Biomedical subjects

S H Gehrke

Publications and source records attributed to S H Gehrke.

9 recordsLinked to original sources

PubMed2000

Adsorption of plasma proteins on to poly(ethylene oxide)/poly(propylene oxide) triblock copolymer films: a focus on fibrinogen.

Triblock copolymers of the form PEO(alpha)PPO(beta)PEO(alpha) [where PEO is poly(ethylene oxide) and PPO is poly(propylene oxide)] have many biomedical applications, many of which depend on the surface properties of the copolymers and the influence that those properties have on the adsorption of proteins. As a tool to help us better understand, predict and exploit the influence of these triblock copolymers on protein adsorption, we developed a model system in which well-defined monolayers of the copolymers are supported by solid, hydrophobic, microscopic beads. At the bead/water interface, the copolymers all form stable films in which the nominal molecular areas correspond to those of the molecules when they are packed rather tightly at the air/water interface. Beads coated with condensed films of copolymers that contain short PEO segments and elicit appreciable inflammation absorb appreciable quantities of plasma proteins, including fibrinogen, from aqueous solution. Beads coated with fibrinogen aggregate when they are stirred in the presence of thrombin, a consequence of interbead fibrin formation. Beads coated with condensed films of copolymers that contain long PEO segments and elicit little inflammation absorb little plasma protein, and they do not aggregate in the presence of thrombin. Our data and observations are consistent with the prevailing notion that the utility of triblock copolymers as agents for modifying the surface properties of blood-contacting surfaces derives from the influence of the copolymers on the adsorption of plasma proteins. In this regard, the ability of the copolymers to influence fibrinogen-mediated adhesive events may be particularly important. As to the mechanism of protein resistance, our data support the proposal that sibling PEO segments of copolymers in condensed films fold back across their parental PPO cores, limiting access of proteins to the hydrophobic cores themselves.

Adsorption↗

PubMed1998

Enhanced loading and activity retention of bioactive proteins in hydrogel delivery systems.

A simple, general and effective technique is developed for increasing the loading of bioactive macromolecules into hydrogels using the principles of aqueous two-phase extraction. Model proteins, ovalbumin and alpha-amylase, were loaded into hydrated gels by soaking the gels in a buffered solution of protein containing 12 wt% PEG-10000 and 0.22 M salt (KCl, KBr or KI). The PEG and salts were expected to enhance protein sorption according to aqueous two-phase extraction heuristics. In the absence of the solution additives, the gels absorbed little protein. But protein loading up to 270 mg ovalbumin/g polymer and 67 mg alpha-amylase/g polymer was obtained when the PEG and salt were added; loading was not significantly dependent upon salt type. Ovalbumin release from hydrated gels was diffusion-controlled. The diffusion coefficient was 1.10 (-7) cm2/s, consistent with protein absorption into the gel rather than adsorption onto the surface. Release kinetics of both proteins from dried, glassy gels matched conventional behavior for release of absorbed drugs from glassy polymers. Finally, alpha-amylase activity was retained even after drying the loaded gel at 65 degreesC, conditions which denatured the enzyme when not absorbed in the gel.

Algorithms↗

PubMed1998

Protein sorption and recovery by hydrogels using principles of aqueous two-phase extraction.

Use of the thermodynamic principles of aqueous two-phase extraction (ATPE) to drive protein into a crosslinked gel is developed as a protein isolation and separation technique, and as a protein loading technique for drug delivery applications. A PEG/dextran gel system was chosen as a model system because PEG/dextran systems are widely used in aqueous two-phase extraction and dextran gels (Sephadex(R)) are common chromatographic media. The effects of polymer concentrations and molecular weights, salts, and pH on the partitioning of ovalbumin matched ATPE heuristics and data trends. Gel partition coefficients (Cgel/Csolution) increased with increasing PEG molecular weight and concentration and decreasing dextran concentration (increased gel swelling). The addition of PEG to the buffer solution yielded partition coefficients more than an order of magnitude greater than those obtained in systems with buffer alone, or added salt. A combined salt/PEG system yielded an additional order of magnitude increase. For example, when ovalbumin solution (2.3 mg/mL) was equilibrated with Sephadex(R) G-50 at pH 6.75, the partition coefficients were 0.13 in buffer, 0.11 in buffer with 0.22M KI, 2.3 in 12 wt% PEG-10,000 and 32.0 in 12 wt% PEG-10, 000 with 0.22M KI. The effect of anions and cations as well as ionic strength and pH on the partitioning of ovalbumin also matched ATPE heuristics. Using the heuristics established above, partition coefficients as high as 80 for bovine serum albumin and protein recoveries over 90% were achieved. In addition, the wide range of partition coefficients that were obtained for different proteins suggests the potential of the technique for separating proteins. Also, ovalbumin sorption capacities in dextran were as high as 450 mg/g dry polymer, and the sorption isotherms were linear over a broad protein concentration range.

Adsorption↗

PubMed1998

Polymer erosion and drug release characterization of hydroxypropyl methylcellulose matrices.

Polymer erosion of matrices of similarly substituted hydroxypropyl methylcellulose (HPMC) polymers was examined, and drug release in terms of diffusion and erosion contributions was characterized, focusing on matrices containing either polymer alone or a drug content of 25% level with no added excipients. A novel approach was utilized to separate diffusional and erosional contributions to drug release. Diffusional drug release was determined by fitting release data versus (time)0.45, and the drug release due to erosion was quantified by subtracting the percent predicted for diffusional drug release from the total drug release at each specific time point. Drug release resulting from polymer erosion was linear versus time and was found to be a function of the number average molecular weight of the polymer. In contrast, diffusional release rates were comparable for all HPMC grades studied and, thus, were independent of number average molecular weight of the polymers studied. Under stirring conditions of 10-100 rpm as well as static condition, the detachment of individual polymer chains at the matrix surface occurred at a faster rate relative to diffusion away from the matrix surface. The erosion study indicated that polymer diffusion of the HPMC polymer chains through the aqueous diffusion layer was the rate-limiting step for polymer erosion. In general, polymer erosion was found to be inversely related to the polymer number average molecular weight. A scaling law was used to relate polymer erosion rate with the respective polymer number average molecular weight. Similar relationships were obtained for matrices with and without drug at a stirring rate of 100 rpm.

Chemistry, Pharmaceutical↗

PubMed1997

Fibrinogen-dependent adherence of macrophages to surfaces coated with poly(ethylene oxide)/poly(propylene oxide) triblock copolymers.

The role of fibrinogen in the adherence of macrophages to polymer surfaces was studied using a human cell line (THP-1 cells) and polystyrene-divinylbenzene beads coated with poly(ethylene oxide)/poly(propylene oxide) copolymers of the form PEO alpha PPO beta PEO alpha. The amphiphilic character of the surface of the beads was varied using a series of copolymers with constant PPO core lengths but different PEO segments. Fibrinogen-dependent adherence of monocytes/macrophages to the modified beads was then assessed. The adherence of THP-1 cells to copolymer-coated beads correlates well with the amount of fibrinogen bound to the beads. Those beads coated with the most hydrophobic surfactant molecules bound the most fibrinogen and the most cells. On these surfaces, the concentration of fibrinogen was less than half that of the protein on unmodified beads. Despite the lower amount of bound fibrinogen, the number of adherent cells was 37% greater than the number of adherent cells on fibrinogen-coated, copolymer-free beads. Beads coated with the most hydrophilic surfactants bound just 10% the amount of fibrinogen bound to unmodified beads. On these surfaces, the number of adherent cells was decreased by approximately 25% with respect to the number of cells bound to beads coated with fibrinogen alone. We propose that the hydrophobic surfactant molecules may act as inflammatory agents by facilitating fibrinogen-dependent cellular adhesion.

Adsorption↗

PubMed1997

Factors determining hydrogel permeability.

Developing hydrogel membranes and coatings of appropriate permeability characteristics is key to the success of a number bioartificial organ technologies. Key principles relevant to the design and application of hydrogels for such applications were reviewed. The first key point is that permeability is a function of both transport and thermodynamic properties, the diffusion coefficient and partition coefficient, respectively, and that these parameters can be evaluated separately. Although the aspect of partitioning often emphasized is size exclusion, this review points out that many other relevant interactions come into play, especially hydrophobic and electrostatic interactions, and that these phenomena can dominate size exclusion. Similarly, while the diffusion coefficient also is strongly dependent upon size, other interactions can also cause diffusivity to deviate from theories which consider only solute size and gel swelling. For example, the heterogeneity of hydrogel networks can result in permeabilities that fail to decline as much as might be anticipated if networks were uniform.

Animals↗

PubMed1994

Evidence for Fickian water transport in initially glassy poly(2-hydroxyethyl methacrylate).

Water sorption which is not classically Fickian has been observed in a variety of polymers. Deviation from Fickian kinetics is widely assumed to be caused by rate-limiting polymer relaxation, despite minimal proof of this. To the contrary, the evidence accumulated in this work indicates that water transport in initially glassy poly(2-hydroxyethyl methacrylate) (PHEMA), an important water-swellable biomedical polymer, is controlled by Fickian diffusion. First of all, the fractional water uptake is initially linear and independent of sample thickness when plotted against the square root of time over initial thickness, as expected for a Fickian process. Furthermore, the moving solvent front also advanced with the square root of time. Temperature, polymer thermal history and initial solvent concentration all affected the sorption kinetics of PHEMA in manners consistent with a Fickian process. The invariably Fickian sorption mechanism is believed to be the consequence of the water molecule's small size and affinity for hydrophilic, swellable polymers.

Adsorption↗

PubMed1994

Development of an azopolymer based colonic release capsule for delivering proteins/macromolecules.

The colon, high in microbial contents with degradation ability of azo bonds, seems to be suitable for site-specific delivery of drugs. The degradation of azoaromatic hydrogel depends on the degree of swelling. However, the degree of swelling of azopolymers and the ability of the azoaromatic polymers to protect peptide drugs against the action of digestive enzymes still remains to be established. An azopolymer was synthesized and membranes were cast to investigate the degree of swelling. An azopolymer-coated capsule suitable for delivering peptides/proteins to the colon was developed by in vitro evaluation using vitamin B12 as a model drug to screen various formulations. For in vitro dissolution the half-changed method was used to mimic the pH-time profile in the GI tract. It was shown that the release of vitamin B12 is dependent on the degree of swelling which increases as pH increases. An in vivo study was also performed with insulin in azopolymer-coated capsules in 6 normal beagle dogs. The results suggest that this azopolymer-coated capsule is capable of protecting peptide/macromolecules against digestive enzymes. However, to obtain a promising peroral insulin capsule, more formulation work is required to achieve better absorption availability.

Administration, Oral↗

PubMedDate not supplied

Protein isolation by solution-controlled gel sorption.

Illustrated are the principles for isolating proteins from solution by sorption into a polymer gel phase, driven by the addition of a water-soluble polymer to the protein solution. The separation is shown to be analogous to conventional two-phase aqueous extraction. However, the use of a gel phase rather than a solution for absorbing the protein makes separation of the protein from the polymer and the recycling of the gel phase much simpler. The model system used was linear poly(ethylene glycol) (PEG) and dextran gel. Increasing the molecular weight and concentration of the PEG favored sorption by the gel of ovalbumin, bovine serum albumin, cytochrome c, and hemoglobin. The proteins could be quantitatively recovered by immersing the gel in PEG-free solution.

Chemistry Techniques, Analytical↗