Effect of chitosan-gluconic acid conjugate/poly(vinyl alcohol) cryogels as wound dressing on partial-thickness wounds in diabetic rats.

We previously developed chitosan cryogels from chitosan-gluconic acid conjugate without using toxic additives for wound care. In this study, we improved physiological characteristics of the previous cryogels by incorporating poly(vinyl alcohol) that also form cryogels. Mechanical strength of the cryogels was more than two times higher than that of the previous cryogels. Furthermore, the incorporation of poly(vinyl alcohol) enhanced water retention and resistance to degradation of the gels by lysozyme. The cryogels retained the favorable biological properties of the previous cryogels that they accelerate infiltration of inflammatory cells into wound sites. Time period for repairing 50 % of initial area of partial-thickness skin wound treated with the cryogels (4.0 ± 1.1 days) was shorter than those with gauze (6.5 ± 0.3 days) or a commercial hydrogel dressing (5.7 ± 0.3 days). Finally, we confirmed that incorporation of basic fibroblast growth factor into the cryogels was effective to further accelerate wound healing (2.7 ± 1.0 days). These results demonstrate that the cryogels in this study are promising for wound care.

Rapidly serum-degradable hydrogel templating fabrication of spherical tissues and curved tubular structures.

Development of the techniques for fabricating three-dimensional tissues still poses significant challenges for tissue engineering. We used hydrogels obtained from phenol-substituted amylopectin (AP-Ph) as templates for preparing multicellular spherical tissues (MSTs) and endothelialized curved tubular structures in type I collagen gel. AP-Ph hydrogel microparticles of diameter 200 µm and fibers of diameter 500 µm disappeared within hours of soaking in a serum-containing medium. HeLa cells and human endothelial cells were enclosed in the microparticles and hydrogel fibers, respectively, and then embedded in Ca-alginate microcapsules or the collagen gel. The enclosed cells were released in cavities formed by hydrogel degradation in the serum-containing medium. The released HeLa cells in the spherical cavities grew and formed MSTs, eventually filling the cavities. The spherical tissues were easily harvested by liquefying the Ca-alginate hydrogel microcapsule membrane by chelation using sodium citrate. The released endothelial cells grew on the tubular cavity surfaces and formed tubular structures. An endothelial cell network was formed by cell migration into the collagen gel. These results demonstrate the potential of serum-degradable AP-Ph hydrogels in constructing three-dimensional tissues.

Fabrication of capillary-like network in a matrix of water-soluble polymer using poly(methyl methacrylate) microfibers.

Poly(methyl methacrylate) (PMMA) microfibers were used as a template for development of a capillary-like network in agarose hydrogel. Microfibers with diameter 10-20 µm, which is comparable to the diameter of native capillary vessels, were fabricated using a wet spinning technique. The microfibers were embedded in agarose gel and dissolved by immersing the gel in dichloromethane. The resultant microchannels in the gel had the same diameter as the microfibers, and allowed an aqueous solution to be perfused through the gel. The methodology is promising for fabricating a capillary-like network in tissue engineering scaffolds of various water-soluble polymers.

In situ simultaneous protein-polysaccharide bioconjugation and hydrogelation using horseradish peroxidase.

We propose the peroxidase-catalyzed simultaneous conjugation and hydrogelation of polysaccharide and protein derivatives, each possessing phenolic hydroxyl (Ph) moieties, as a novel route for obtaining protein-polysaccharide conjugate hydrogels. We used alginate, gelatin, and albumin derivatives bearing Ph moieties (Alg-Ph, Gela-Ph, and Alb-Ph) to demonstrate the feasibility. The gelation time of conjugate gels decreased with decreasing H(2)O(2) concentration and with increasing horseradish peroxidase concentration. Gelation time was controllable from a few seconds to 6 min. The repulsion force detected at 40% compression of a conjugate gel obtained from a mixture of Alg-Ph and Gela-Ph at 1.0% (w/v), respectively, was more than 2.8 times larger than that detected for gels produced from 3.0% (w/v) Gela-Ph or 2.0% (w/v) Alg-Ph alone. Cell adhesiveness of gels was tunable by changing the type of protein derivative. A gel from Gela-Ph and Alg-Ph showed higher cell adhesiveness than Alg-Ph gel, but a gel produced from Alb-Ph and Alg-Ph showed a lower cell adhesiveness than Alg-Ph gel. The conjugate gel was degradable by degrading alginate molecules using the nonproteolytic enzyme alginate lyase. The tunable gelation, mechanical properties, and cell adhesiveness of polysaccharide-protein conjugate hydrogels obtained through peroxidase-catalyzed gelation indicates great potential for a wide range of applications, such as scaffolds for tissue engineering and carriers for drug delivery system.

Hematin is an alternative catalyst to horseradish peroxidase for in situ hydrogelation of polymers with phenolic hydroxyl groups in vivo.

Hematin, an iron-containing porphyrin used in the management of porphyria attacks, was evaluated as an alternative catalyst to horseradish peroxidase (HRP) for in situ gelation of polymers with phenolic hydroxyl (Ph) moieties in vivo. An aqueous solution of gelatin derivative with Ph moieties was gellable in the presence of both hematin and H2O2. A total of 98.6% adhesion of L929 fibroblast cells 4 h after seeding and their similar morphology to those on substrate coated with unmodified gelatin indicated no obvious substrate cytotoxicity. High cytocompatibility of the gelation process under conditions inducing gelation within 20 s was demonstrated by 95.0% viability of enclosed cells in vitro. Furthermore, no adverse effects of hematin were found compared with HRP by histological observation of cutaneous tissue surrounding in situ formed gels. The versatility of hematin for gelation of a variety of polymers possessing Ph groups was demonstrated by the gelation of a carboxymethyl cellulose derivative.

In situ gellable oxidized citrus pectin for localized delivery of anticancer drugs and prevention of homotypic cancer cell aggregation.

The aim of this study was to develop in situ gellable hydrogels composed of periodate oxidized citrus pectin (OP) for localized anticancer drug delivery and evaluate the potential of OP to inhibit cancer metastasis. Doxorubicin (Dox) was coupled to OP by imine bonds. Adipic dihydrazide (ADH) was used for cross-linking of the Dox-OP conjugates. The Dox-OP conjugate solution gelled within 2 min after addition of ADH. The release rate of Dox from the hydrogels was controllable by an additive amount of ADH. The released Dox retained anticancer activity. OP was shown to have a potency to prevent homotypic cancer cell aggregation compared to unmodified citrus pectin, strongly suggesting that OP released from hydrogels in vivo will inhibit cancer metastasis. These results indicate that OP hydrogels have the potential to prevent progression of primary cancer by the released Dox and generation of metastatic cancer by the released OP.

Immobilization of Pseudomonas cepacia lipase onto electrospun polyacrylonitrile fibers through physical adsorption and application to transesterification in nonaqueous solvent.

The lipase of Pseudomonas cepacia was immobilized onto electrospun polyacrylonitrile (PAN) fibers and used for the conversion of (S)-glycidol with vinyl n-butyrate to glycidyl n-butyrate in isooctane. The rate of reaction with the adsorbed lipase was 23-fold higher than the initial material. After 10 recyclings, the initial reaction rate was 80% of the original rate. This system of enzyme immobilization is therefore suitable for carrying out transesterification reactions in nonaqueous solvents.

Development of subsieve-size capsules and application to cell therapy.

Reduction in the diameter of cell-enclosing capsules has a practical application in cell therapy as it induces beneficial effects such as higher molecular exchangeability between the enclosed cells and the ambient environment, as well as higher mechanical stability and biocompatibility. Subsieve-size capsules are capsules of less than 100 microm in diameter, which are approximately one tenth the size of conventional cell-enclosing microcapsules. Such small capsules can be prepared using the emulsion system obtained via the jetting process in which a cell-suspending polymer solution is extruded into an ambient coflowing water-immiscible liquid from a needle several hundred micrometers in diameter. The capsule size can be controlled by changing the velocity of the polymer solution and the ambient water-immiscible liquid. The emulsification process does not significantly affect viability of mammalian cells enclosed in the resultant subsieve-size capsules. In this chapter we will review the technique of subsieve-size capsule production and the effects of diameter reduction on the enclosed cells and properties of the capsules.

Production of cell-enclosing hollow-core agarose microcapsules via jetting in water-immiscible liquid paraffin and formation of embryoid body-like spherical tissues from mouse ES cells enclosed within these microcapsules.

We developed agarose microcapsules with a single hollow core templated by alginate microparticles using a jet-technique. We extruded an agarose aqueous solution containing suspended alginate microparticles into a coflowing stream of liquid paraffin and controlled the diameter of the agarose microparticles by changing the flow rate of the liquid paraffin. Subsequent degradation of the inner alginate microparticles using alginate lyase resulted in the hollow-core structure. We successfully obtained agarose microcapsules with 20-50 microm of agarose gel layer thickness and hollow cores ranging in diameter from ca. 50 to 450 microm. Using alginate microparticles of ca. 150 microm in diameter and enclosing feline kidney cells, we were able to create cell-enclosing agarose microcapsules with a hollow core of ca. 150 microm in diameter. The cells in these microcapsules grew much faster than those in alginate microparticles. In addition, we enclosed mouse embryonic stem cells in agarose microcapsules. The embryonic stem cells began to self-aggregate in the core just after encapsulation, and subsequently grew and formed embryoid body-like spherical tissues in the hollow core of the microcapsules. These results show that our novel microcapsule production technique and the resultant microcapsules have potential for tissue engineering, cell therapy and biopharmaceutical applications.

Oxidized alginate-cross-linked alginate/gelatin hydrogel fibers for fabricating tubular constructs with layered smooth muscle cells and endothelial cells in collagen gels.

Hydrogel fibers that possessed a cell-adhesive surface and were degradable via enzymatic reactions were developed for fabricating tubular constructs with smooth muscle cell (SMC) and endothelial cell (EC) layers, similar to native blood vessels, in collagen gels. The fibers were prepared by soaking hydrogel fibers prepared from a solution of sodium alginate and gelatin containing bovine ECs (BECs) in medium containing oxidized alginate (AO). BECs soaked in 8.0% (w/v) AO showed no reduction in viability within 3 h of soaking. Furthermore, mouse SMCs (MSMCs) adhered and proliferated on the AO-cross-linked hydrogels. Based on these results, we prepared AO-cross-linked hydrogel fibers containing BECs, covered their surface with MSMCs, and embedded them in collagen gels. We then degraded the fibers using alginate lyase to obtain channels in the collagen gels. Histological analysis of the released ECs using a specific fluorescent dye revealed the formation of tubular structures with layered BECs and MSMCs.

The mixed coculture effect of primary rat hepatocytes and bone marrow cells is caused by soluble factors derived from bone marrow cells.

Heterospheroids consisting of hepatocytes and bone marrow cells (BMCs) are formed by the mixed coculture of these cells and enhance the expression and maintenance of the liver-specific functions of hepatocytes. Not only the soluble factors derived from these cells, but also functional organoid (heterospheroid) formation, are considered to underlie this coculture effect. Therefore, in the present study, we aimed to clarify the mechanism of this co-culture effect. We performed hepatocyte monoculture with conditioned media prepared from hepatocyte cultures, BMC cultures and a coculture of hepatocytes and BMCs. When using any type of conditioned medium, no hepatocyte spheroids formed, and the hepatocytes formed a monolayer. In addition, an effect for these conditioned media was shown in terms of the albumin production and ammonia metabolism activities of the hepatocytes; conditioned medium from BMCs showed the strongest effect. The monocultured hepatocytes in the conditioned medium derived from BMCs showed equivalent albumin production and ammonia metabolism activities to the cocultured spheroids of hepatocytes and BMCs. Therefore, it was determined that the effect of the coculture of hepatocytes and BMCs was caused by soluble factors derived from BMCs.

Synthesis and characterization of both ionically and enzymatically cross-linkable alginate.

Alginate with phenol moieties in the polymer side chains was synthesized through the conjugation reaction of alginate and tyramine. Immersing an aqueous solution of the alginate containing horseradish peroxidase into a solution containing H(2)O(2) caused the solution to gel via peroxidase-catalyzed oxidative coupling of the phenols. In addition, alginate prepared under appropriate reaction conditions retained the attractive properties associated with unmodified alginate; spherical gel beads were formed by dropping an aqueous alginate solution (1.0wt.%) into a solution containing calcium ions. The oxidative coupling of the phenols was effective for suppressing the destabilization of the alginate gel resulting from a loss of bonding between the divalent cations and alginate. The mechanical properties of the resultant gels were influenced by the preparation conditions of the alginate and the type of cross-linking.

Synthesis of enzymatically-gellable carboxymethylcellulose for biomedical applications.

We synthesized a carboxymethylcellulose with phenol moieties by covalently incorporating tyramine into carboxymethylcellulose using aqueous-phase carbodiimide activation chemistry. The resulting hydrogel was obtained from an aqueous solution of the conjugate via the horseradish peroxidase-catalyzed oxidation reaction of phenols by consuming H(2)O(2), where the gelation speed depended on the concentrations of enzyme and H(2)O(2). The viability of the mammalian cells enclosed within the hydrogel prepared from 1.5% (w/v) conjugate solution containing 5 units/ml horseradish peroxidase and 1 mM H(2)O(2), was 80% after 24 h. These results demonstrate that this carboxymethylcellulose with phenol moieties has potential for biomedical applications including tissue-engineering.

Synthesis of an agarose-gelatin conjugate for use as a tissue engineering scaffold.

We synthesized a conjugate in which gelatin was covalently crosslinked to agarose using 1,1-carbonyldiimidazole (CDI) in dimethyl sulfoxide in order to obtain gels with cellular adhesiveness that showed a sol-to-gel transition, but no gel-to-sol transition, around body temperature. The gelatin content of the conjugate increased by 2.7-fold when the concentration of CDI was increased from 1.3 to 32.7 mM. Aqueous solutions of the conjugate gelled upon cooling from 40 degrees C to 20 degrees C, but did not remelt at 37 degrees C. The percentage of adhered cells after 4 h of culture on a gel created from a conjugate containing about 25 wt% gelatin was similar to that for cells grown on tissue culture dishes. The adhered cells proliferated on the conjugate gel during culture for a further 5 d. In addition, the conjugate used in this study did not result in mechanical instability of the resultant gel compared to that of an unmodified agarose gel. These results demonstrate that this agarose-gelatin conjugate is a good candidate material for tissue engineering.

Expression of a liver-specific function by a hepatoblastoma cell line cocultured with three-dimensional endothelialized tubes in collagen gels.

A human hepatoblastoma cell line, Hep G2, showed albumin production activity, a hepatic function, when cocultured with three-dimensional endothelialized tubes in collagen gels. The albumin production rate of the collagen-based liver-like constructs increased with increasing length of the endothelialized tube in the construct.

Novel technique for fabricating double-layered tubular constructs consisting of two vascular cell types in collagen gels used as templates for three-dimensional tissues.

Double-layered tubular constructs consisting of two vascular cell types in collagen gels were fabricated using a previously developed technique [Takei et al., Biotechnol. Bioeng., 95, 1-7 (2006)]. Histological examination suggested that shear stress (4.0 dyn/cm2) on the luminal surface of the constructs induced morphological changes in their walls.

Biocompatibility of subsieve-size capsules versus conventional-size microcapsules.

Biocompatibility of cell-enclosing capsules, defined as suppression of pericapsular cellular reactions, is one of the factors governing the success of enclosed cell transplantation in in vivo cell therapy. Agarose capsules of subsieve size, less than 100 microm in diameter, and conventional size, approximately 300-1,000 microm in diameter, were implanted into the peritoneal cavity and epididymal fat pads of mice and rats, respectively, to determine the effect of a reduction in diameter to subsieve size. The degree of cellular reaction to the subsieve-size capsules was much lower than that of the conventional-size microcapsules, independent of implantation site. The frequency of overgrown subsieve-size capsules retrieved from the peritoneal cavities was less than 5% in contrast to approximately 20% for capsules 387 microm in diameter. In addition, no increase in floating cells, which are generated through capsule stimulation, was observed in the peritoneal cavity only with subsieve-size capsules. From these results, we concluded that subsieve-size capsules are more biocompatible than microcapsules of conventional size.

Subsieve-size agarose capsules enclosing ifosfamide-activating cells: a strategy toward chemotherapeutic targeting to tumors.

Localized activation of the prodrug ifosfamide in or close to tumors by implanting encapsulated ifosfamide-activating cells is an efficacious strategy for tumor therapy. The aim of this study was to evaluate the feasibility of subsieve-size agarose capsules for enclosing the cells in this application. Compared with many conventional microcapsules, subsieve-size agarose capsules are about one-tenth the size and have both higher mechanical stability and allow better molecular exchangeability than other systems. Cells that have been genetically modified to express cytochrome P450 2B1 enzyme were encapsulated in subsieve-size agarose capsules of approximately 90 microm in diameter and implanted into preformed tumors in nude mice. Living cells were detected for >1 month after encapsulation in vitro and showed enzymatic activity (i.e., they were able to activate ifosfamide). More significant regression of preformed tumors was observed in the recipients implanted with cell-enclosing capsules compared with those implanted with empty capsules. These results suggest that the strategy of using subsieve-size agarose capsules enclosing cytochrome P450 2B1-expressing cells is feasible for tumor therapy by chemotherapeutic targeting in combination with ifosfamide administration.

Development of mammalian cell-enclosing subsieve-size agarose capsules (<100 microm) for cell therapy.

Agarose capsules were prepared using a droplet breakup method in a coflowing stream. Subsieve-size capsules 76+/-9 microm in diameter were obtained by extruding 4 wt% agarose solution from a needle (300 microm inner diameter) at a velocity of 1.2 cm/s into an ambient liquid paraffin flow of 20.8 cm/s. Increasing the flow rate of the liquid paraffin and decreasing that of the agarose solution resulted in a decreased resultant capsule diameter. Reduction in diameter from several hundred micrometers to subsieve-size (<100 microm) enhanced molecular exchange and mechanical stability. Measurements based on the percentage of intact mitochondria in the cells demonstrated that the viability of the enclosed cells was independent of capsule diameter. No significant difference was observed between the viabilities of cells enclosed in capsules with diameters of 79+/-8 and 351+/-41 microm (p=0.43). Compared with cells seeded in a tissue culture dish, the cells enclosed in the subsieve-size capsules showed 89.2% viability.

Higher viscous solution induces smaller droplets for cell-enclosing capsules in a co-flowing stream.

Mechanical strength of cell-enclosing capsules governs the success of the transplantation of enclosed cells in vivo for cell therapy. Mechanical strength closely correlates with the concentration and molecular weight of the polymers present in the aqueous solution that end up in the capsules, and the viscosity of the aqueous polymer solution also depends on these two factors. Three aqueous solutions differing in viscosity (1.0, 36, and 194 mPa s) were extruded from a needle (300 microm inner diameter) at a velocity of 1.2 cm/s into an ambient co-flowing liquid paraffin laminar stream. Smaller droplets were obtained from a higher viscous solution. At a liquid paraffin velocity of 23.5 cm/s, the diameter of droplets obtained from the highest viscous solution (194 mPa s)) was 44 +/- 4 microm, and it represented 40% and 20% of that from droplets in solutions of 36 and 1.0 mPa s viscosity, respectively. The cells enclosed in these droplets maintained more than 95% viability during the droplet breakup process independent of the viscosity of the aqueous solution (p > 0.50). In addition, retrieved cells from the droplets showed the same proliferation profiles as the cells that were not subjected to the droplet breakup process, on tissue culture dishes (p > 0.13).