Gel microparticles composed of lightly cross-linked poly(acrylic acid) networks, onto which polyether chains (Pluronic F127) are grafted, are introduced. The hydrophobic poly(propylene oxide) chains aggregate within the microgel structure, and the resulting aggregates are capable of solubilizing hydrophobic drugs, such as taxol. At temperatures where the Pluronic chains are not aggregated, the microgels behave like networks without spatial heterogeneity. Upon formation of aggregates within hydrogels, their equilibrium swelling diminishes, and the swelling behavior indicates non-Gaussian chain distribution. The kinetics of gel swelling shows unusual temperature dependence of the effective diffusion coefficient, indicative of chain rearrangement within a certain temperature range. The microgels exhibit high ion-exchange capacity for cationic hydrophilic drugs. The potential for the newly obtained microgels to be used as drug carriers is discussed.

The study dealt with microgels that consist of loosely cross-linked poly(acrylic acid) onto which poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO−PPO−PEO) copolymers such as Pluronic F127 (average composition EO99PO67EO99) or L92 (EO8PO52EO8) segments are grafted. Microgels based on the more hydrophobic Pluronic L92 exhibited highly porous structure, while the microgels containing Pluronic F127 were generally larger and possessed smooth surfaces, a homogeneous structure, and lower ion-exchange capacity. A good correlation was observed between the microgel surface potential obtained from the potentiometric titration data and the independently measured electrophoretic ζ-potential. The differences in the microgel ion-exchange capacity account for the differences in the capacity of the microgels to absorb weakly basic anticancer drugs such as mitomycin, mitoxantrone, and doxorubicin. The uptake of hydrophobic drugs such as Taxol, estradiol, progesterone, and camptothecin was dictated by the content of PPO in the microgels, which determines their solubilizing capacity. The exclusion of proteins of varying molecular weight by the microgels reveals the effective pore size, which is below 7.5 nm in the F127-based microgels but is on the order of tens of nanometers in the L92-based microgels.

Effects of sodium dodecyl sulfate (SDS) on surface tension and rheology of aqueous solutions of an associative polymer, poly(ethylene oxide)-b-poly(propylene oxide)-b-(poly(ethylene oxide))-g-poly(acrylic acid) (Pluronic-PAA) have been studied. SDS starts to bind to the Pluronic-PAA copolymer at a concentration that is 25-fold lower than the critical micellization concentration (CMC) of the surfactant. The data indicate predominant binding between SDS and polyether segments of Pluronic-PAA. Incorporation of SDS into the Pluronic micelles makes the polymer less surface-active, as the surface tension increases. The Pluronic-PAA chains collapse when SDS binds, because the surfactant enhances hydrophobic intramolecular associations.

The interactions of DNA (salmon testes) with two new cationic block copolymers made of poly(2-dimethylaminoethyl) methacrylate and poly(ethylene oxide), PEO-pDMAEMA, or poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), L92-pDMAEMA, were studied with the aim to understand their different in vitro transfection efficiencies when used as nonviral delivery vectors. PEO-pDMAEMA does not show surface activity while L92-pDMAEMA is as surface active as its parent Pluronic L92. Surface tension, titration microcalorimetry, ethidium bromide displacement, and zeta-potential measurements were carried out in phosphate buffers at pH 5 and 7. The association of L92-pDMAEMA with DNA was strongly exothermic at both pHs; the critical aggregation concentration (CAC) corresponded to a N/P ratio of 0.3, the maximum energy evolved was reached for N/P ratios of 0.82 and 1.27 at pH 5 and pH 7, respectively, and the saturation occurred for N/P ratios close to 2. The presence of L92 in the structure of this new block copolymer apparently did not modify the thermodynamic parameters of the interaction with DNA. In contrast, the interaction with PEO-pDMAEMA was significantly less exothermic, and CAC and saturation occurred for N/Ps equal to 0.43 and 1.37, respectively. The strong affinity of L92-pDMAEMA for DNA was reflected in its capacity to displace ethidium bromide and in the jump in the values of the zeta potential when N/P is near 1. Above the N/P ratio at which electroneutral polyplexes are formed, only at pH 5 an excess of L92-pDMAEMA is incorporated in the complexes, resulting in positively charged complexes. The profile of the zeta-potential values obtained for mixtures of L92-pDMAEMA with Pluronic P123 showed a shift to a lower N/P ratio, owing to an easier interaction of L92-pDMAEMA molecules with DNA in the presence of P123. Additionally, a visual inspection of the systems indicates that P123 contributes to stabilize/solubilize the DNA/cationic polymer aggregates, by avoiding the typical phase separation near the charge neutralization point. The information obtained can be particularly useful to optimize the conditions to form efficient polyplexes for gene delivery systems.