically crosslinked HP/PVA47 (50/50, w/w) scaffolds. Moreover, the HP/PVA125 cryogel had the best ability for the adipogenic differentiation of cells. The overall results demonstrated that the HP/PVA composite hydrogels or cryogels are suitable biomaterials for tissue engineering applications.

Concentration of Aqueous Residue from Isada Krill Using Freezing-Thawing Technique

Concentration of Aqueous Residue from Isada Krill Using Freezing-Thawing Technique

Antimicrobial starch-citrate hydrogel for potential applications as drug delivery carriers

Antimicrobial starch-citrate hydrogels were prepared by physical crosslinking of starch citrate, Poly(vinyl alcohol (PVA) and Polyethylene glycol (PEG) via the freeze–thaw technique. This starch-citrate hydrogel exhibited excellent antimicrobial activity against various bacteria strains such as Escherichia coli (gram negative), Staphylococcus pyogenes (gram negative), Salmonella thypimurium (gram positive) and Streptococcus aureus (gram positive). Most notably it showed antimicrobial activity against the multiple antibiotic resistant (M.A.R) superbugs (Klebsiella pneumoniae). Penicillin G loaded starch-citrate hydrogel was observed to exhibit a sustain release profile over 7 days. Penicillin G loaded starch-citrate hydrogel also exhibited synergistic antimicrobial activities against Salmonella thypimurium, Escherichia coli (E-coli) (O15:H7) and E-coli control (ATC 2925). The potential application of starch-citrate hydrogel as both antimicrobial agent and drug delivery carriers is therefore envisaged.

Co-encapsulation of Rifampicin and Isoniazid in Crude Soybean Lecithin Liposomes

Despite the well-known anti-mycobacterial actions of isoniazid (INH) and rifampicin (RIF), the clinical success of tuberculosis (TB) therapy requires prolonged administration of multiple drugs in high doses, which often result in frequent adverse effects and low patient adherence. Although liposomes are promising candidates for controlled delivery of anti-TB drug, the high cost of synthetic and highly purified natural lipids currently used in liposomal technology might preclude the universal application of therapeutic liposomes. This work aimed at evaluating the potential of a cost-effective lipid material, crude soybean lecithin (CL), to co-encapsulate RIF and INH for liposomal dual delivery. RIF was encapsulated in CL-liposomes with/without cholesterol using film hydration method, after which INH was incorporated using a freeze-thawing technique. Dynamic light scattering, differential scanning calorimetry, X-ray diffraction and dialysis were used for liposome characterization. Liposomes containing CL alone (CLL) exhibited 90 % encapsulation efficiency for RIF and 59 % for INH. The mean size and surface charge of CLL were 1114 nm and -63 mV respectively. In addition, CLL showed a controlled release profile for the co-encapsulated drugs. CLL would be promising vehicles for macrophage-targeting drug delivery. The present findings demonstrate the feasibility of using CL for preparation of combination products for liposomal delivery. Keywords: Tuberculosis, isoniazid, rifampicin, drug delivery, liposomes, soybean lecithin.

Bentonite Reinforced Tough Composite Hydrogels as Potential Artificial Articular Cartilage

Novel tough composite hydrogels were prepared from inorganic bentonite(IB), polyvinyl alcohol(PVA) and polyethylene glycol(PEG) by means of a freeze-thaw technique, during which IB acted as multifunctional physically crosslinking junction and a filler to bridge the 3D network hydrogel; while the physical adsorption between IB and the polymer chains served as sacrificial bonds and increased the energy dissipation efficiency. The effects of different content of IB(wIB) on the morphological, thermal, swelling, and mechanical properties of the hydrogels were investigated. It was found that the added IB promoted the material crosslinking and stability, and the mechanical properties of the hydrogels were significantly improved with increasing wIB. The highest tensile stress of the hydrogel was achieved(1.1 MPa) when wIB was 5%. The synthesized hydrogels with high mechanical strength and low friction coefficient are potential candidate materials for artificial cartilage.

Electroactive Hydrogels Made with Polyvinyl Alcohol/Cellulose Nanocrystals.

This paper reports a nontoxic, soft and electroactive hydrogel made with polyvinyl alcohol (PVA) and cellulose nanocrystal (CNC). The CNC incorporating PVA-CNC hydrogels were prepared using a freeze–thaw technique with different CNC concentrations. Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy results proved the good miscibility of CNCs with PVA. The optical transparency, water uptake capacity and mechanical properties of the prepared hydrogels were investigated in this study. The CNC incorporating PVA-CNC hydrogels showed improved displacement output in the presence of an electric field and the displacement increased with an increase in the CNC concentration. The possible actuation mechanism was an electrostatic effect and the displacement improvement of the hydrogel associated with its enhanced dielectric properties and softness. Since the prepared PVA-CNC hydrogel is nontoxic and electroactive, it can be used for biomimetic soft robots, actively reconfigurable lenses and active drug-release applications.

Investigation of the effects of orientation on freeze/thawed Polyvinyl alcohol hydrogel properties

Although hydrogels produced via freeze-thawing (F/T) technique have been thoroughly studied in literature, stretching hydrogels in between freeze-thawing cycles could change its properties for drug delivery systems. The PVA hydrogels and Caffeine-contained PVA hydrogels were created by freezing for 20 min using liquid nitrogen and 4 h thawing at 4 °C. The results revealed that the PVA/CAF hydrogel with two F/T cycles followed by two stretching (S) cycles delivered the best response. Furthermore, this sample had the highest degree of crystallinity and Young’s modulus of 36% and 1462 MPa, respectively, which makes this hydrogel very stiff in comparison to all other samples. Moreover, it fits in the Hixson-Crowell drug release model with the highest drug of releasing rate of 15 min, and the highest swelling degree of 470%. Therefore, the study suggests that the number of F/T cycles and stretching cycles influences the properties of the resulting hydrogels and the caffeine release rate.

Kinetics and thermodynamics of swelling and dissolution of PVA gels obtained by freeze-thaw technique

PVA hydrogels obtained by freeze-thaw technique find important applications in regenerative medicine and controlled drug delivery technologies. The novelty of this contribution is the establishment of the cross-dependency of kinetics and thermodynamics parameters of swelling and dissolution of PVA hydrogels with the freeze-thaw process main variables. The work also establishes cross-correlations between the freeze-thaw process conditions, the gel microstructure and the properties of obtained hydrogel/hydrosoluble PVA materials. The conditions of the freeze-thaw process were selected by following an experimental design. The time evolution of the water up-take of PVA hydrogels was measured and used as main raw data generator. A 4-D surface response of the swelling and dissolution parameters in the space of freeze-thaw process variables was obtained and analyzed in terms of the resulting hydrogel and hydrosoluble polymeric structures. The 4-D surface response in the space of freeze-thaw process variables can predict very precisely the swelling and dissolution of PVA systems and their conditions for developing fully crosslinked hydrogel networks to fully hydrosoluble polymeric bodies.

Innovative application of PVA hydrogel for the forming of porous Si3N4 ceramics via freeze-thaw technique

The generation of PVA hydrogel through freeze-thaw technique has been utilized innovatively to complete the forming process of porous Si3N4 ceramics in the present work. In comparison with gelcasting method, green body with comparable strength has been achieved by this novel forming process, which is owing to the strong function of PVA gel caused by the freeze-thaw cycles. Herein, Si3N4 hollow spheres are employed as pore-forming agent in order to achieve closed pores. The morphology of green body illustrates that pores originate from both hollow spheres and the space occupied by ice crystals, and the latter gives rise to the increase of porosity around 10%. Studies on the mechanical, dielectric and thermal properties of porous Si3N4 ceramics dependent on freeze-thaw cycle and freezing time have been performed. It is found that porous ceramics treated for freeze-thaw 2 cycles with freezing time of 24 h present excellent properties, which are promising to be applied as radome materials.

New cryogels based on polymers and zeolite L for controlled Enalapril maleate release

New composite cryogels for controlled release of Enalapril Maleate (EM) were developed based on two biodegradable polymers, poly(vinyl alcohol) (PVA) and pullulan (PU) as polymer matrix, and zeolite L (ZL) as inorganic filler. The cryogels were prepared by freeze-thaw technique using different concentrations of the components, the EM being introduced directly during the cryogel preparation. The samples were characterized by FTIR spectroscopy, scanning electron microscopy and water vapor sorption-desorption isotherms. The presence of PU and ZL in the cryogels led to the modification of FTIR characteristic absorption bands of EM and contributed to the modification of the cryogel morphology. The sample moisture absorption was in the range 5.28–18.20%, determined at a relative humidity RH = 82%. Cryogels based on PVA, PU and ZL components showed a longer EM release period compared to cryogels containing only PVA. The analysis of in vitro EM release kinetics was performed using three predictable models: zero and first order kinetics, and Korsmeyer-Peppas model. Korsmeyer-Peppas model best fitted the release data.

Extract of mouse embryonic stem cells induces the expression of pluripotency genes in human adipose tissue-derived stem cells.

Objective(s):In some previous studies, the extract of embryonic carcinoma cells (ECCs) and embryonic stem cells (ESCs) have been used to reprogram somatic cells to more dedifferentiated state. The aim of this study was to investigate the effect of mouse ESCs extract on the expression of some pluripotency markers in human adipose tissue-derived stem cells (ADSCs).Materials and Methods:Human ADSCs were isolated from subcutaneous abdominal adipose tissue and characterized by flow cytometric analysis for the expression of some mesenchymal stem cell markers and adipogenic and osteogenic differentiation. Frequent freeze-thaw technique was used to prepare cytoplasmic extract of ESCs. Plasma membranes of the ADSCs were reversibly permeabilized by streptolysin-O (SLO). Then the permeabilized ADSCs were incubated with the ESC extract and cultured in resealing medium. After reprogramming, the expression of some pluripotency genes was evaluated by RT-PCR and quantitative real-time PCR (qPCR) analyses.Results:Third-passaged ADSCs showed a fibroblast-like morphology and expressed mesenchymal stem cell markers. They also showed adipogenic and osteogenic differentiation potential. QPCR analysis revealed a significant upregulation in the expression of some pluripotency genes including OCT4, SOX2, NANOG, REX1 and ESG1 in the reprogrammed ADSCs compared to the control group.Conclusion:These findings showed that mouse ESC extract can be used to induce reprogramming of human ADSCs. In fact, this method is applicable for reprogramming of human adult stem cells to a more pluripotent sate and may have a potential in regenerative medicine.

Oil recovery from waste cutting fluid via the combination of suspension crystallization and freeze-thaw processes

A set of experiments was conducted so as to detect the influences of concerned factors including freezing temperature, stirring speed and pH adjustment on combined method of suspension crystallization and freeze-thaw technique for waste cutting fluid treatment and oil recovery. The optimal operating conditions of freezing temperature and stirring speed were confirmed by the experiment result to be −8 °C and 300 rpm respectively, while the chemical oxygen demand (COD) removal efficiency was above 90% without pH adjustment. The concentrates from the suspension crystallization were further treated by freeze-thaw method, and the oil recovery rate could reach to 95%. The micrograms of concentrates from suspension crystallization showed that the size of oil droplets in concentrates was about 148 μm in diameter, which became significantly larger after the suspension crystallization process. As a consequence, higher possibility for oil droplets to gather and separate was provided by their expanded size. The results of infrared spectroscopy studies indicated that the oil recovered from waste cutting fluid precipitates was mainly composed of esters. Up to 2.85 kg fat was recovered from 1 m3 of waste cutting fluid, while the required power of the combined freezing process is theoretically 42 kW h. Based on the results, a freezing/concentration concept for oil recovery from waste cutting fluid is proposed.

Magnetically responsive gels based on crosslinked gelatin: An overview on the synthesis, properties, and their potential in water remediation

ABSTRACTEfficient adsorbent ferrogels based on gelatin and dextrose were synthetized using freeze-thawing technique. Two different methodologies of magnetic particles incorporation were developed. An exhaustive morphologic and thermal characterization of hydro and ferrogels was achieved. The performance of ferrogel in water remediation was tested using Cd(II) as model contaminant. The adsorption ability was evaluated as a function of pH, initial heavy metal concentration, and contact time. The maximum adsorption capacity was 35 mg of Cd(II)/g of adsorbent. Ferrogel was regenerated by washing with distilled water, maintaining efficiency of almost 30% after the fourth cycle.

Freeze–Thaw-Induced Gelation of Hyaluronan: Physical Cryostructuration Correlated with Intermolecular Associations and Molecular Conformation

Physically cross-linked hydrogels from hyaluronan (hyaluronic acid, HA) were prepared by a freeze–thaw technique at low pH. The effect of the freezing–thawing of HA solutions on the formation of physical cryogels is typical for the processes of noncovalent cryostructuration that takes the advantages of mild fabrication conditions and the absence of organic solvents and toxically cross-linking agents. The effects of processing steps (freezing time and number of freeze–thaw cycles), HA molecular weight (Mw), and the addition of typical polycarboxylic and polyhydric small molecules such as dicarboxylic acids and polyols on the formation of HA cryotropic hydrogels were investigated. Results verified that long freezing time and repeated freeze–thaw cycles benefited the alignment of polymer chains in the unfrozen liquid microphase, thereby promoting the formation of intermolecular aggregations and dense fibrillar network structures. High Mw of HA endowed the cryogel with strong mechanical strength. The influences of various small molecules on the cryogelation of HA revealed the different intermolecular association patterns in the gel network. Both succinic and glutaric acids participated in HA cryogelation, whereas oxalic, malic, and tartaric acids as well as some polyols (glycol, butanediol, and glycerol) inhibited the cryostructuration of HA. Hydrogen bonding and intermolecular interactions in acidic cryogels and in neutral cryogels obtained by in situ neutralizing the acidic cryogel were discussed at the molecular level in correlation with intermolecular associations and molecular conformation. A gelation mechanism for HA cryogel was proposed. In addition, experimental findings showed that the neutral HA cryogels possessed enhanced thermostability, resistance to acid decomposition, and enzyme degradation which are essentially important properties for biomaterials.