Rheological and thermal characteristics of a two phase hydrogel system for potential wound healing applications

Hydrogels fabricated from single polymers have been extensively investigated for wound healing applications. However, in many cases a single polymer cannot meet divergent demands in terms of both properties and performance. In this work, a two phase hydrogel was prepared by physically imbedding a xerogel in the core of a freeze thawed hydrogel. The outer hydrogel was prepared by freeze thawing poly (vinyl alcohol) (PVA) and poly (acrylic acid) (PAA) while the xerogels were prepared by UV polymerisation of 1-vinyl-2-pyrrolidinone (NVP). The rheological results indicated that the two phase hydrogels over a period of 2 weeks formed a strong interface and demonstrated greater physical strength. This suggested that the inner gel containing PVP diffused into the PVA/PAA hydrogel, which in turn increased hydrogen bonding, resulting in the overall increase in the stiffness of the gel. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) confirmed hydrogen bonding had occurred between the constituents of the two phase hydrogels. Thermal analysis suggested that T _g of each of the samples was above 80 °C, which indicated no impact on the behaviour of the gel at body temperature, but did however, give an indication of the stiffness of the dry polymer.     

Poly (vinyl alcohol) hydrogels for pH dependent colon targeted drug delivery

Oral drug administration is convenient with pH dependent drug delivery system since the drug has to pass through different pH environments in gastro intestinal (GI) tract. The pH dependent swelling/shrinking behavior of hydrogel drug carrier controls the drug release without affecting the function of drug. pH dependent hydrogels of poly (vinyl alcohol) (PVA) were prepared by cross linking with maleic acid (MA). The hydrogels were characterized by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, DSC, porosimetry, SEM, TEM, biocompatibility study and by measuring their swelling behavior in water, simulated gastric fluid (SGF) and intestinal fluid (SIF). Swelling of the hydrogels was found to be highest in SIF (pH: 7.5) and lowest in SGF (pH: 1.2) resembling that required in colon targeted drug delivery systems. Since the swelling behavior of the gel is pH dependent, these hydrogels were studied for colon targeted drug delivery in an in-vitro set-up resembling the condition of GI tract. The ratio of PVA and MA in the hydrogel was varied to study the effect on the drug diffusion rate. For drug delivery study, vitamin B₁₂ and salicylic acid were used as model drugs. The hydrogel, loaded with model drugs vitamin B₁₂ and salicylic acid also demonstrated colon specific drug release with a relatively higher drug release in SIF (pH: 7.5) than that in SGF (pH: 1.2).     

Thermoresponsive hyperbranched copolymer with multi acrylate functionality for in situ cross-linkable hyaluronic acid composite semi-IPN hydrogel

Thermoresponsive polymers have been widely used for in situ formed hydrogels in drug delivery and tissue engineering as they are easy to handle and their shape can easily conform to tissue defects. However, non-covalent bonding and mechanical weakness of these hydrogels limit their applications. In this study, a physically and chemically in situ cross-linkable hydrogel system was developed from a novel thermoresponsive hyperbranched PEG based copolymer with multi acrylate functionality, which was synthesized via an ‘one pot and one step’ in situ deactivation enhanced atom transfer radical co-polymerization of poly(ethylene glycol) diacrylate (PEGDA, M_n = 258 g mol⁻¹), poly(ethylene glycol) methyl ether methacrylate (PEGMEMA, M_n = 475 g mol⁻¹) and (2-methoxyethoxy) ethyl methacrylate (MEO₂MA). This hyperbranched copolymer was tailored to have the lower critical solution temperature to form physical gelation around 37°C. Meanwhile, with high level of acrylate functionalities, a chemically cross-linked gel was formed from this copolymer using thiol functional cross-linker of pentaerythritol tetrakis (3-mercaptopropionate) (QT) via thiol-ene Michael addition reaction. Furthermore, a semi-interpenetrated polymer networks (semi-IPN) structure was developed by combining this polymer with hyaluronic acid (HA), leading to an in situ cross-linkable hydrogel with significantly increased porosity, enhanced swelling behavior and improved cell adhesion and viability both in 2D and 3D cell culture models.     

Thermoreversible behavior of κ-carrageenan and its apatite-forming ability in simulated body fluid

Osteoconductive materials with self-setting ability have received much attention because their properties allow developing injectable materials for bone defects. Thermosensitive hydrogel with ability of bone-like apatite formation in a body environment is a candidate of injectable bone fillers with osteoconductivity because the apatite formation on materials is an essential to show osteoconduction. The present study focused on the development of a thermosensitive hydrogel through modifications of the sulphonic groups of the polysaccharide, κ-carrageenan, with potassium chloride (KCl) and calcium chloride (CaCl₂). We found that the gelation temperature of κ-carrageenan solutions increased with increasing amounts of K⁺ ions. Apatite formation was observed on the gel after exposure to simulated body fluid for 0.5 day when the gel was prepared with a molar ratio of Ca²⁺/sulfonic groups = 1.5. These results indicate that a thermosensitive κ-carrageenan hydrogel with apatite-forming ability was obtained through the incorporation of K⁺ and Ca²⁺ ions into the solution.     

FTIR spectroscopy characterization of poly (vinyl alcohol) hydrogel with different hydrolysis degree and chemically crosslinked with glutaraldehyde

In this work, poly (vinyl alcohol) (PVA) hydrogels with different degree of hydrolysis (DH) were prepared by chemical crosslinking with glutaraldehyde (GA). The nanostructure of the resulting hydrogels was investigated by Fourier Transform Infrared Spectroscopy (FTIR) and Synchrotron small-angle X-ray scattering characterization (SAXS). In vitro tests were performed by swelling ratio assays in different pH solutions. The infrared spectra of the crosslinked PVA showed absorption bands of the acetal bridges resulted from the reaction of the GA with the OH groups from PVA. Also the FTIR spectroscopy was used to determine the crystallinity of the PVA film based on the relative intensity of the vibration band at 1141 cm^− 1. The results have showed an increase of hydrogel crystallinity with higher DH of PVA. SAXS patterns have clearly indicated important modifications on the PVA semicrystalline structure when it was crosslinked by GA. The swelling ratio was significantly reduced by chemically crosslinking the PVA network. PVA-derived hydrogel with chemically modified network was found to be pH-sensitive, indicating a high potential to be used in drug delivery polymer system.     

Release of Ftorafur from pH-sensitive hydrogels with hyperbranched poly(4-vinylbenzyl chloride) moieties

Drug release behavior of a hydrogel is related to its transport mechanism, which is dominated by structure of the hydrogel. Therefore, we prepared pH-sensitive poly(4-vinylpyridine) (P4VP) hydrogels with hyperbranched poly(4-vinylbenzyl chloride) (PVBC; M_n = 2391 g/mol, PDI = 1.87, the minimum percent linearity = 12.4%) moieties (P4VP-PVBC) by atom transfer radical polymerizations (ATRP) in two steps. A PVBC moiety provides the hydrogel with a microenvironment, which may encapsulate guest molecules like drug. The presence of the microenvironment could affect drug transport in the hydrogel matrices. To understand this, we used Ftorafur as drug molecule, and investigated release behavior of the P4VP-based hydrogels. Diffusion and transport mechanism of Ftorafur in the P4VP-based hydrogels was analyzed by early-time and late-time approximation diffusion coefficients. It was found that the transport behavior of Ftorafur was related to the presence of the PVBC moiety and external pH. The presence of the PVBC moiety could sustain release of Ftorafur.     

Recognition of four structurally resembled benzoic acid derivatives by albumin-crosslinked poly(acrylamide) hydrogel

Poly(acrylamide) hydrogels crosslinked by bovine serum albumin (BSA) were prepared by the introduction of vinyl groups into BSA and subsequent co-polymerization with acrylamide (AAm). The hydrogels were loaded with four structurally resembled benzoic acid derivatives such as salicylic acid, o-anisic acid, salicylamide and sodium benzoate, and their release from the hydrogel was investigated. The affinity of these four compounds for BSA gradually decreased in a following order; salicylic acid > o-anisic acid > salicylamide > sodium benzoate. The amounts of compounds loaded on a hydrogel of high BSA content and the duration of their release were found to be dependent on the affinity for BSA clearly reflecting the subtle difference in BSA affinity of each compound. Therefore, this hydrogel would be used not only as a sustained drug release carrier, but also a facile tool to evaluate the binding of various compounds to serum albumin.     

Preparation and characterization of hybrid pH-sensitive hydrogels of chitosan-co-acrylic acid for controlled release of verapamil

In the present work crosslinked hydrogels based on chitosan (CS) and acrylic acid (AA) were prepared by free radical polymerization with various feed compositions using N,N methylenebisacrylamide (MBA) as crosslinking agent. Benzoyl peroxide was used as catalyst. Fourier transform infrared spectra (FTIR) confirmed the formation of the crosslinked hydrogels. This hydrogel is formed due to electrostatic interaction between cationic groups in CS and anionic groups in AA. Prepared hydrogels were used for dynamic and equilibrium swelling studies. For swelling behavior, effect of pH, polymeric and monomeric compositions and degree of crosslinking were investigated. Swelling studies were performed in USP phosphate buffer solutions of varying pH 1.2, 5.5, 6.5 and 7.5. Results showed that swelling increased by increasing AA contents in structure of hydrogels in solutions of higher pH values. This is due to the presence of more carboxylic groups available for ionization. On the other hand by increasing the chitosan content swelling increased in a solution of acidic pH, but this swelling was not significant and it is due to ionization of amine groups present in the structure of hydrogel. Swelling decreased with increase in crosslinking ratio owing to tighter hydrogel structure. Porosity and sol-gel fraction were also measured. With increase in CS and AA contents porosity and gel fraction increased, whereas by increasing MBA content porosity decreased and gel fraction increased. Furthermore, diffusion coefficient (D) and the network parameters i.e., the average molecular weight between crosslinks (M_c), polymer volume fraction in swollen state (V_2s), number of repeating units between crosslinks (M_r) and crosslinking density (q) were calculated using Flory-Rehner theory. Selected samples were loaded with a model drug verapamil. Release of verapamil depends on the ratios of CS/AA, degree of crosslinking and pH of the medium. The release mechanisms were studied by fitting experimental data to model equations and calculating the corresponding parameters. The result showed that the kinetics of drug release from the hydrogels in both pH 1.2 and 7.5 buffer solutions was mainly non-Fickian diffusion.     

Poly (vinyl alcohol)/poly (acrylamide‐ co ‐diallyldimethyl ammonium chloride) semi‐IPN hydrogels for ciprofloxacin hydrochloride drug delivery

Herein, the authors developed a new and potential semi‐interpenetrating polymer network (semi‐IPN) hydrogels of poly vinyl alcohol (PVA), acryl amide and diallyldimethyl ammonium chloride employing chemical cross‐linker N, N''‐methylene bisacrylamide (NNMBA) and ammonium persulphate as an initiator by radical polymerisation. To analyse the copolymer formation between two monomers and IPN cross‐linking reaction, the resulting hydrogel was characterised by Fourier transform infrared spectroscopy and the surface morphology was analysed using scanning electron microscopy. Differential scanning calorimetry and X‐ray diffraction studies were also carried out for investigating drug loading and distribution and swelling experiments were carried out for the uptake of water. In vitro release of ciprofloxacin hydrochloride from hydrogel was performed at intestinal conditions. The amount of PVA, NNMBA and total monomer concentration was found to strongly control the drug release behaviour from the hydrogels.Inspec keywords: hydrogels, polymer blends, biomedical materials, drug delivery systems, polymerisation, Fourier transform infrared spectra, surface morphology, scanning electron microscopy, differential scanning calorimetry, X‐ray diffraction, swelling, biological organs, ammonium compoundsOther keywords: PVA‐poly(acrylamide‐co‐diallyldimethyl ammonium chloride) semiIPN hydrogels, ciprofloxacin hydrochloride drug delivery, semiinterpenetrating polymer network hydrogels, polyvinyl alcohol, acryl amide, diallyldimethyl ammonium chloride, chemical crosslinker N,N''‐methylene bisacrylamide, ammonium persulphate, radical polymerisation initiator, NNMBA, copolymer formation, IPN crosslinking reaction, Fourier transform infrared spectroscopy, surface morphology, scanning electron microscopy, differential scanning calorimetry, X‐ray diffraction, drug loading, drug distribution, swelling, water uptake, in vitro ciprofloxacin hydrochloride release, intestinal conditions, total monomer concentration, drug release behaviour     

Hydroxybutyl chitosan thermo-sensitive hydrogel: a potential drug delivery system

Drug delivery mediated by hydrogel has shown great promise in controlled drug release field. We report here the development of a hydroxybutyl chitosan (HBC) thermo-sensitive gel to deliver doxorubicine hydrochloride (DOX·HCl) for cancer treatment. Concentrated HBC aqueous solution could transform into hydrogel within 30 s after injection under physiological temperature in non-chemical fashion. The properties of the HBC gels including chemical structure, surface morphology, and rheologic properties were studied. Gelation temperature and gelation time of HBC could be adjusted by HBC concentrations. The gel erosion rate in vivo was faster than solubilization rate in vitro. The mild inflammatory response caused by implantation of the hydrogel was acceptable. The DOX·HCl (1 mg/ml) loaded HBC gels displayed slow release rates that were independent of the HBC concentration, and significantly reduced viability of 4T-1 cells compared with the HBC gels after 1 day incubation. These results indicate that thermo-sensitive HBC hydrogels have promising potential as an injectable drug carrier for pharmaceutical applications.     

Effects of crosslinking ratio,model drugs,and electric field strength on electrically controlled release for alginate-based hydrogel

The drug release characteristics of calcium alginate hydrogels, (Ca-Alg), under an electric field assisted transdermal drug delivery system were systematically investigated. The Ca-Alg hydrogels were prepared by the solution-casting using CaCl₂ as a crosslinking agent. The diffusion coefficients and the release mechanism of the anionic model drugs, benzoic acid and tannic acid, and a cationic model drug, folic acid on the Ca-Alg hydrogels were determined and investigated using a modified Franz-Diffusion cell in an MES buffer solution of pH 5.5, at a temperature of 37°C, for 48 h. The influences of the crosslinking ratio, —the mole of the crosslinking agent to the mole of the alginate monomer—mesh size, model drug size, drug charge, electric field strength, and electrode polarity were systematically studied. The drug diffusion coefficient decreased with an increasing crosslinking ratio and drug size for all of the model drugs. The drug diffusion coefficient is precisely controlled by an applied electric field and the electrode polarity depending on the drug charge, suitable for a tailor-made transdermal drug delivery system.     

Mucoadhesive Polymeric Hydrogels for Nasal Delivery of Acyclovir

The study evaluated different mucoadhesive polymeric hydrogels for nasal delivery of acyclovir. Gels containing poly-N-vinyl-2-pyrrolidone (PVP) were prepared with crosslinking achieved by irradiation with a radiation dose of 15 kGy being as efficient as 20 kGy. Gels containing chitosan and carbopol were also evaluated. The mucoadhesive properties of gels were measured by a modification of a classical tensile experiment, employing a tensile tester and using freshly excised sheep nasal mucosa. Considering the mucoadhesive force, chitosan gel and gel prepared with 3% PVP in presence of polyethylene glycol (PEG) 600 were the most efficient. The in vitro drug release depended on the gel composition. Higher release rates were obtained from PVP gels compared to chitosan or carbopol gels. The release rate of drug from PVP gels was increased further in presence of PEG or glycerol. Histopathological investigations proved that the PVP was a safe hydrogel to be used for mucosal delivery. The PEG in gel formulations caused less damages to the nasal mucosal compared to formulation containing glycerol.     

In vitro release profile of anti-ulcer drug rabeprazole from biocompatible psyllium-PVA hydrogels

The present article discusses the synthesis, characterization and haemocompatibility behaviour of the psyllium-PVA hydrogels prepared by chemical method in the presence of N,N′-methylenebisacrylamide. These hydrogels have been characterized by Fourier Transform infrared spectroscopy, thermo gravimetric analysis, swelling and drug release studies. The release of model drug rabeprazole sodium from the drug loaded hydrogels occurred through non-Fickian diffusion mechanism. Psyllium itself acts as anti-ulcer agent and release of rabeprazole from the drug loaded hydrogels may enhance the curing potential of the drug delivery device. The haemocompatibility was evaluated by studying the blood interactions with hydrogels with reference to thrombogenicity and haemolytic potential. Thrombogenicity results indicate that hydrogels are non-thrombogenic as the weight of clot formed and thrombus percentage for hydrogels was less than the positive control. The haemolytic index has been observed <5%. These observations indicate that these hydrogels are haemo-compatible and hence could be used for oral administration of antiulcer drugs.     

Low-cost removal of organic pollutants with nickel nanoparticle loaded ordered macroporous hydrogel as high performance catalyst

A facile route for the in situ preparation of catalytically active Ni nanoparticles (NPs) in ordered macroporous hydrogel (OMH) has been developed. The hydrogel was fabricated based on polystyrene colloid template. The electronegativity of amide and carboxyl groups on the poly(acrylamide-co-acryl acid) chains of the hydrogel caused strong binding of Ni²⁺ ions which made them distribute uniformly inside the hydrogel. When immersed in NaBH₄ aqueous solution, the Ni²⁺ ions on the hydrogel were reduced to Ni NPs. The resultant Ni NPs loaded OMH showed good catalytic activity for the reduction of a common organic pollutant, 4-nitrophenol, with NaBH₄. A kinetic study of the catalytic reaction was carried out. The rate constant per unit weight could reach 0.53 s⁻¹ g⁻¹, which is much better than many common hydrogel loaded nickel catalysts. Moreover, the current catalyst can be easily separated and recovered with stable catalytic activity.     

In situ-forming and pH-responsive hydrogel based on chitosan for vaginal delivery of therapeutic agents

One of the important routes of drug administration for localized delivery of contraceptives and cervical cancer treatment agents is vaginal canal. Due to the low pH of vagina, a pH-responsive drug delivery system was developed. This hydrogel was synthesized based on a mucoadhesive biopolymer, chitosan (CS), that promotes the interaction between the hydrogel and mucosal surface of the vagina, potentially increasing the residence time of the system. This injectable hydrogel was formed via acid-labile Schiff-base linkages between free amine groups and aldehyde functionalities on modified chitosan. A novel approach was taken to add aldehyde functionalities to chitosan using a two-step reaction. Two types of slow and fast degrading hydrogels were prepared and loaded with iron (II) gluconate dihydrate, a non-hormonal spermicide, and doxorubicin hydrochloride, an anti-cancer drug. The release profiles of these drugs at different pH environments were assessed to determine the pH-dependent release mechanism. Mechanical properties, swell-ability and degradation rate of these matrices were studied. The cross-linking density of the hydrogel as well as pH changes played an important role in the characteristic of these hydrogels. The hydrogels degraded faster in lower pH, while the hydrogel with lower cross-linking density showed longer gelation time and faster degradation rate compared to the gel with higher cross-linking density. In vitro cytotoxicity assessment of these hydrogels in 48?h indicated the non-toxic effect of these hydrogels toward mesenchymal stem cells (MSCs) in the test period.

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Investigation of swellable poly (N-isopropylacrylamide) based hydrogels for drug delivery

The thermo-sensitive properties of poly (N-isopropylacrylamide) (PNIPA) hydrogels are modified by the addition of hydrophilic acrylamide comonomers and an interpenetrating network of sodium alginate for drug delivery applications near 37 °C. A mathematical model is presented to describe the mass transport kinetics during the hydrogel drug delivery process, which is accompanied by a volume change during phase transition. In this model, the transport in the polymer matrix is described by Fick's second law in cylindrical coordinates, with concentration dependent diffusion coefficients. The moving boundary problems caused by the polymer matrix swelling are also solved by numerical simulation. The models show that the Trypan blue release from the modified PNIPA-based hydrogels is strongly concentration dependent. The sodium alginate component is also shown to effectively facilitate the diffusion process. The results from the simulation are in good agreement with the measurements of diffusion and swelling observed from in vitro experiments. The implications of this work are also discussed for practical drug delivery systems.     

Preparation and characterization of keratin-based biocomposite hydrogels prepared by electron beam irradiation

The biocompatible and highly porous keratin-based hydrogels were prepared using electron beam irradiation (EBI). The conditions for keratin-based hydrogel formation were investigated depending on several conditions, including the presence of poly(vinyl alcohol) (PVA), concentration of keratin solution, EBI dose, and poly(ethylene imine) (PEI) additives. The pure keratin (human hair and wool) aqueous solution was not gelled by EBI, while the aqueous keratin solutions blended with PVA were gelled at an EBI dose of more than 90 kGy. Furthermore, in the presence of PEI, the aqueous keratin solution blended with PVA could be gelled at a considerably lower EBI dose, even at 10 kGy. This finding suggests that the PEI additives significantly influence the rate of gelation and that PEIs function as an accelerator during gelation. The resulting keratin-based hydrogels were characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), gel fraction, degree of swelling, gel strength, and kinetics of swelling analyses.     

Poly (vinyl alcohol) hydrogels for pH dependent colon targeted drug delivery

Oral drug administration is convenient with pH dependent drug delivery system since the drug has to pass through different pH environments in gastro intestinal (GI) tract. The pH dependent swelling/shrinking behavior of hydrogel drug carrier controls the drug release without affecting the function of drug. pH dependent hydrogels of poly (vinyl alcohol) (PVA) were prepared by cross linking with maleic acid (MA). The hydrogels were characterized by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, DSC, porosimetry, SEM, TEM, biocompatibility study and by measuring their swelling behavior in water, simulated gastric fluid (SGF) and intestinal fluid (SIF). Swelling of the hydrogels was found to be highest in SIF (pH: 7.5) and lowest in SGF (pH: 1.2) resembling that required in colon targeted drug delivery systems. Since the swelling behavior of the gel is pH dependent, these hydrogels were studied for colon targeted drug delivery in an in-vitro set-up resembling the condition of GI tract. The ratio of PVA and MA in the hydrogel was varied to study the effect on the drug diffusion rate. For drug delivery study, vitamin B12 and salicylic acid were used as model drugs. The hydrogel, loaded with model drugs vitamin B12 and salicylic acid also demonstrated colon specific drug release with a relatively higher drug release in SIF (pH: 7.5) than that in SGF (pH: 1.2).     

A comparative study of chitosan and poloxamer based thermosensitive hydrogel for the delivery of PEGylated melphalan conjugates

Abstract

Objective: Although the melphalan (ML) used extensively for the management of breast cancer, its clinical application is limited due to significant hemolytic activity. In the present work, a comparative analysis of two distinct in situ-based thermogelling polymers of PEGylated ML was performed.

Methods: Briefly, the PEGylated conjugate of the melphalan (MLPEG 5000) for local and sustained drug release action is loaded into two different thermogelling polymeric systems, namely chitosan- and poloxamer-based systems. The synthesized conjugate was loaded to a chitosan (MLP 5000) and poloxamer-based (MPX-CG) thermogelling injectable hydrogels. These thermogelling hydrogels were evaluated for in vitro hydrolysis, in vitro hemolytic activity. and in vitro anticancer activity.

Results: The lower percent cumulative hydrolysis was witness for both the hydrogels. MPX-CG and MLP 5000 hydrogels as predicted had shown lower percent cumulative hydrolysis of 3.31?±?0.1 and 1.67?±?0.1 after 6?h. The percentage hemolysis of MPX-CG and MLP 5000 even at a concentration of 32?µg/ml was found to be 39.23?±?1.24% and 34.23?±?2.24%, observed at 1?h, respectively. Both the hydrogels showed similar anticancer pattern, the MPX-CG hydrogel showed low cell viability of 8.4?±?1.1% at a concentration of 150?µM and the MLP-5000 hydrogel showed slight higher cell viability (13.12?±?5.4%) as compared with MPX-CG hydrogel.

Conclusion: Hence, from the present study it can be well understood that both the chitosan- and the poloxamer-based thermogelling hydrogel proves to be an effective drug delivery systems for the delivery of the PEGylated conjugates.     

Performance improvement and comparison of mass recovery in CaCl2/activated carbon adsorption refrigerator and silica gel/LiCl adsorption chiller driven by low grade waste heat

One CaCl₂/activated carbon-ammonia adsorption refrigerator and one silica gel/LiCl-methanol chiller was designed and tested. The comparison of performance improvement of mass recovery process on the two adsorption systems was studied. The results show that the COP (coefficient of performance) and SCP (specific of cooling power) can be improved by 15.4% and 10.5% by mass recovery process in silica gel/LiCl-methanol chiller, while they can be improved by 53.8% and 51.5% in CaCl₂/acitvated carbon-ammonia refrigerator, because the latter has larger pressure difference between the hot and cold bed. Both the CaCl₂/acitvated carbon-ammonia refrigerator and the silica gel/LiCl-methanol chiller can provide continuous and stable cooling capacity. Under nominal working condition, the evaporator temperature, COP and SCP can reach −21 °C, 0.26 and 474 W kg⁻¹ in CaCl₂/activated carbon-ammonia refrigerator, and they are 15 °C, 0.41 and 244 W kg⁻¹ in the silica gel/LiCl-methanol chiller.