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PubMed2000

Diffusion Coefficients of Three Organic Solutes in Aqueous Sodium Dodecyl Sulfate Solutions.

Tracer diffusion coefficients of phenol, toluene, and benzoic acid in aqueous solutions of sodium dodecyl sulfate (SDS) were measured by the Taylor dispersion technique. In addition, the viscosities and densities of the SDS solutions were measured. For phenol and toluene, the effect of micelle formation on the diffusion coefficient is pronounced. When the SDS concentration is below the critical micelle concentration (cmc), the diffusion coefficients are almost independent of the SDS concentration. However, above the cmc there is a rapid decrease in the diffusion coefficients, and the apparent diffusion coefficients of the two solutes are the weighted average of free solute diffusion and the micelle diffusion. A model is presented to describe the diffusion behavior of the two solutes in aqueous micellar solutions of SDS. The interaction between the two solutes and the micelles has been investigated and the fraction of each solute that is solubilized by the micelles is estimated from the measured apparent diffusion coefficient. For benzoic acid, the diffusion coefficient is dependent on the joint contribution of the benzoic acid molecules that are solubilized by the micelles as well as the corresponding benzoate ions. The effect of micelle formation on the diffusion coefficient of benzoic acid is not as pronounced as for phenol and toluene. Copyright 2000 Academic Press.

Journal Article↗

PubMed2005

The influence of excipients on the diffusion of ibuprofen and paracetamol in gastric mucus.

The aim of this study was to examine the diffusion of commonly administered analgesics, ibuprofen and paracetamol, through gastric mucus. As ibuprofen and paracetamol are often formulated with alkalising excipients, or are commonly co-administered with antacids that have been demonstrated to alter their absorption, diffusion was also studied in the presence of a range of soluble and insoluble antacids or buffering agents. The effect of pH, which has been demonstrated to modify the properties of mucus, was also studied. Mucus was a significant barrier to diffusion for both drugs, compared to an unstirred aqueous layer with diffusion rates significantly lower in the presence of a mucus barrier for both drugs; ibuprofen diffusion also demonstrated a significant increase in the lag time. Paracetamol diffusion was not significantly affected by addition of any antacid, whereas ibuprofen rates were affected and the diffusion lag time for ibuprofen was significantly reduced in all cases. Isolated increases in pH increased the rate and reduced the lag time for ibuprofen diffusion. It was shown that mucus acts as a passive barrier in the case of paracetamol diffusion, and an interactive barrier to ibuprofen diffusion. Changes in mucus viscosity at different pH values may be responsible for the observed changes in ibuprofen diffusion rate.

Acetaminophen↗

PubMed2003

Arterial remodeling patterns before intervention predict diffuse in-stent restenosis: an intravascular ultrasound study.

OBJECTIVES: The aim of this retrospective study was to determine the predictors of diffuse in-stent restenosis (ISR) among the lesions causing the first ISR by intravascular ultrasound (IVUS) studies. BACKGROUND: Although some predictors of diffuse ISR have been reported, parameters on IVUS relating to diffuse ISR are not well characterized. METHODS: We classified 52 ISR lesions that had undergone successful stent implantation and led to restenosis into two types--focal and diffuse ISR--using quantitative coronary angiography. Restenosis was defined as > or =50% diameter stenosis, and diffuse ISR as lesion length > or =10 mm at follow-up. The remodeling index (RI) was defined as the vessel area at the target lesion divided by that of averaged reference segments. RESULTS: There were no significant differences in patient, angiographic, and procedural characteristics between the focal (n = 25) and diffuse (n = 27) ISR groups. Baseline RI was significantly greater in the diffuse ISR group (1.03 +/- 0.18 vs. 0.88 +/- 0.24, p = 0.0159). Negative remodeling, defined as RI <0.9, was detected in 60% of the focal ISR group and in only 26% of the diffuse ISR group. By logistic regression analysis, baseline RI was the only independent predictor of diffuse ISR (p = 0.0341). Moreover, volumetric analyses revealed that lesions developing into diffuse ISR had less capacity to compensate for further plaque growth. CONCLUSIONS: Among the first ISR lesions, baseline positive remodeling was the most powerful predictor of diffuse ISR. Measuring pre-interventional arterial remodeling patterns by IVUS may be helpful to stratify lesions at higher risk.

Aged↗

PubMed1998

Quantitative diffusion coefficient maps using fast spin-echo MRI.

In this work, we have evaluated the performance of a diffusion-sensitive fast spin-echo (FSE) pulse sequence. The proposed pulse sequence utilises velocity-compensating diffusion-encoding gradients and includes the collection of navigator echoes. Spoiler gradients were inserted in the slice-selecting direction to minimise effects from stimulated echoes. Calculations of the b values showed that cross-terms between imaging gradients and diffusion gradients only led to a marginal increase of b values. Pixel-wise calculation of apparent diffusion coefficient (ADC) maps was performed numerically, considering cross-terms between diffusion-encoding and imaging gradients. The sequences investigated used echo train lengths of 16, 8 and 4 echoes and were encoded in either the slice-, frequency- or phase-encoding direction. In order to allow for higher b values a pulse-sequence version using non-motion compensating diffusion-encoding gradients was written. Phantom measurements were performed and the diffusion coefficients of water and acetone were reasonable. Seven healthy volunteers (age 28-50 years) were examined and apparent diffusion coefficient values agreed well with expected values. Diffusion-weighted images, apparent diffusion coefficient maps and images corresponding to the trace of the diffusion tensor of good quality were retrieved in vivo.

Adult↗

PubMed2005

Measuring the mutual diffusion coefficient for dodecyl acrylate in low molecular weight poly(dodecyl acrylate) with laser line deflection (Wiener's Method) and the fluorescence of pyrene.

Diffusion of small molecules into glassy polymers is quite complicated and almost always non-Fickian. Little work has been done with the diffusion of low molecular weight polymers that are liquids at room temperature (such as poly(dodecyl acrylate)) into their miscible monomers. We have studied three molecular weights under 20 000 to determine if poly(dodecyl acrylate) diffusion into dodecyl acrylate could be treated with Fick's law and if so to determine the values of the diffusion coefficients. We compare two methods for measuring the diffusion of dodecyl acrylate into poly(dodecyl acrylate): We used laser line deflection (Wiener's method) and improved upon the method from published reports. We also used the dependence of pyrene's fluorescence on the viscosity to measure the concentration distribution, and thus to extract the diffusion coefficient. After an initial relaxation period, diffusion in all cases followed Fick's law with a single concentration-independent diffusion coefficient. Comparison of the diffusion coefficients obtained by both methods yielded the same order of magnitude for the diffusion coefficients (10(-7) cm2/s) and showed the same trend in the dependence on the average molecular weight of the polymer (a decrease in the diffusion coefficient with an increase in the molecular weight).

Journal Article↗

PubMed2005

Diffusion at the liquid-vapor interface of an aqueous ionic solution utilizing a dual simulation technique.

The recently proposed dual simulation technique [J. Phys. Chem. B 2004, 108, 6595.], with slight modification, was used to determine the diffusion coefficients for a variety of regions of a 2.2 M sodium chloride aqueous solution with a vapor-liquid interface. The diffusion of all species was shown to be isotropic far away from the interface, but at different regions in the interface, the diffusion coefficients parallel and perpendicular to the interface did not agree for water and chloride. Specifically, interfacial water diffusion parallel to the interface was significantly higher than diffusion perpendicular to the interface. Chloride ions showed even larger anisotropicity in its diffusion coefficient at the interface, with its perpendicular diffusion being similar to its bulk value, but parallel diffusion being much higher, corresponding to the region of highest chloride ion concentration. The origin for this was found to be hydrogen bonds with waters which are highly oriented perpendicular to the interface, somewhat impeding chloride ion diffusion perpendicular to the interface. While sodium ion diffusion increased at the interface, its interfacial concentration is low in that region, and its diffusion was fairly isotropic throughout all regions.

Journal Article↗

PubMed2004

The mutual diffusion coefficient for (meth)acrylate monomers as determined with a nuclear microprobe.

The value of the mutual diffusion coefficient DV of two acrylic monomers is determined with nuclear microprobe measurements on a set of polymer films. These films have been prepared by allowing the monomers to diffuse into each other for a certain time and subsequently applying fast ultraviolet photo-polymerization, which freezes the concentration profile. The monomer diffusion profiles are studied with a scanning 2.1 MeV proton microprobe. Each monomer contains a marker element, e.g., Cl and Si, which are easily detected with proton induced x-ray emission. From the diffusion profiles, it is possible to determine the mutual diffusion coefficient. The mutual diffusion coefficient is dependent of concentration, which is concluded from the asymmetry in the Cl- and Si-profiles. A linear dependence of the mutual diffusion coefficient on the composition is used as a first order approximation. The best fits are obtained for a value of b=(0.38+/-0.15), which is the ratio of the diffusion coefficient of 1,3-bis(3-methacryloxypropyl)-1, 1,3,3-tetramethyldisiloxane in pure 2-chloroethyl acrylate and the diffusion coefficient of 2-chloroethyl acrylate in pure 1,3-bis(3-methacryloxypropyl)-1,1,3,3-tetramethyldisiloxane. Under the assumption of a linear dependence of the mutual diffusion coefficient DV on monomer composition, it follows that DV = (2.9+/-0.6)10(-10) m(2)/s at a 1:1 monomer ratio. With Flory-Huggins expressions for the monomer chemical potentials, one can derive approximate values for the individual monomer diffusion coefficients.

Journal Article↗

PubMed2005

Measurements of molecular and thermal diffusion coefficients in ternary mixtures.

Thermal diffusion coefficients in three ternary mixtures are measured in a thermogravitational column. One of the mixtures consists of one normal alkane and two aromatics (dodecane-isobutylbenzene-tetrahydronaphthalene), and the other two consist of two normal alkanes and one aromatic (octane-decane-1-methylnaphthalene). This is the first report of measured thermal diffusion coefficients (for all species) of a ternary nonelectrolyte mixture in literature. The results in ternary mixtures of octane-decane-1-methylnaphthalene show a sign change of the thermal diffusion coefficient for decane as the composition changes, despite the fact that the two normal alkanes are similar. In addition to thermal diffusion coefficients, molecular diffusion coefficients are also measured for three binaries and one of the ternary mixtures. The open-end capillary-tube method was used in the measurement of molecular diffusion coefficients. The molecular and thermal diffusion coefficients allow the estimation of thermal diffusion factors in binary and ternary mixtures. However, in the ternaries one also has to calculate phenomenological coefficients from the molecular diffusion coefficients. A comparison of the binary and ternary thermal diffusion factors for the mixtures comprised of octane-decane-1-methylnaphthalene reveals a remarkable difference in the thermal diffusion behavior in binary and ternary mixtures.

Journal Article↗

PubMed2006

Messages diffuse faster than messengers.

In many cell-signaling pathways, information is transmitted by the diffusion of messenger molecules. Diffusion coefficients characterize the messenger's spatial range and the characteristic times of signal propagation. Inside cells, particles usually diffuse in the presence of immobile binding sites (or traps). It is well known that binding to traps results in an effective diffusion coefficient that is smaller than the free coefficient in media free of traps. To measure effective diffusion coefficients in cells, "tagged" particles are often used. Radioactive calcium was used in a giant squid axon and in cytosolic extracts of Xenopus laevis oocytes. Fluorescence recovery after photobleaching yields diffusion coefficients from observations of the distribution of fluorescently labeled proteins. In the absence of traps, free diffusion coefficients give both the rate at which single-particle mean square displacements increase and the rate at which information in the form of inhomogeneities in particle concentration spread out with time. We show here that, in the presence of traps, information diffuses faster than single particles. Thus, messages diffuse faster than messengers. Tagged-particle experiments give the single-particle diffusion coefficients and, thus, can underestimate the rate of diffusive signal propagation.

Animals↗

PubMed2003

Crossover in diffusion equation: anomalous and normal behaviors.

Ubiquitous phenomena exist in nature where, as time goes on, a crossover is observed between different diffusion regimes (e.g., anomalous diffusion at early times which becomes normal diffusion at long times, or the other way around). In order to focus on such situations we have analyzed particular relevant cases of the generalized Fokker-Planck equation integral dgamma(')tau(gamma('))[ partial differential (gamma('))rho(x,t)]/ partial differential t(gamma('))= integral dmu(')dnu'D(mu('),nu('))[ partial differential (mu('))[rho(x,t)](nu('))]/ partial differential x(mu(')), where tau(gamma(')) and D(mu('),nu(')) are kernels to be chosen; the choice tau(gamma('))=delta(gamma(')-1) and D(mu('),nu('))=delta(mu(')-2)delta(nu(')-1) recovers the normal diffusion equation. We discuss in detail the following cases: (i) a mixture of the porous medium equation, which is connected with nonextensive statistical mechanics, with the normal diffusion equation; (ii) a mixture of the fractional time derivative and normal diffusion equations; (iii) a mixture of the fractional space derivative, which is related with Lévy flights, and normal diffusion equations. In all three cases a crossover is obtained between anomalous and normal diffusions. In cases (i) and (iii), the less diffusive regime occurs for short times, while at long times the more diffusive regime emerges. The opposite occurs in case (ii). The present results could be easily extended to more complex situations (e.g., crossover between two, or even more, different anomalous regimes), and are expected to be useful in the analysis of phenomena where nonlinear and fractional diffusion equations play an important role. Such appears to be the case for isolated long-ranged interaction Hamiltonians, which along time can exhibit a crossover from a longstanding metastable anomalous state to the usual Boltzmann-Gibbs equilibrium one. Another illustration of such crossover occurs in active intracellular transport.

Journal Article↗

PubMed1994

Anisotropy of water diffusion in the myocardium of the rat.

Pulsed field gradient nuclear magnetic resonance methods combined with nuclear magnetic resonance imaging were used to determine the water diffusion anisotropy in perfused rat hearts at 37 degrees C. It was found that the observed diffusion coefficient D(app) (apparent diffusion coefficient) depends on the orientation of the applied gradient g. When g is parallel to the epicardial surface, the observed diffusivity is D(app) parallel = 1.8 +/- 0.4 x 10(-9) m2.s-1, whereas when g is perpendicular to it, diffusivity is D(app) perpendicular = 2.5 +/- 0.5 x 10(-9) m2.s-1. To better characterize this directional dependence, images of the second-order diffusion tensor D of the myocardium were obtained. These data demonstrate several essential features of cardiac myoarchitecture, including the helicity of fiber orientation with respect to the ventricular axis and the variation of fiber pitch angle with transmural depth. Diffusion anisotropy may be quantified in a coordinate-independent manner by the eigenvalues of the diffusion tensor. In the myocardial midwall, these eigenvalues were E1 = 3.29 +/- 0.57, E2 = 2.01 +/- 0.42, and E3 = 0.77 +/- 0.58 x 10(-9) m2.s-1 (mean +/- SD). These data suggest that myocardial water diffusion is essentially unrestricted parallel to the myofibers. They further show that failure to measure the complete diffusion tensor may lead to substantial underestimates of diffusion anisotropy in the myocardium.

Animals↗

PubMed1999

Diffusion MRI in patients with transient ischemic attacks.

BACKGROUND AND PURPOSE: Diffusion MRI has established value in patients with ischemic stroke but has not been systematically investigated in patients with transient ischemic attack (TIA). METHODS: Clinical, conventional MRI, and diffusion MRI data were collected on 42 consecutive patients with symptoms of cerebral TIA. TIA imaging data were compared with those from a contemporaneous group of 23 completed stroke patients. RESULTS: Twenty of the 42 TIA patients (48%) demonstrated neuroanatomically relevant focal abnormalities on diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) imaging. When present, DWI/ADC signal changes in TIA patients were less pronounced and smaller in volume than those in completed stroke patients. TIA symptom duration was significantly longer for DWI-positive than for DWI-negative patients, 7.3 versus 3.2 hours. Diffusion MRI information changed the suspected anatomic and vascular TIA localization and the suspected etiologic mechanism in over one third of patients with diffusion MRI abnormalities. Of the 20 TIA patients with identifiable lesions on diffusion MRI, 9 had follow-up imaging studies; of these, 4 did not show a relevant infarct on follow-up imaging. CONCLUSIONS: Diffusion MRI demonstrates ischemic abnormalities in nearly half of clinically defined TIA patients. The percentage of patients with a DWI lesion increases with increasing total symptom duration. In nearly half, the diffusion MRI changes may be fully reversible, while in the remainder the diffusion MRI findings herald the development of a parenchymal infarct despite transient clinical symptoms. Finally, diffusion imaging results have significant clinical utility, frequently changing the presumed localization and etiologic mechanism.

Adult↗

PubMed2001

Evaluating pediatric brain tumor cellularity with diffusion-tensor imaging.

OBJECTIVE: MR imaging of central nervous system (CNS) malignancies falls short of a definitive evaluation. Tissue diagnosis remains the gold standard. Diffusion-tensor MR imaging measures the apparent diffusion coefficient and diffusion anisotropy of water in tissue. The purpose of this study was to test the hypothesis that the apparent diffusion coefficient may improve the MR imaging evaluation of newly diagnosed CNS neoplasms. We examined the relationship between the apparent diffusion coefficient, anisotropy, and tumor cellularity in 12 pediatric patients. MATERIALS AND METHODS: On the basis of histopathologic evaluation, tumors in this case series were segregated into three types: low-grade gliomas, embryonal tumors, and nonembryonal high-grade tumors. Mean apparent diffusion coefficient and anisotropy values obtained from the solid components of each tumor were compared with cellularity, total cellular area, and total nuclear area derived from biopsy material. RESULTS: The apparent diffusion coefficient ratio (tumor to normal brain) correlated well with tumor classification (p = 0.001). Anisotropy was decreased similarly in all tumor classifications. The absolute apparent diffusion coefficient correlated well with cellularity (p = 0.014) and total nuclear area (p = 0.005) per high-power field. The correlation between apparent diffusion coefficient and total cellular area per high-power field was not statistically significant. CONCLUSION: The apparent diffusion coefficient may be predictive of tumor classification and may be a useful tool in characterizing tumor cellularity and total nuclear area. These parameters are not available in standard MR imaging. Therefore, diffusion-tensor imaging may enhance the diagnostic process in pediatric CNS malignancies.

Adolescent↗

PubMedDate not supplied

[Diffusion in bone tissue of antibiotics].

DIFFICULT ASSESSMENT: Bone and joint infections are difficult to treat. Therapeutic success depends greatly on the diffusion of antibiotics into bone tissue. Few studies have been devoted to this subject and the variable nature of those reported hinders interpretation. Bone biopsies are generally obtained during orthopedic procedures. Antibiotic administration routes vary although intravenous infusion predominates. Agar gel diffusion is generally used for antibiotic assays but methodology varies depending of the study. The most recent reports use high-performance liquid chromatography. DIFFUSION STUDIES: The different studies examining antibiotic diffusion in bone tissue describe three classes: good diffusion (greater than 30%), moderate diffusion (between 15% and 30%), and low diffusion (less than 15%). Antibiotics in the good diffusion class include fluoroquinolones, teicoplanin, macrolides, rifampicin and trimethoprime. Antibiotics with moderate bone diffusion are ureidopenicillins, second and third generation cephalosporins, aminoglycosides, clindamycin, fosfomycin and vancomycin. Those with low bone diffusion are aminopenicillins, penicillin M and first generation cephalosporins. No data is available on the bone diffusion of pristinamycin. DATA INTERPRETATION: The clinical impact of these classifications must be interpreted with precaution when considering bone and joint infections as they were established on the basis of pharmacokinetic studies and not clinical trials. They would however appear to be useful in guiding antibiotic prophylaxis for orthopedic surgery in protocols with administration conditions and concentration goals similar to the experimental conditions. PRACTICAL ATTITUDES: These laboratory results could be used in clinical practice by comparing the MIC50 of the germs regularly encountered in bone infections (staphylococci, streptococci including enterococci, Gram negative bacilli including P. aeruginosa and H. influenzae) with concentrations obtained in the different studies, i.e. by calculating the inhibitor coefficient (IQ) of each antibiotic for each susceptible germ. This gives a classification by efficacy (excellent IQ > 10, good 1 < IQ < 10, poor IQ < 1) useful for guiding antibiotic choice in the difficult situation of bone and joint infection.

Anti-Bacterial Agents↗

PubMed2005

Diffusion MRI in multiple sclerosis.

Diffusion imaging is a quantitative, MR-based technique potentially useful for the study of multiple sclerosis (MS), due to its increased pathologic specificity over conventional MRI and its ability to assess in vivo the presence of tissue damage occurring outside T2-visible lesions, i.e., in the so-called normal-appearing white and gray matter. The present review aims at critically summarizing the state-of-the-art and providing a background for the planning of future diffusion studies of MS. Several pieces of evidence suggest that diffusion-weighted and diffusion tensor MRI are sensitive to MS damage and able to detect its evolution over relatively short periods of time. Although a significant relationship between diffusion-weighted MRI findings and MS clinical disability was not found in the earliest studies, with improved diffusion imaging technology correlations between diffusion abnormalities and MS clinical aspects are now emerging. However, the best acquisition and postprocessing strategies for MS studies remain a matter of debate and the contribution of newer and more sophisticated techniques to diffusion tensor MRI investigations in MS needs to be further evaluated. Although changes in diffusion MRI indices reflect a net loss of structural organization, at present we can only speculate on their possible pathologic substrates in the MS brain. Postmortem studies correlating diffusion findings with histopathology of patients with MS are, therefore, also warranted.

Autopsy↗

PubMed2002

Modelling of the enzymatic kinetically controlled synthesis of cephalexin: influence of diffusion limitation.

In this study the influence of diffusion limitation on enzymatic kinetically controlled cephalexin synthesis from phenylglycine amide and 7-aminodeacetoxycephalosporinic acid (7-ADCA) was investigated systematically. It was found that if diffusion limitation occurred, both the synthesis/hydrolysis ratio (S/H ratio) and the yield decreased, resulting in lower product and higher by-product concentrations. The effect of pH, enzyme loading, and temperature was investigated, their influence on the course of the reaction was evaluated, and eventually diffusion limitation was minimised. It was found that at pH >or=7 the effect of diffusion limitation was eminent; the difference in S/H ratio and yield between free and immobilised enzyme was considerable. At lower pH, the influence of diffusion limitation was minimal. At low temperature, high yields and S/H ratios were found for all enzymes tested because the hydrolysis reactions were suppressed and the synthesis reaction was hardly influenced by temperature. The enzyme loading influenced the S/H ratio and yield, as expected for diffusion-limited particles. For Assemblase 3750 (the number refers to the degree of enzyme loading), it was proven that both cephalexin synthesis and hydrolysis were diffusion limited. For Assemblase 7500, which carries double the enzyme load of Assemblase 3750, these reactions were also proven to be diffusion limited, together with the binding-step of the substrate phenylglycine amide to the enzyme. For an actual process, the effects of diffusion limitation should preferably be minimised. This can be achieved at low temperature, low pH, and high substrate concentrations. An optimum in S/H ratio and yield was found at pH 7.5 and low temperature, where a relatively low reaction pH can be combined with a relatively high solubility of 7-ADCA. When comparing the different enzymes at these conditions, the free enzyme gave slightly better results than both immobilised biocatalysts, but the effect of diffusion limitation was minimal.

Catalysis↗

PubMed2006

Diffusivity of oxygen in aerobic granules.

This work for the first time estimated apparent oxygen diffusivity (D(app)) of two types of aerobic granules, acetate-fed and phenol-fed, by probing the dissolved oxygen (DO) level at the granule center with a sudden change in the DO of the bulk liquid. With a high enough flow velocity across the granule to minimize the effects of external mass transfer resistance, the diffusivity coefficients of the two types of granules were estimated with reference to a one-dimensional diffusion model. The carbon source has a considerable effect on the granule diameter (d) and the oxygen diffusivity. The diffusivity coefficients were noted 1.24-2.28 x 10(-9) m2/s of 1.28-2.50 mm acetate-fed granules, and 2.50-7.65 x 10(-10) m2/s of 0.42-0.78 mm phenol-fed granules. Oxygen diffusivity declined with decreasing granule diameter, in particular, the diffusivity of acetate-fed granules is proportional to the size, whereas the diffusivity of phenol-fed granules is proportional to the square of granule diameter. The existence of large pores in granule, evidenced by FISH-CLSM imaging, was proposed to correspond to the noted size-dependent oxygen diffusivity. The phenol-fed granules exhibited a higher excellular polymer (ECP) content than the acetate-fed granules, hence yielding a lower oxygen diffusivity.

Acetates↗

PubMed1993

Diffusion and concentration profiles of drugs in gels.

A versatile membraneless method was used to study the diffusion of acetaminophen, ibuprofen, indomethacin, theophylline, and chlorpheniramine in thermoreversible gels. Two independent ways to calculate the diffusion coefficients and to verify Fickian transport are presented; the most sensitive criterion for Fickian transport being an agreement between the concentration profile for the drug in the gel and the free diffusion model. The diffusion of acetaminophen, ibuprofen, and indomethacin was studied at different temperatures in 1% (w/w) agarose gels. The diffusion coefficients for these drugs were found to be essentially the same as in water, and the apparent diffusion activation energies are close to that for self diffusion of water (4.62 kcal/mol), indicating a common mechanism for the diffusion of these drugs in the gel. The diffusivity of chlorpheniramine was also studied in 4% (w/w) agarose gels or with part of the agarose substituted with other polymers (e.g., chitosan and sodium alginate). These two oppositely charged polymers, mixed together, were found to occupy an "equivalent polymer volume" that was three times larger than the same amount of each of the constituents. When chitosan and gelatin-B were mixed into the agarose gel, non-Fickian transport resulted. Such non-Fickian transport was also observed with theophylline diffusing in a mixture of agarose, chitosan, and sodium alginate at a low pH.

Acetaminophen↗