Hydrogels, and particularly biopolymeric hydrogels, have recently received tremendous interest as... more Hydrogels, and particularly biopolymeric hydrogels, have recently received tremendous interest as controlled release systems for their peculiar features such as high biocompatibility, biodegradability, bioadhesivity, chemical and thermal resistance and good mechanical properties. Among biopolymers, the exocellular microbial polysaccharide scleroglucan appears to be particularly well suited for the formulation of monolithic hydrogel matrices for controlled drug release. In this work we
This paper deals with the rheological properties under continuous shear flow conditions of aqueou... more This paper deals with the rheological properties under continuous shear flow conditions of aqueous systems of hydroxypropyl guar gum (HPG) and three derivatives, of the same or lower molecular weight (MW) than HPG, characterized by different contents of long-chain hydrophobic pendants. The shear-dependent behavior of HPG and of the derivative with low molecular weight and low degree of hydrophobization (LMLH)
A waxy crude oil which gels below a threshold temperature has been investigated under static and ... more A waxy crude oil which gels below a threshold temperature has been investigated under static and dynamic conditions, using a combination of rheological methods, optical microscopy, and DSC. Particular attention is given in this work to the influence of the mechanical history on gel strength and to describing the time-dependent rheological behavior. The gels display a strong dependence of the yield stress and moduli on the shear history, cooling rate, and stress loading rate. Of particular interest is the partial recovery of the gel structure after application of small stress or strain (much smaller than the critical values needed for flow onset) during cooling, which can be used to reduce the ultimate strength of the crude oil gel formed below the pour point. A second focus of this study is to further develop the physical interpretation of the mechanism by which wax crystallization produces gelation. Gelation of the waxy crude oil studied is suggested to be the result of the association between wax crystals, which produces an extended network structure, and it is shown that the system displays features common to attractive colloidal gels, for one of which, fumed silica (Aerosil 200) in paraffin oil, rheological data are reported. The colloidal gel model provides a simple and economical basis for explaining the response of the gelled oil to various mechanical perturbations and constitutes a fruitful basis from which to develop technologies for controlling the gelation phenomenon, as suggested by the rheological results reported.
Microemulsions are particularly interesting for pharmaceutical applications as oil-in-water vesic... more Microemulsions are particularly interesting for pharmaceutical applications as oil-in-water vesicular minicarriers for drug delivery systems, since they can be thermodynamically stable over a rather wide temperature range and easily prepared with no signi®cant energy consumption. Their stability is essentially due to the appropriate selection of the surfactant destined to be disposed at the water/oil interface and to reduce the interfacial tension. Very often the presence of a co-surfactant, usually an alcohol, is required to get an optimal formulation, in particular to reduce the amount of surfactant needed. In this manner, the interfacial tension is generally lowered below 1 mN/m so that oil and/or water domains ranging from few to one hundred nanometers spontaneously form without requesting any additional external energy . Even if the water/oil interface is very large, the interfacial tension is quite low and then the mixing entropy can be enough to compensate for the positive interfacial free energy associated with the formation of such a ®ne dispersion. Microemulsions can be characterized by many structures, such as oil droplets in water, water
In this paper we have concentrated on the characterization of calcium alginate hydrogels loaded w... more In this paper we have concentrated on the characterization of calcium alginate hydrogels loaded with a model drug (myoglobin) by means of a mechanical approach; in addition, release tests of myoglobin from alginate hydrogels were performed. At a fixed temperature, relaxation tests (mechanical study) were carried out on matrices constituted by different polymer concentrations. The interpretation of the relaxation behavior of the different matrices was conducted using the generalized Maxwell model; as a result of this investigation it was possible to conclude that for polymer concentrations greater than 0.5 g/ 100 mL the matrices behaved as solid materials. In addition, it was observed that the mechanical properties of the matrices increased with polymer concentration. With regard to the release tests, the diffusion coefficient of myoglobin in the matrix in relation to polymer concentrations was determined. The mechanical and release data where then analyzed by Flory's theory and by a modified free-volume theory, respectively, to estimate the network mesh size ξ. The comparison between the mesh sizes obtained by the two approaches showed a satisfactory agreement for polymer concentrations greater than
Two methyl end-capped oligo(ethylene glycol) esters (1a,b) of benzofulvene derivative BF1 were sy... more Two methyl end-capped oligo(ethylene glycol) esters (1a,b) of benzofulvene derivative BF1 were synthesized and induced to polymerize spontaneously by solvent removal to give poly-1a,b showing both NMR and absorption/emission spectra very similar to those of poly-BF1. Poly-1a,b showed relatively high molecular weight and the tendency to depolymerize to a different degree as a function of the temperature in the presence of solvents, while they exhibited appreciable stability in the absence of solvent. Poly-1b, bearing a longer oligo(ethylene glycol) side chain, featured an amphiphilic character and interacted with a number of organic solvents to produce transparent gel aggregates, and with water to give a quite compact physical gel. Rheological studies performed on the hydrogel suggested strong gel characteristics and the combination of rheology and NMR transverse relaxation measurements allowed the pore size distribution in the hydrogel to be defined. Finally, biological studies performed with poly-1b solutions showed neither cytotoxicity nor cell viability impairment suggesting potential biocompatibility features for this polymer. In conclusion, poly-1b can be considered a promising polymer for the preparation of hydrogels potentially useful in a range of biological and biotechnological applications such as drug delivery, molecular recognition, biosensing, protein and DNA separation, micro-and nanofluidics, as well as tissue engineering.
A waxy crude oil which gels below a threshold temperature has been investigated under static and ... more A waxy crude oil which gels below a threshold temperature has been investigated under static and dynamic conditions, using a combination of rheological methods, optical microscopy, and DSC. Particular attention is given in this work to the influence of the mechanical history on gel strength and to describing the time-dependent rheological behavior. The gels display a strong dependence of the yield stress and moduli on the shear history, cooling rate, and stress loading rate. Of particular interest is the partial recovery of the gel structure after application of small stress or strain (much smaller than the critical values needed for flow onset) during cooling, which can be used to reduce the ultimate strength of the crude oil gel formed below the pour point. A second focus of this study is to further develop the physical interpretation of the mechanism by which wax crystallization produces gelation. Gelation of the waxy crude oil studied is suggested to be the result of the association between wax crystals, which produces an extended network structure, and it is shown that the system displays features common to attractive colloidal gels, for one of which, fumed silica (Aerosil 200) in paraffin oil, rheological data are reported. The colloidal gel model provides a simple and economical basis for explaining the response of the gelled oil to various mechanical perturbations and constitutes a fruitful basis from which to develop technologies for controlling the gelation phenomenon, as suggested by the rheological results reported.
Water-based suspensions of mullite that are used to form porous bodies through the impregnation o... more Water-based suspensions of mullite that are used to form porous bodies through the impregnation of sponges were investigated. The rheological properties of the suspensions are strongly influenced by the amount of magnesia added, as well as the solid loading and the preparation procedure. Amounts of magnesia higher than 0.15 wt% change both the steady shear behavior from nearly Newtonian to plastic (with an increasing apparent yield stress) and time dependent, as well as the viscoelastic properties (the storage modulus increases more than 5 orders of magnitude). Similar effects are shown for long milling procedures, or solids loadings higher than 25 vol%, while the elastic properties are not strictly related to the total volume fraction. The results of the rheological characterization, under either continuous or dynamic conditions, are discussed in relation to the properties of the structure built up within the suspension. The density of the cast bodies increases with decreasing magnesia content.
The phase separation of paraffinic solids from crude oil constitutes a major operating issue for ... more The phase separation of paraffinic solids from crude oil constitutes a major operating issue for the petroleum industry, in view of the fact that the association of wax crystals into a disordered spanning network can lead to severe flow assurance problems. Although the presence of water in crude oil is quite common, its impact on waxy crude oil gelation and rheology has not received much more attention in the literature.
This paper presents a new model suitable to describe the drug release from drug delivery systems ... more This paper presents a new model suitable to describe the drug release from drug delivery systems constituted by an ensemble of drug loaded crosslinked polymer particles. The model accounts for the main factors affecting the drug release such as the particle size distribution, the physical state and the concentration profile of the drug inside the polymeric particles, the viscoelastic properties of the polymer-penetrant system and the dissolution-diffusion properties of the loaded drug. In order to check the validity of the model, release experiments were performed by using crosslinked polyvinylpyrrolidone (PVP) particles and two different model drugs, MAP (medroxyprogesterone acetate) and TEM (Temazepam). MAP and TEM were chosen because of their completely different dissolution behaviours in water. In particular, TEM undergoes a phase transition to the crystalline state upon dissolution when it is loaded in the polymeric network in the amorphous state. The comparison with the experimental results confirms that the most important factors determining the drug release kinetics can be properly accounted for.
In this paper attention is focused on the determination of the drug diffusion coefficient in a sw... more In this paper attention is focused on the determination of the drug diffusion coefficient in a swollen polymeric membrane referring to a recent mathematical model (linear model). The main advantage deriving from its use is that, despite its analytical nature and its ability to account for the most important aspects characterising a permeation experiment, it can also be applied in the case of thick membranes. To check the model reliability, a comparison is made with a more complex numerical model and with a largely employed model in terms of data fitting quality. To this purpose, particular care is devoted to the experimental and theoretical tools employed to calculate the auxiliary parameters required by the three models, and with the aim of getting a drug diffusion coefficient value as accurate as possible. Theophylline was chosen as model drug owing to its wide employment in the pharmaceutical field. Membranes were prepared with sodium alginates hydrogels at three different polymer concentrations. The present analysis demonstrates the reliability of the linear model and reveals that the theophylline diffusion coefficient is not significantly affected by the polymer concentration. Indeed, such a parameter is reflected in different membrane thicknesses rather than in different mesh sizes of the polymeric network.
... Emerson, DW; Gaj, D.; Grigorian, C.; Turek, JE “Intraresin reactions of a,w-Alkanediamines wi... more ... Emerson, DW; Gaj, D.; Grigorian, C.; Turek, JE “Intraresin reactions of a,w-Alkanediamines with Sulfochlorinated Copoly-(styrene-divinylbenzene)”. ... Romano Lapasin* and Sabrina Pricl Istituto di Chimica Applicata ed Industriale, Uniuersitd di Trieste, 34127 Trieste, Italy Marco ...
Starches were isolated from five improved rice varieties developed by West African Rice Developme... more Starches were isolated from five improved rice varieties developed by West African Rice Development Agency (WARDA) namely FARO 32, FARO 51, FARO 52, FARO 54 and NERICA. Starch yield and amylose content varied between 73.77e70.02% and 22.88e24.48% respectively. Starches were polyhedral in appearances and within the size range 1.5e6.1 mm. The X-ray diffraction patterns of the starches show a peak centered on 2q ¼ 15.1 , a doublet on 17.1 and 18.1 , and another single peak at 23.12 . However, NERICA shows no doublet but a single peak at 2q ¼ 17.1 with a small shoulder. The peak viscosity of the starches ranged between 147.48 and 209.17 RVA corresponding to FARO 52 and NERICA respectively. A neat distinction is observed between the marked shear thinning of FARO 52 and the apparently plastic behavior of the other samples. Important differences appear in the low shear region where the viscosity increases in the following order: FARO 52 < FARO 54 < FARO 51 < FARO 32 < NERICA. The mechanical spectra exhibit similar profiles as the storage modulus (G 0 ) prevails over the viscous component (G 00 ) and is weakly dependent on the frequency. However, the storage modulus increases in the same order observed for low shear viscosity. The percentage retrogradation was between 61.9 and 86.6% and NERICA starch showed the least retrogradation indication. NERICA starch exhibited highest swelling and solubility, while the least was observed in FARO 52. Rheology and functional properties are dependent of amylose composition. This study provides knowledge for the utilization of starches isolated from improved rice varieties that would be relevant for both domestic and industrial applications.
Microencapsulation is a powerful technique commonly used for the protection of a wide range of bi... more Microencapsulation is a powerful technique commonly used for the protection of a wide range of biomolecules (small molecules and protein) and cells of bacterial, yeast and animal origin. In this work, solutions of mixed biopolymers are investigated as excipients for the formulation of a model system. The influence of the different components is studied from the viscoelastic behaviour of the starting solutions to the thermal characterisation of the gel beads therefrom produced. Rheological characterisation displays an almost regular trend for the several combination of solutes and for the frequency dependence, but some peculiarities emerge when both the model protein lysozyme and the cosolvent ethanol are present in the mixture; for the latter system a delayed melting behaviour of water appears in the gel beads. Changes in the temperature dependence of water evaporation from the beads are taken as an evidence of the rate of release from the beads.
Hydrogels, and particularly biopolymeric hydrogels, have recently received tremendous interest as... more Hydrogels, and particularly biopolymeric hydrogels, have recently received tremendous interest as controlled release systems for their peculiar features such as high biocompatibility, biodegradability, bioadhesivity, chemical and thermal resistance and good mechanical properties. Among biopolymers, the exocellular microbial polysaccharide scleroglucan appears to be particularly well suited for the formulation of monolithic hydrogel matrices for controlled drug release. In this work we
This paper deals with the rheological properties under continuous shear flow conditions of aqueou... more This paper deals with the rheological properties under continuous shear flow conditions of aqueous systems of hydroxypropyl guar gum (HPG) and three derivatives, of the same or lower molecular weight (MW) than HPG, characterized by different contents of long-chain hydrophobic pendants. The shear-dependent behavior of HPG and of the derivative with low molecular weight and low degree of hydrophobization (LMLH)
A waxy crude oil which gels below a threshold temperature has been investigated under static and ... more A waxy crude oil which gels below a threshold temperature has been investigated under static and dynamic conditions, using a combination of rheological methods, optical microscopy, and DSC. Particular attention is given in this work to the influence of the mechanical history on gel strength and to describing the time-dependent rheological behavior. The gels display a strong dependence of the yield stress and moduli on the shear history, cooling rate, and stress loading rate. Of particular interest is the partial recovery of the gel structure after application of small stress or strain (much smaller than the critical values needed for flow onset) during cooling, which can be used to reduce the ultimate strength of the crude oil gel formed below the pour point. A second focus of this study is to further develop the physical interpretation of the mechanism by which wax crystallization produces gelation. Gelation of the waxy crude oil studied is suggested to be the result of the association between wax crystals, which produces an extended network structure, and it is shown that the system displays features common to attractive colloidal gels, for one of which, fumed silica (Aerosil 200) in paraffin oil, rheological data are reported. The colloidal gel model provides a simple and economical basis for explaining the response of the gelled oil to various mechanical perturbations and constitutes a fruitful basis from which to develop technologies for controlling the gelation phenomenon, as suggested by the rheological results reported.
Microemulsions are particularly interesting for pharmaceutical applications as oil-in-water vesic... more Microemulsions are particularly interesting for pharmaceutical applications as oil-in-water vesicular minicarriers for drug delivery systems, since they can be thermodynamically stable over a rather wide temperature range and easily prepared with no signi®cant energy consumption. Their stability is essentially due to the appropriate selection of the surfactant destined to be disposed at the water/oil interface and to reduce the interfacial tension. Very often the presence of a co-surfactant, usually an alcohol, is required to get an optimal formulation, in particular to reduce the amount of surfactant needed. In this manner, the interfacial tension is generally lowered below 1 mN/m so that oil and/or water domains ranging from few to one hundred nanometers spontaneously form without requesting any additional external energy . Even if the water/oil interface is very large, the interfacial tension is quite low and then the mixing entropy can be enough to compensate for the positive interfacial free energy associated with the formation of such a ®ne dispersion. Microemulsions can be characterized by many structures, such as oil droplets in water, water
In this paper we have concentrated on the characterization of calcium alginate hydrogels loaded w... more In this paper we have concentrated on the characterization of calcium alginate hydrogels loaded with a model drug (myoglobin) by means of a mechanical approach; in addition, release tests of myoglobin from alginate hydrogels were performed. At a fixed temperature, relaxation tests (mechanical study) were carried out on matrices constituted by different polymer concentrations. The interpretation of the relaxation behavior of the different matrices was conducted using the generalized Maxwell model; as a result of this investigation it was possible to conclude that for polymer concentrations greater than 0.5 g/ 100 mL the matrices behaved as solid materials. In addition, it was observed that the mechanical properties of the matrices increased with polymer concentration. With regard to the release tests, the diffusion coefficient of myoglobin in the matrix in relation to polymer concentrations was determined. The mechanical and release data where then analyzed by Flory's theory and by a modified free-volume theory, respectively, to estimate the network mesh size ξ. The comparison between the mesh sizes obtained by the two approaches showed a satisfactory agreement for polymer concentrations greater than
Two methyl end-capped oligo(ethylene glycol) esters (1a,b) of benzofulvene derivative BF1 were sy... more Two methyl end-capped oligo(ethylene glycol) esters (1a,b) of benzofulvene derivative BF1 were synthesized and induced to polymerize spontaneously by solvent removal to give poly-1a,b showing both NMR and absorption/emission spectra very similar to those of poly-BF1. Poly-1a,b showed relatively high molecular weight and the tendency to depolymerize to a different degree as a function of the temperature in the presence of solvents, while they exhibited appreciable stability in the absence of solvent. Poly-1b, bearing a longer oligo(ethylene glycol) side chain, featured an amphiphilic character and interacted with a number of organic solvents to produce transparent gel aggregates, and with water to give a quite compact physical gel. Rheological studies performed on the hydrogel suggested strong gel characteristics and the combination of rheology and NMR transverse relaxation measurements allowed the pore size distribution in the hydrogel to be defined. Finally, biological studies performed with poly-1b solutions showed neither cytotoxicity nor cell viability impairment suggesting potential biocompatibility features for this polymer. In conclusion, poly-1b can be considered a promising polymer for the preparation of hydrogels potentially useful in a range of biological and biotechnological applications such as drug delivery, molecular recognition, biosensing, protein and DNA separation, micro-and nanofluidics, as well as tissue engineering.
A waxy crude oil which gels below a threshold temperature has been investigated under static and ... more A waxy crude oil which gels below a threshold temperature has been investigated under static and dynamic conditions, using a combination of rheological methods, optical microscopy, and DSC. Particular attention is given in this work to the influence of the mechanical history on gel strength and to describing the time-dependent rheological behavior. The gels display a strong dependence of the yield stress and moduli on the shear history, cooling rate, and stress loading rate. Of particular interest is the partial recovery of the gel structure after application of small stress or strain (much smaller than the critical values needed for flow onset) during cooling, which can be used to reduce the ultimate strength of the crude oil gel formed below the pour point. A second focus of this study is to further develop the physical interpretation of the mechanism by which wax crystallization produces gelation. Gelation of the waxy crude oil studied is suggested to be the result of the association between wax crystals, which produces an extended network structure, and it is shown that the system displays features common to attractive colloidal gels, for one of which, fumed silica (Aerosil 200) in paraffin oil, rheological data are reported. The colloidal gel model provides a simple and economical basis for explaining the response of the gelled oil to various mechanical perturbations and constitutes a fruitful basis from which to develop technologies for controlling the gelation phenomenon, as suggested by the rheological results reported.
Water-based suspensions of mullite that are used to form porous bodies through the impregnation o... more Water-based suspensions of mullite that are used to form porous bodies through the impregnation of sponges were investigated. The rheological properties of the suspensions are strongly influenced by the amount of magnesia added, as well as the solid loading and the preparation procedure. Amounts of magnesia higher than 0.15 wt% change both the steady shear behavior from nearly Newtonian to plastic (with an increasing apparent yield stress) and time dependent, as well as the viscoelastic properties (the storage modulus increases more than 5 orders of magnitude). Similar effects are shown for long milling procedures, or solids loadings higher than 25 vol%, while the elastic properties are not strictly related to the total volume fraction. The results of the rheological characterization, under either continuous or dynamic conditions, are discussed in relation to the properties of the structure built up within the suspension. The density of the cast bodies increases with decreasing magnesia content.
The phase separation of paraffinic solids from crude oil constitutes a major operating issue for ... more The phase separation of paraffinic solids from crude oil constitutes a major operating issue for the petroleum industry, in view of the fact that the association of wax crystals into a disordered spanning network can lead to severe flow assurance problems. Although the presence of water in crude oil is quite common, its impact on waxy crude oil gelation and rheology has not received much more attention in the literature.
This paper presents a new model suitable to describe the drug release from drug delivery systems ... more This paper presents a new model suitable to describe the drug release from drug delivery systems constituted by an ensemble of drug loaded crosslinked polymer particles. The model accounts for the main factors affecting the drug release such as the particle size distribution, the physical state and the concentration profile of the drug inside the polymeric particles, the viscoelastic properties of the polymer-penetrant system and the dissolution-diffusion properties of the loaded drug. In order to check the validity of the model, release experiments were performed by using crosslinked polyvinylpyrrolidone (PVP) particles and two different model drugs, MAP (medroxyprogesterone acetate) and TEM (Temazepam). MAP and TEM were chosen because of their completely different dissolution behaviours in water. In particular, TEM undergoes a phase transition to the crystalline state upon dissolution when it is loaded in the polymeric network in the amorphous state. The comparison with the experimental results confirms that the most important factors determining the drug release kinetics can be properly accounted for.
In this paper attention is focused on the determination of the drug diffusion coefficient in a sw... more In this paper attention is focused on the determination of the drug diffusion coefficient in a swollen polymeric membrane referring to a recent mathematical model (linear model). The main advantage deriving from its use is that, despite its analytical nature and its ability to account for the most important aspects characterising a permeation experiment, it can also be applied in the case of thick membranes. To check the model reliability, a comparison is made with a more complex numerical model and with a largely employed model in terms of data fitting quality. To this purpose, particular care is devoted to the experimental and theoretical tools employed to calculate the auxiliary parameters required by the three models, and with the aim of getting a drug diffusion coefficient value as accurate as possible. Theophylline was chosen as model drug owing to its wide employment in the pharmaceutical field. Membranes were prepared with sodium alginates hydrogels at three different polymer concentrations. The present analysis demonstrates the reliability of the linear model and reveals that the theophylline diffusion coefficient is not significantly affected by the polymer concentration. Indeed, such a parameter is reflected in different membrane thicknesses rather than in different mesh sizes of the polymeric network.
... Emerson, DW; Gaj, D.; Grigorian, C.; Turek, JE “Intraresin reactions of a,w-Alkanediamines wi... more ... Emerson, DW; Gaj, D.; Grigorian, C.; Turek, JE “Intraresin reactions of a,w-Alkanediamines with Sulfochlorinated Copoly-(styrene-divinylbenzene)”. ... Romano Lapasin* and Sabrina Pricl Istituto di Chimica Applicata ed Industriale, Uniuersitd di Trieste, 34127 Trieste, Italy Marco ...
Starches were isolated from five improved rice varieties developed by West African Rice Developme... more Starches were isolated from five improved rice varieties developed by West African Rice Development Agency (WARDA) namely FARO 32, FARO 51, FARO 52, FARO 54 and NERICA. Starch yield and amylose content varied between 73.77e70.02% and 22.88e24.48% respectively. Starches were polyhedral in appearances and within the size range 1.5e6.1 mm. The X-ray diffraction patterns of the starches show a peak centered on 2q ¼ 15.1 , a doublet on 17.1 and 18.1 , and another single peak at 23.12 . However, NERICA shows no doublet but a single peak at 2q ¼ 17.1 with a small shoulder. The peak viscosity of the starches ranged between 147.48 and 209.17 RVA corresponding to FARO 52 and NERICA respectively. A neat distinction is observed between the marked shear thinning of FARO 52 and the apparently plastic behavior of the other samples. Important differences appear in the low shear region where the viscosity increases in the following order: FARO 52 < FARO 54 < FARO 51 < FARO 32 < NERICA. The mechanical spectra exhibit similar profiles as the storage modulus (G 0 ) prevails over the viscous component (G 00 ) and is weakly dependent on the frequency. However, the storage modulus increases in the same order observed for low shear viscosity. The percentage retrogradation was between 61.9 and 86.6% and NERICA starch showed the least retrogradation indication. NERICA starch exhibited highest swelling and solubility, while the least was observed in FARO 52. Rheology and functional properties are dependent of amylose composition. This study provides knowledge for the utilization of starches isolated from improved rice varieties that would be relevant for both domestic and industrial applications.
Microencapsulation is a powerful technique commonly used for the protection of a wide range of bi... more Microencapsulation is a powerful technique commonly used for the protection of a wide range of biomolecules (small molecules and protein) and cells of bacterial, yeast and animal origin. In this work, solutions of mixed biopolymers are investigated as excipients for the formulation of a model system. The influence of the different components is studied from the viscoelastic behaviour of the starting solutions to the thermal characterisation of the gel beads therefrom produced. Rheological characterisation displays an almost regular trend for the several combination of solutes and for the frequency dependence, but some peculiarities emerge when both the model protein lysozyme and the cosolvent ethanol are present in the mixture; for the latter system a delayed melting behaviour of water appears in the gel beads. Changes in the temperature dependence of water evaporation from the beads are taken as an evidence of the rate of release from the beads.
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