Blends of natural and synthetic polymers: A new route to novel biomaterials

Blends of natural and synthetic polymers were studied for potential applications in the biomedical field. Collagen and hyaluronic acid were mixed in aqueous solution with poly(vinyl alcohol) and poly(acrylic acid). The properties of the blends were studied by differential scanning calorimetry and dynamic mechanical thermal analysis. Some methods were also investigated to enhance the miscibility of the polymers in these blends.     

From self-organizing polymers to nanohybrid and biomaterials

Block copolymers form a large number of superlattices with characteristic dimensions in the range of a few nanometers up to several micrometers by self-organization. The interplay of supramolecular physics and chemistry opens up new approaches to the production of inorganic, organic, and biological structures and to their integration into functional units. Possible applications in the fields of materials science and molecular biology are being investigated. Block copolymers find numerous applications from the production of inorganic nanoparticles (metals, semiconductors, magnets) and mesoporous materials up to take-up/release systems in chemo- and gene therapy.     

Adhesion of filamentous microbial cells on paper covered with ionic polymers by radiation polymerization

The adhesion of filamentous microbial cells such as Trichoderma reesei was studied by using carriers covered with polymers which were prepared by the radiation polymerization of ionic monomers. The weight of the cells adhering to the carriers increased with increasing cationic monomer content, indicating that the surface of the polymers prepared from cationic and hydrophobic monomers is suitable for the adhesion of the cells. The production of cellulase in the cells adhered to polymers from cationic monomers was higher than that in cells adhered to polymers prepared from anionic monomers. The growth of the cells adhered to the surfaces of the polymers was affected by the hydrophilicity of the polymers.     

Adhesion of modified carbamide-formaldehyde oligomer

The calculation of the work of adhesion is presented for modified adhesive compositions based on Carbamide-formaldehyde resin used to develop plate materials. The modification of synthetic resins by alcohols is proven to reduce their surface tension to the level of the substrate, as well as to increase their ability to stick to a surface, the homogeneity of the distribution due to the particles of filling agents, and the work of adhesion.     

Multiphase electropatterning of cells and biomaterials

Tissues formed by cells encapsulated in hydrogels have uses in biotechnology, cell-based assays, and tissue engineering. We have previously presented a 3D micropatterning technique that rapidly localizes live cells within hydrogels using dielectrophoretic (DEP) forces, and have demonstrated the ability to modulate tissue function through the control of microscale cell architecture. A limitation of this method is the requirement that a single biomaterial must simultaneously harbor biological properties that support cell survival and function and material properties that permit efficient dielectrophoretic patterning. Here, we resolve this issue by forming multiphase tissues consisting of microscale tissue sub-units in a 'local phase' biomaterial, which, in turn, are organized by DEP forces in a separate, mechanically supportive 'bulk phase' material. We first define the effects of medium conductivity on the speed and quality of DEP cell patterning. As a case study, we then produce multiphase tissues with microscale architecture that combine high local hydrogel conductivity for enhanced survival of sensitive liver progenitor cells with low bulk conductivity required for efficient DEP micropatterning. This approach enables an expanded range of studies examining the influence of 3D cellular architecture on diverse cell types, and in the future may improve the biological function of inhomogeneous tissues assembled from a variety of modular tissue sub-units.     

Chitosan based surfactant polymers designed to improve blood compatibility on biomaterials

We developed chitosan based surfactant polymers that could be used to modify the surface of existing biomaterials in order to improve their blood compatibility. These polymers consist of a chitosan backbone, PEG side chains to repel non-specific protein adsorption, and hexanal side chains to facilitate adsorption and proper orientation onto a hydrophobic substrate via hydrophobic interactions. Since chitosan is a polycationic polymer, and it is thrombogenic, the surface charge was altered to determine the role of this charge in the hemocompatibility of chitosan. Charge had a notable effect on platelet adhesion. The platelet adhesion was greatest on the positively charged surface, and decreased by almost 50% with the neutralization of this charge. A chitosan surface containing the negatively charged SO(3)(-) exhibited the fewest number of adherent platelets of all surfaces tested. Coagulation activation was not altered by the neutralization of the positive charge, but a marked increase of approximately 5-6 min in the plasma recalcification time (PRT) was displayed with the addition of the negatively charged species. Polyethylene (PE) surfaces were modified with the chitosan surfactant resulting in a significant improvement in blood compatibility, which correlated to the increasing PEG content within the polymer. Adsorption of the chitosan surfactants onto PE resulted in approximately an 85-96% decrease in the number of adherent platelets. The surfactant polymers also reduced surface induced coagulation activation, which was indicated by the PEG density dependent increase in PRTs. These results indicate that surface modification with our chitosan based surfactant polymers successfully improves blood compatibility. Moreover, the inclusion of either negatively charged SO(3)(-) groups or a high density of large water-soluble PEG side chains produces a surface that may be suitable for cardiovascular applications.     

Polymeric materials as biomaterials under particular consideration of biodegradable polymers

Biomaterials may be defined as artificial materials which fulfill the mechanical requirements and interact with the biosystem they are in contact with in same way as a natural material would react in the same place. While the requirements of mechanical properties can be reached by suitable organo-polymeric and inorganic materials the interfacial biocompatiblity is neither understood in all its complexity nor can be fulfilled by any of the applied materials. Surface modification and characterization with greatest scrutiny and the observation of the answer of selected parameters of the biosystem are a subject of utmost interest. A few examples will be presented. In the long range, however, it has to be considered that any material is degraded and hence should present continuously a renewable biocompatible surface. On the other hand, materials are desired which deliberately are biodegradable. Presently available materials are polylactides and copolymers. An alternative could be presented by polydepsipeptides because of two reasons, (i) the local concentration of acid formed upon degradation would be reduced as compared to polylactides which in certain cases might be advantageous and (ii) the aminoacid units could carry side groups with bioactive molecules attached. Therefore, a new method of acylation of an aminoacid with a hydroxyacid is presented as well as the cyclisation to result in the cyclic depsipeptide and the polymerisation to yield the polydepsipeptide. The microstructure of the polymers, the thermal properties and the degradation behaviour is presented.     

Stimuli-responsive hydrogels based on polysaccharides incorporated with thermo-responsive polymers as novel biomaterials

In recent years, intelligent hydrogels which can change their swelling behavior and other properties in response to environmental stimuli such as temperature, pH, solvent composition and electric fields, have attracted great interest. The hydrogels based on polysaccharides incorporated with thermo-responsive polymers have shown unique properties such as biocompatibility, biodegradability, and biological functions in addition to the stimuli-responsive characters. These "smart" hydrogels exhibit single or multiple stimuli-responsive characters which could be used in biomedical applications, including controlled drug delivery, bioengineering or tissue engineering. This review focuses on the recent developments and future trends dealing with stimuli-responsive hydrogels based on grafting/blending of polysaccharides such as chitosan, alginate, cellulose, dextran and their derivatives with thermo-sensitive polymers. This review also screens the current applications of these hydrogels in the fields of drug delivery, tissue engineering and wound healing.     

Functionalized polynorbornene dielectric polymers: Adhesion and mechanical properties

Within the microelectronics industry, there is an ongoing trend toward miniaturization coupled with higher performance. High glass-transition temperature polynorbornenes exhibit many of the key performance criteria necessary for these demanding applications. However, homopolynorbornene exhibits poor adhesion to common substrate materials, including silicon, silicon dioxide, aluminum, gold, and copper. In addition, this homopolymer is extremely brittle, yielding less than 1% elongation-to-break values. To address these issues, the homopolymer was functionalized to improve adhesive and mechanical properties. Attaching triethoxysilyl groups to the polymer backbone substantially improved the adhesion, but at the cost of increasing the dielectric constant because of the polarity of the functional group. Alkyl groups were also added to the backbone, which decreased the rigidity of the system, and resulted in significantly higher elongation-to-break values and a decrease in residual stress. The addition of an alkyl group slightly decreased the dielectric constant of the polymer as a result of an increase in molar volume. The coefficient of thermal expansion and modulus are also reported for the polynorbornene functionalized with triethoxysilyl groups using a multiple substrate approach. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3003–3010, 1999     

Adhesion of protective coatings with modified filler

The effect of physical (ultrasonic) and chemical (exposure in 3% solution of acetic acid) activation of filler on the adhesion strength of the protective coatings based on styrene–acrylic dispersions to different substrates is studied. It is shown that modification of kaolins allows one to increase the adhesion strength by 40–50% in the case of the physical activation and by 1.1–1.2 times in the case of chemical activation. The best results are achieved with the use of E-21 dispersion as the polymer matrix.     

Properties of modified polymers based on syndiotactic 1,2-polybutadiene

The influence of the chemical modification on the properties of syndiotactic 1,2-polybutadiene derivatives that contain various functional groups in macromolecules and show promise for practical use was studied. The dependence of the glass transition point and the fluidity and rheological properties of modified polydiene melts on the nature of the functional group in the macrochain and on the degree of the polymer functionalization was revealed. The possibility of directional synthesis of modified polymer products with the required operation characteristics on the basis of syndiotactic 1,2-polybutadiene was demonstrated.     

Effect of hydrophobically modified polymers on shear-induced multilamellar vesicles

The effects of polymer concentration, polymer molecular weight, and hydrophobe substitution level of modified poly(acrylic acid) polymers on the formation, size, and viscoelastic properties of shear-induced multilamellar vesicles (onions) are studied by rheology and light diffraction. The onions are close-packed, space-filling vesicles formed by shearing aqueous lamellar phases of C12E5 surfactant to produce phases with sufficient order and size uniformity (O(1-3 microm)) to diffract light. The addition of hydrophobically modified polymers enhances the rate of formation, uniformity, and stability independent of hydrophobe substitution level. Onion size decreases with increasing shear rate as observed for pure surfactant onion systems, but the shear-rate dependence is changed by the polymer. The onion phase has a plateau modulus that increases with polymer concentration but is independent of hydrophobe substitution level or molecular weight. The model presented by Panizza et al. that relates the plateau modulus of the onion phase to membrane rigidity and the compression modulus is consistent with independent measurements of membrane properties from SANS.     

Adhesion of modified epoxy matrices to reinforcing fibers

The adhesive strength of joints between fibers and epoxy matrices modified with three types of modifiers—active diluents, thermostable rigid-chain thermoplasts, and dispersed fillers—is studied. It is shown that the introduction of modifiers increases interface strength in several cases in a particular concentration interval. The introduction of thermostable thermoplasts into epoxides is more effective. Possible mechanisms of adhesive-strength synergism for each of the modification types are discussed.     

Adhesion of high polymers. II. Wettability of elastomers

Data on wettability of elastomers should be considered basic to the understanding of all phases of elastomer adhesion. However, no such data in the form of critical surface tensions were available for elastomers other than polydimethylsiloxane. For this study, 18 elastomers were selected to determine the effects of functional groups, of geometrical and structural isomerisms, of copolymerization, and of the induced orientation upon wettability. Most results support the constitutive law of wettability established by Shafrin and Zisman. The effect of structural isomerisms in the form of a vinyl side group and cyclization is discussed. An equation for the calculation of critical surface tension of a copolymer or of a mixture of isomers is proposed as follows: where N_i is the mole-fraction of the individual monomer in the copolymer and \documentclass{article}\pagestyle{empty}\begin{document}$ \gamma _{c_i } $\end{document} is the critical surface tension of each homopolymer. Most elastomer adhesion studies conducted in the past were concerned with the diffusion theory of adhesion. This study further supports the conclusion on the role of diffusion and adsorption in adhesion advanced in Part I, especially with respect to the physical state of polymer at the time of application. The wettability data in this study could shed some light upon major basic mechanisms involved in elastomer reinforcement.     

Adhesion and tack of polymers: Influence of mechanical properties and surface tensions

The adhesion of various polymers used as model adhesives, polyisobutylene, polyacrylates etc. has been investigated by means of an apparatus measuring the adhesive failure energyw in dependence on contact time, contact pressure, rate of separation, and temperature. The adhesive failure energy of adhesive joints formed with low contact pressure during a short contact time is called tack. After a sufficiently long contact time and with a high bonding pressure an adhesive joint exhibits its maximum energy of separationw _m .The viscoelastic properties of the model adhesives were characterized by creep experiments in dependence on time and temperature. The surface tension of the polymer adhesives and adherents could be determined by contact angle measurements. Adhesion measurements of polyisobutylene on a number of adherents were carried out in air and in various liquids in order to obtain information about the influence of surface tension on tack and maximum adhesive failure energy. w _m can be written as the product of two terms: the thermodynamic work of adhesionW _A which is related to the surface and interfacial tensions of adhesive and adherent and a dimensionless function dependent on temperature and rate of separation which describes the viscoeleastic properties of the adhesive and which obeys the rate-temperature superposition principle known from linear viscoelasticity. The tack is related to incomplete bond formation and cannot be described in the same manner. It is, however, strongly dependent on the viscoelastic properties of the adhesive showing a maximum at about 50 to 70 °C above the glass transition temperature. It is, moreover, influenced by the compliance in the plateau range above the glass transition which is determined by the entanglement network of the polymer. Wetting of the adherent by the adhesive is a further important condition for high tack values which is fulfilled if the adherent has a higher surface tension than the adhesive.     

High-performance liquid chromatography on silica modified with temperature-responsive polymers

Summary The separation selectivity of temperature-responsive poly(N-isopropylacrylamide)-modified silica as a packing material for high performance liquid chromatography was investigated with steroids, alkaloids, and substituted anilines as solutes. The elution profiles of the solutes depended on the temperature of the column and the methanol content of the mobile phase, indicating that the separation selectivity could be controlled by the column temperature or the mobile phase composition.     

Binding of surfactants to hydrophobically modified polymers

Recent progress in the understanding of the binding of surfactants to hydrophobically modified polymers (HMP), and the consequences of such binding, is reviewed. HMP are water-soluble polymers onto which low proportions of hydrophobic sidechains (hydrophobes) have been grafted. In an aqueous environment, the HMP hydrophobes associate among themselves and with added surfactant molecules into micelle-like aggregates. An HMP may therefore be considered as a ‘modified surfactant’, and the binding of surfactants to HMP is analogous to the mixed micellisation in mixed surfactant solutions. The binding isotherm gives the concentration of free (monomeric) surfactant and the stoichiometry of the HMP/surfactant complex at different total compositions. In mixtures involving ionic surfactants, it is found that the free surfactant often dominates, and gives important contributions to the ionic strength. Characteristic properties of HMP/surfactant mixtures may be related to stoichiometries of the mixed complexes. Thus, the maximum in solution viscosity, which is commonly found in HMP/surfactant mixtures, occurs at a similar hydrophobe stoichiometry (ratio of bound surfactant to HMP hydrophobe) for many different systems, although the total concentrations of surfactant at the maximum may vary by orders of magnitude, depending on the surfactant cmc. The solubility of a complex of oppositely charged HMP and surfactant is related to the charge stoichiometry of the complex. The phase separation/redissolution phenomena occurring in the bulk solution influence the HMP adsorption to surfaces and the forces between surfaces with adsorbed HMP.