Synthetic pH sensitive polyampholyte hydrogels: A preliminary study

pH Responsiveness of hydrogels formed by telechelic polyampholytes

We investigate the influence of pH on the rheological and structural properties of hydrogels formed by hydrophobic association of the sticky ends of the triblock terpolymer poly(methyl methacrylate)-b-poly(2-(diethylamino)ethyl methacrylate-co-methacrylic acid)-b-poly(methyl methacrylate) (PMMA-b-P(DEA-co-MAA)-b-PMMA). The middle block is a weak polyampholyte having a pH dependent charge density and sign, which enables tuning of the rheological and structural properties by pH variation. Small-angle neutron scattering (SANS) studies of solutions in D₂O at 0.05 wt% and pH 3.0 reveal clusters of interconnected spherical micelles having PMMA cores, stabilized by repulsive ionic interactions in the middle polyampholyte block. With increasing pH, the degree of ionization of the DEA units decreases, whereas the one of the MAA units increases, resulting in a complete loss of the correlation between these micelles. At a concentration of 3 wt% at low pH values, the system forms a gel with charged fuzzy spheres from PMMA interacting via a screened Coulomb potential. With increasing pH, the gel disintegrates due to the decrease in the effective charge on the micelles. At both concentrations, the hydrophobic aggregation of micelles is observed near the isoelectric point. At pH 3.0-7.4, the autocorrelation functions measured by rotational dynamic light scattering at 3 wt% exhibit a decay steeper than single exponential, which confirms that the gels are frozen, presumably due to the glassy PMMA cores and hydrophobic interpolyelectrolyte complexes. At pH 11, the diffusion of single micelles is observed in addition to the frozen dynamics.

Stimulus-responsive "smart" hydrogels as novel drug delivery systems

Recently, there has been a great deal of research activity in the development of stimulus-responsive polymeric hydrogels. These hydrogels are responsive to external or internal stimuli and the response can be observed through abrupt changes in the physical nature of the network. This property can be favorable in many drug delivery applications. The external stimuli can be temperature, pH, ionic strength, ultrasonic sound, electric current, etc. A majority of the literature related to the development of stimulus-responsive drug delivery systems deals with temperature-sensitive poly(N-isopropyl acrylamide) (pNIPAAm) and its various derivatives. However, acrylic-based pH-sensitive systems with weakly acidic/basic functional groups have also been widely studied. Quite recently, glucose-sensitive hydrogels that are responsive to glucose concentration have been developed to monitor the release of insulin. The present article provides a brief introduction and recent developments in the area of stimulus-responsive hydrogels, particularly those that respond to temperature and pH, and their applications in drug delivery.

Thermosensitive polymers as carriers for DNA delivery

Copolymers of 2-(dimethylamino) ethyl methacrylate (DMAEMA) and N-isopropylacryl amide (NIPAAm) of various monomer ratios and molecular weights were evaluated as carrier systems for DNA delivery. All copolymers, even with a low DMAEMA content of 15 mol%, were able to bind to DNA at 25 degrees C. Light-scattering measurements indicate that complexation is accompanied by precipitation of the (co)polymer in the complex caused by a drop of the lower critical solution temperature of the (co)polymer. The (co)polymer/plasmid ratio at which complexes with a size of around 200 nm were formed increased with increasing NIPAAm content of the copolymer and was independent of molecular weight of the (co)polymer. However, complexes containing (co)polymers of low molecular weight or high NIPAAm content prepared at 25 degrees C aggregated rapidly when the temperature was raised to 37 degrees C, whereas complexes containing (co)polymers of high molecular weight or lower NIPAAm content were relatively stable at 37 degrees C. The zeta potential of the complexes was also independent of molecular weight of the (co)polymer and increased with increasing (co)polymer/plasmid ratio until a plateau value was reached. The (co)polymer/plasmid ratio at which this plateau was reached increased with increasing NIPAAm content. The plateau values decreased from around 26 mV to around 13 mV when the NIPAAm content of the copolymer was increased from 0 to 85 mol%. The cytotoxicity of the complexes strongly decreased with increasing NIPAAm content and was independent of molecular weight of the (co)polymer. The transfection efficiency of complexes with poor stability was in general much lower than that of complexes with good stability. The transfection efficiency as a function of the (co)polymer/plasmid ratio showed a bell-shaped curve. The (co)polymer/plasmid ratio at which the transfection efficiency was maximal increased with increasing NIPAAm content, while the maximum transfection efficiency strongly decreased with increasing NIPAAm content of the copolymer. The results of this study show that the formation of stable (co)polymer/plasmid complexes with a size of around 200 nm is a prerequisite for efficient transfection. Furthermore, the transfection efficiency and cytotoxicity strongly decreased with decreasing zeta potential. Therefore, besides the size, the zeta potential can also be used as a characteristic to predict the behavior of this type of (co)polymer/plasmid complexes in transfection. Copolymers of DMAEMA and NIPAAm provided with a homing device may be interesting carrier systems for gene targeting because these copolymers can condense DNA to small particles, and the resulting complexes show a low cytotoxicity and aspecific transfection.

Synthetic strategies for the preparation of precursor polymers and of microcapsules suitable for cellular entrapment by polyelectrolyte complexation of those polymers

The production of microcapsules suitable for the entrapment of mammalian cell by means of polyelectrolyte complexation has, of a necessity, led to the development of novel strategies for the preparation of relatively bioinert polymers which complex efficiently under unique conditions to produce a mechanically resilient membrane with efficient transport properties. In this communication we relate a brief overview of capsule-membrane forming systems for the immunoisolation (or potential immunoisolation) of mammalian cells, which are based upon the complexation of polyelectrolyte (PE) polymers; with emphasis on precursor synthesis and relationships between precursor polymer structure and capsule membrane stability.

Microcapsules through polymer complexation. Part 3: Encapsulation and culture of human Burkitt lymphoma cells in vitro

Methacrylic acid (MAA) based polyelectrolytes were complexed with protonated or quaternized dimethylaminoethyl methacrylate (DMAEMA) containing polyelectrolytes to form microcapsules in vitro. Anchorage independent human Burkitt lymphoma (Raji) cells were successfully cultured in the presence of dissolved MAA containing polymer. Capsule morphology was investigated by light microscopy and by scanning electron microscopy (SEM). Capsules based on quaternized DMAEMA containing polymer were found to be more stable than capsules containing protonated DMAEMA functionality. Raji cells were successfully encapsulated in both systems and divided to confluence; thereafter sufficient pressure was exerted to burst open the capsules. Cells released from these capsules appeared to suffer no discernible trauma and were successfully isolated and subcultured to confluence.