NAD-sensitive hydrogel for the release of macromolecules

Role of Mechanical Factors in Applications of Stimuli-Responsive Polymer Gels - Status and Prospects

Due to their unique characteristics such as multifold change of volume in response to minute change in the environment, resemblance of soft biological tissues, ability to operate in wet environments, and chemical tailorability, stimuli responsive gels represent a versatile and very promising class of materials for sensors, muscle-type actuators, biomedical applications, and autonomous intelligent structures. Success of these materials in practical applications largely depends on their ability to fulfill application-specific mechanical requirements. This article provides an overview of recent application-driven development of covalent polymer gels with special emphasis on the relevant mechanical factors and properties. A short account of mechanisms of gel swelling and mechanical characteristics of importance to stimuli-responsive gels is presented. The review highlights major barriers for wider application of these materials and discusses latest advances and potential future directions toward overcoming these barriers, including interpenetrating networks, homogeneous networks, nanocomposites, and nanofilamentary gels.

Myogenic stem cell-laden hydrogel scaffold in wound healing of the disrupted external anal sphincter

OBJECTIVE

To evaluate the effect of myogenic stem cell-laden hydrogel scaffold on contractile function and histomorphology of the external anal sphincter (EAS) after transection without repair.

METHODS

Eighty female rats underwent anal sphincter transection without repair. After 2 weeks, animals were injected at the transection site with: nothing (non-repaired control, NRC group); a polyethylene glycol-based hydrogel matrix scaffold combined with phosphate-buffered saline (PBS/hydrogel group); a hydrogel matrix scaffold combined with myogenic stem cells (stem cell/hydrogel group): or type I collagen (collagen) group. 4 (n = 40) or 12 (n = 40) weeks later, the anal sphincter complexes were dissected out and analyzed for contractile function, disruption, and striated muscle volume. Time-matched unoperated controls (UOC) were utilized for each of the two time points (n = 20).

RESULTS

After 4 weeks, maximal electrical field-stimulated (EFS) contractions were significantly decreased in all four non-repaired treatment groups compared with UOC. However, EFS-stimulated contractions, tetanic force generation, and twitch tension were improved in non-repaired EAS injected with stem cell/hydrogel group relative to the NRC, PBS/hydrogel, or collagen groups. NRC and sphincters injected with PBS/hydrogel deteriorated further by 12 weeks, while those receiving stem cell/hydrogel maintained improved contractile function at varying frequencies and voltages. Striated muscle volume increased from 4 to 12 weeks for PBS/hydrogel and stem cell/hydrogel animals. At 12 weeks, stem cell/hydrogel animals had greater sphincter striated muscle volumes compared with all other treatment groups.

CONCLUSION

In this animal model, sustained improvement of contractile responses in non-repaired EAS treated with biogel scaffold and myogenic stem cells suggests that a biologically compatible matrix may facilitate stem cell survival, differentiation, or function leading to recovery of contractile function even after persistent disruption.

Partitioning of charged and neutral dextran-dye derivatives in biphasic cellulose nanocrystal suspensions

The partitioning behaviour of dye-labeled dextrans of high molecular weight in aqueous suspensions of native cellulose nanocrystals was studied. Cellulose concentrations lie in the isotropic–nematic coexistence region. Blue dextrans of various molecular weights and degrees of substitution of dye molecules (anionic Cibacron blue 3G-A) were investigated. Increasing the total concentration of blue dextran and degree of dye substitution led to increasing partition coefficients. Increasing dextran molecular weight resulted in higher partition coefficients, in agreement with theory. Partition coefficients were larger than predicted theoretically using a second virial coefficient approximation. Electrostatic and entropic contributions to the partition coefficient of blue dextran are discussed. Dextrans labeled with neutral fluorescein isothiocyanate did not partition preferentially in this system.Key words: partition coefficient, cellulose nanocrystals, dextrans, degree of substitution, polyelectrolyte.

Smart Polymeric Gels: Redefining the Limits of Biomedical Devices

This review describes recent progresses in the development and applications of smart polymeric gels, especially in the context of biomedical devices. The review has been organized into three separate sections: defining the basis of smart properties in polymeric gels; describing representative stimuli to which these gels respond; and illustrating a sample application area, namely, microfluidics. One of the major limitations in the use of hydrogels in stimuli-responsive applications is the diffusion rate limited transduction of signals. This can be obviated by engineering interconnected pores in the polymer structure to form capillary networks in the matrix and by downscaling the size of hydrogels to significantly decrease diffusion paths. Reducing the lag time in the induction of smart responses can be highly useful in biomedical devices, such as sensors and actuators. This review also describes molecular imprinting techniques to fabricate hydrogels for specific molecular recognition of target analytes. Additionally, it describes the significant advances in bottom-up nanofabrication strategies, involving supramolecular chemistry. Learning to assemble supramolecular structures from nature has led to the rapid prototyping of functional supramolecular devices. In essence, the barriers in the current performance potential of biomedical devices can be lowered or removed by the rapid convergence of interdisciplinary technologies.

A smart membrane based on an antigen-responsive hydrogel

Hydrogel membranes have been fabricated that incorporate antibody/antigen moieties. The permeability of large solutes through these membranes is dependent on the presence of soluble antigen that can compete with the internal interactions between antibody and antigen leading to an increase in gel mesh size. Specifically, the membrane's structure is based on a dextran backbone grafted with a fluorescein isothiocyanate (FITC) antigen and a sheep anti-FITC IgG antibody. The backbone is covalently cross-linked by conjugated divinyl sulfone (DVS) groups. The gel structure is additionally stabilized by affinity crosslinks formed by biospecific interactions between the bound IgG and FITC. FTIR spectra of the gel are consistent with formation of covalent bonds between cysteine groups in the IgG and DVS groups in the dextran. Results obtained using isothermal titration calorimetry (ITC) confirmed the competitive interaction binding between IgG-FITC-dextran and free sodium fluorescein at pH 5.0. Scanning electron microscopy (SEM) of samples prepared using cryofixation and cryofracturing techniques showed that observed changes in permeability correlate with free fluorescein-dependent structural changes in the gel. Three-dimensional images obtained from confocal laser scanning microscopy show that these changes occur throughout the gel and indicate that SEM results are not artifacts of sample preparation. The permeability of these gels, as shown by blue-dextran (12 kDa) diffusion, increases in response to the presence of free fluorescein of the external medium, which causes competitive displacement of the affinity cross-links. Sequential addition and removal of sodium fluorescein showed that these permeability changes are reversible.

Induced phase separation in low-ionic-strength cellulose nanocrystal suspensions containing high-molecular-weight blue dextrans

Spontaneous entropic phase separation phenomena occur in a wide range of systems containing highly anisotropic colloidal particles. Among these are aqueous suspensions of negatively charged cellulose I nanocrystals produced by sulfuric acid hydrolysis of native cellulose, which phase separate into isotropic and chiral nematic liquid-crystalline phases. Phase separation of an isotropic phase from a completely ordered nanocrystal suspension may be induced by the addition of salts or nonadsorbing macromolecules. In previous work (Edgar, C. D.; Gray, D. G. Macromolecules 2002, 35, 7400-7406), an isotropic phase was found to form over a period of several days when blue dextran (a sulfonated triazine dye, Cibacron blue 3G-A, covalently attached to high-molecular-weight dextran chains) was added to initially ordered suspensions. Here we report work showing that the observed phase separation was associated with the charged dye molecules attached to the dextran. The Cibacron blue 3G-A dye attached to blue dextran was found to induce greater phase separation than free (unbound) dye; at increasing ionic strength, depletion attractions due to the blue dextran increasingly contribute to the phase separation.

A novel pH- and ionic-strength-sensitive carboxy methyl dextran hydrogel

A fast and simple method for the preparation of pH-sensitive hydrogel membranes for drug delivery and tissue engineering applications has been developed using carbodiimide chemistry. The hydrogels were formed by the intermolecular cross-linking of carboxymethyl dextran (CM-dextran) using 1-ethyl-(3-3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS). Infrared spectra of the hydrogels suggest the formation of ester bonds between the hydroxyl and carboxyl groups in the CM-dextran. The porosity of the hydrogels produced, as shown by protein diffusion, increases in response to changes in the pH and the ionic strength of the external medium. The results show pH-dependent swelling behaviour arising from the acidic pedant groups in the polymer network. The diffusion of the protein lysozyme through the hydrogel membranes increased with increases in both pH (5.0-9.0) and ionic strength. The effect of changes of pH and ionic strength on the hydrogel's permeability was shown to be reversible. Scanning electron microscopy of these hydrogels showed that pH-dependent changes in permeability are mirrored by morphological changes in gel structure.