Haesun Park

Gastric retention properties of superporous hydrogel composites

In many applications, usefulness of conventional hydrogels is limited by their slow swelling. To improve the swelling property of the conventional hydrogels, we have synthesized superporous hydrogels (SPHs) which swell fast to equilibrium size in minutes due to water uptake by capillary wetting through numerous interconnected open pores. The swelling ratio was also large in the range of hundreds. The mechanical strength of the highly swollen SPHs was increased by adding a composite material during the synthesis. The composite material used in the synthesis of SPH composites was Ac-Di-Sol((R)) (croscarmellose sodium). The gastric retention property of the prepared SPH composites was tested in dogs both in fasted and fed conditions. The SPH composites were placed in a hard gelatin capsule (size 000) for oral administration. All dogs tested were fasted for 36 h before experiments. Under the fasted condition, the SPH composite remained in the stomach for 2-3 h after before breaking into two pieces and being emptied. When food was given before the experiment just once following 36 h of fasting, the SPH composite remained in the stomach for more than 24 h, even though the fed condition was maintained only for the first few hours. Our study indicated that SPH composites possessed three properties necessary for gastric retention: fast swelling; superswelling; and high mechanical strength. While more improvements need to be made, the SPH composites provide the basis for the development of effective long-term gastric retention devices.

Oral vaccination of animals with antigens encapsulated in alginate microspheres.

Most infectious diseases begin at a mucosal surface. Prevention of infection must therefore consider ways to enhance local immunity to prevent the attachment and invasion of microbes. Despite this understanding, most vaccines depend on parenterally administered vaccines that induce a circulating immune response that often does not cross to mucosal sites. Administration of vaccines to mucosal sites induces local immunity. To be effective requires that antigen be administered often. This is not always practical depending on the site where protection is needed, nor comfortable to the patient. Not all mucosal sites have inductive lymphoid tissue present as well. Oral administration is easy to do, is well accepted by humans and animals and targets the largest inductive lymphoid tissue in the body in the intestine. Oral administration of antigen requires protection of antigen from the enzymes and pH of the stomach. Polymeric delivery systems are under investigation to deliver vaccines to the intestine while protecting them from adverse conditions that could adversely affect the antigens. They also can enhance delivery of antigen specifically to the inductive lymphoid tissue. Sodium alginate is a readily available, inexpensive polymer that can be used to encapsulate a wide variety of antigens under mild conditions. Orally administered alginate microspheres containing antigen have successfully induced immunity in mice to enteric (rotavirus) pathogens and in the respiratory tract in cattle with a model antigen (ovalbumin). This delivery system offers a safe, effective means of orally vaccinating large numbers of animals (and perhaps humans) to a variety of infectious agents.

pH-sensitivity of fast responsive superporous hydrogels

Stimuli-sensitive hydrogels (or smart hydrogels) are hydrogels that swell or shrink in response to small changes in environmental conditions in which they are placed. While the extent of swelling or shrinking may be large, the kinetics of such changes is slow, since the diffusion of water into and out of the hydrogel is a slow process. To obtain fast responses, we have prepared superporous hydrogels (SPHs) that can swell or shrink extremely fast regardless of their dimensions. The swelling and shrinking are orders of magnitude faster than expected for a nonporous hydrogel of the same dimensions. Water molecules are taken up into the SPHs by capillary forces, and this makes water uptake much faster than diffusion. The swelling ratio of the poly(acrylamide- co -acrylic acid) (p(AM-co-AA)) SPHs was dependent on the pH and ionic strength of the medium. The effect of pH was most pronounced and the effect of ionic strength was observed at all pH values. SPHs made at pH around 5 showed transient maximum swelling when exposed to pH 1.2 medium due to the transient low hydrogen ion concentration inside the swelling SPHs. The p(AM-co-AA) SPHs showed repeated swelling and shrinking by alternating the medium pH between 1.2 and 7.5, and the changes in swelling ratio was quite fast occurring in a matter of a minute. This fast sensitivity may make the stimuli sensitive hydrogels useful in many applications not previously possible. These materials can be used for applications where a single-piece hydrogel is more advantageous than hydrogel microparticulates.

Morphological characterization of surface-induced platelet activation.

Morphological changes of platelets activated on glass and dimethyldichlorosilane-treated glass were investigated using video microscopy. The platelet morphological changes were quantified by measuring the area and circularity of spreading platelets. In addition, re-organization of cytoskeletal structures of spread platelets was examined. The effects of precoated albumin and fibrinogen on the platelet spreading kinetics were examined as a function of surface protein concentrations. Results showed that platelet shape changes were very sensitive to the surface concentration of precoated proteins. In general, platelets on fibrinogen-precoated surfaces spread fully to a circular shape and developed an extensive inner filamentous zone. In the presence of albumin on the surface, however, platelets could not spread fully and the development of the inner filamentous zone was very poor. For both albumin and fibrinogen, the maximum effects of precoated proteins on platelet shape changes were observed when the surface protein concentration reached the monolayer concentration.

The hydrogel template method for fabrication of homogeneous nano/microparticles

Nano/microparticles have been used widely in drug delivery applications. The majority of the particles are prepared by the conventional emulsion methods, which tend to result in particles with heterogeneous size distribution with sub-optimal drug loading and release properties. Recently, microfabrication methods have been used to make nano/microparticles with a monodisperse size distribution. The existing methods utilize solid templates for making particles, and the collection of individual particles after preparation has not been easy. The new hydrogel template approach was developed to make the particle preparation process simple and fast. The hydrogel template approach is based on the unique properties of physical gels that can undergo sol-gel phase transition upon changes in environmental conditions. The phase reversible hydrogels, however, are in general mechanically too weak to be treated as a solid material. It was unexpectedly found that gelatin hydrogels could be made to possess various properties necessary for microfabrication of nano/microparticles in large quantities. The size of the particles can be adjusted from 200 nm to >50 microm, providing flexibility in controlling the size in drug delivery formulations. The simplicity in processing makes the hydrogel template method useful for scale-up manufacturing of particles. The drug loading capacity is 50% or higher, and yet the initial burst release is minimal. The hydrogel template approach presents a new strategy of preparing nano/microparticles of predefined size and shape with homogeneous size distribution for drug delivery applications.

Oral vaccination with alginate microsphere systems

Abstract Oral vaccination is a simple, efficient way of inducing immunity at mucosal surfaces. The slow development of oral vaccines has been mainly due to the lack of suitable delivery systems. We have used hydrogel microspheres to deliver various vaccines to several animal species by oral administration. Oral delivery of vaccines using alginate microspheres elicited the production of secretory IgA (sIgA) at the mucosal surfaces in mice, rabbits, and cattle. Oral vaccination of chicken resulted in an increased delayed-type hypersensitivity, a cell-mediated immune response, indicating a positive response to the vaccine. Our studies have clearly shown that alginate microspheres are effective for the oral administration of vaccines.

Pore structure of superporous hydrogels

Hydrogels with a fast swelling property have been synthesized using a gas blowing technique. Since those hydrogels possess interconnected pores of which diameters are in the order of a few hundred micrometers, they are called “superporous hydrogels” (SPHs). The fast swelling of SPHs in aqueous solution is due to the absorption of water by capillary pressure through interconnected pores (i.e. open channels). Because of the importance of pore structures on the fast swelling property, effects of surface porosity on the swelling kinetics were examined. The surface chemistry of the polymerization mold made of glass was varied using various silanes, and the surface morphology of the synthesized SPHs was examined by scanning electron microscopy. The porosity was measured using mercury porosimetry. Despite differences in surface morphology and surface porosity of SPHs, the swelling kinetics were not changed significantly. The internal pore structures remained the same as the surface porosity changed. The study indicates that the swelling of SPHs is predominantly determined by the internal pore structures, and small differences in the surface porosity do not alter the overall swelling kinetics of SPHs. Copyright

Study on the prevention of surface-induced platelet activation by albumin coating

To understand how albumin on the surface inhibits surface-induced platelet activation, we adsorbed albumin on dimethyldichlorosilane-coated glass (DDS-glass) and modified the adsorbed albumin by three different methods. The adsorbed albumin was crosslinked with glutaraldehyde, dried and rehydrated, or digested with trypsin. Surface albumin concentration did not change by crosslinking; however, it decreased by about 15% by a simple dry-and-rehydration process. Trypsin digestion reduced the surface albumin concentration by 50%. Platelets were found to adhere and activate on albumin coated DDS-glass, if the adsorbed albumin was modified. The extent of platelet activation was quantified with two numeric parameters, the spread area and circularity. Fibrinogen adsorption to the dried or digested albumin layer resulted in enhancement of platelet activation, while adsorption of more albumin inhibited platelet activation. The results suggest that albumin can inhibit platelet activation as long as it covers the surface completely and remains flexible on the surface. This study indicates that steric repulsion is one of the mechanisms of surface passivation by albumin.

Induction of pulmonary immunity in cattle by oral administration of ovalbumin in alginate microspheres

Respiratory infectious diseases are an important cause of economic losses to the cattle industry. There is a need for an effective, easy to administer vaccine to the critical bacterial pathogens that cause pneumonia in cattle. An orally administered vaccine could be given to a large number of animals without significant stress to the animals and with minimal labor. The purpose of this study was to determine whether the oral administration of a model antigen (ovalbumin) in alginate microspheres could induce pulmonary immunity in cattle. Calves were vaccinated orally with ovalbumin (OVA) following either a subcutaneous (s.c.) or oral priming dose of OVA. Calves primed and boostered by oral administration (oral/oral) of OVA encapsulated in alginate microparticles had increased numbers of antigen-specific IgA ASCs (ASCs) in bronchoalveolar lavage (BAL) fluids. Calves that received a s.c. priming followed by an oral booster inoculation (s.c./oral) of OVA in alginate microspheres had a greater number of anti-OVA IgA, IgG1 and IgG2 ASCs in BALF. S.c./oral calves also had increased numbers of anti-OVA IgG1 ASCs in peripheral blood whereas oral/oral calves had none. S.c./oral calves had increased anti-OVA IgG1, IgG2, and IgA titers in BALF, and IgG1 and IgG2 in serum compared to both oral/oral and sham vaccinated calves. These results indicate that oral administration of antigen encapsulated in alginate microspheres results in a mucosal immune response in the respiratory tract of cattle. Furthermore, s.c. priming both enhanced the IgA response and stimulated an IgG1 and IgG2 response not seen in oral/oral calves. The difference in antibody isotype results suggest that design of the vaccination protocol can direct antibody responses as needed for a specific immunization program.

Effect of compression on fast swelling of poly(acrylamide-co-acrylic acid) superporous hydrogels.

Superporous hydrogels (SPHs) swell to a large size in a very short time. In many applications it is preferred to compress SPHs to reduce the overall dimension in the dried state. The effects of compression on the swelling property of SPHs were examined. The swelling property of the compressed SPHs was dependent on the orientation of the SPHs during compression. If SPHs were compressed in an orientated manner so that they retained interconnected porous structure, they were able to swell to near equilibrium within 10 min of immersion in aqueous fluids. If SPHs were compressed in a manner that did not retain the open pore structure, the swelling rate was greatly reduced. The results showed that the SPHs could be compressed without significant sacrifice of the fast swelling property if compressed in a proper orientation. Because pores were formed owing to the generation of gas which rose from bottom to the top of the container, the compression parallel to the pore formation resulted in preservation of the pore structure, and thus fast swelling property. The ability to compress SPHs, maintaining the fast swelling property, is expected to be useful in various applications including development of gastric retention devices for oral drug delivery.