Thermoresponsive transition of a PEO-b-PNIPAM copolymer: From hierarchical aggregates to well defined ellipsoidal vesicles

Novel ABA block copolymers: preparation, temperature sensitivity, and drug release

A new PEGylated macroiniferter was prepared based on the polycondensation reaction of polyethylene oxide (PEO), methylene diphenyl diisocyanate (MDI), and 1,1,2,2-tetraphenyl-1,2-ethanediol (TPED). The macroiniferter consists of PEO end groups and readily reacts with acrylamides (such as N-isopropylacrylamide, NIPAM) and forms ABA block copolymers (PEO-PNIPAM-PEO). This approach of making amphiphilic ABA block copolymers is robust, versatile, and useful, particularly for the development of polymers for biomedical applications. The resulting amphiphilic PEO-PNIPAM-PEO block copolymers are also temperature sensitive, and their phase transition temperatures are close to human body temperature and therefore they have been applied as drug carriers for cancer treatment. Two PEO-PNIPAM-PEO polymers with different molecular weights were prepared and selected to make temperature-sensitive micelles. As a result of the biocompatibility of these micelles, cell viability tests proved that these micelles have low toxicity toward cancer cells. The resultant polymer micelles were then used as drug carriers to deliver the hydrophobic anticancer drug doxorubicin (DOX), and the results showed that they exhibit significantly higher cumulative drug release efficiency at higher temperatures. Moreover, after loading DOX into the micelles, cellular uptake experiments showed easy uptake and cell viability tests showed that DOX-loaded micelles possess a better therapeutic effect than free DOX at the same dose.

Lamellarity and size distributions in mixed DPPC/amphiphilic poly(2-oxazoline) gradient copolymer vesicles and their temperature response

Mixed liposomes of dipalmitoylphosphatidylcholine (DPPC) and gradient (pseudodiblock) poly(2-methyl-2-oxazoline)-grad-poly(2-phenyl-2-oxazoline) (MPOx) copolymers are investigated by small angle neutron scattering (SANS). All experimental data, from different phospholipid-copolymer compositions, concentrations and temperatures are fitted with one model. This model allows the determination of the separate contributions from vesicular populations of different lamellarity and size. MPOx copolymers are proved to modify both the size and lamellarity of DPPC liposomes. The gradient copolymer with higher hydrophilic content induces shrinkage of the uni- and bi-lamellar DPPC vesicles. The copolymer with lower hydrophilic content causes dramatic changes on the lamellarity of DPPC vesicles by the formation of hexa-lamellar vesicles. The tendency of multi-lamellar vesicles to transform into uni-lamellar ones as temperature increases is more pronounced in the presence of the copolymers. These findings may have direct implications on the drug loading and release properties of liposomes and their interactions with cells.

Tuneable peptide cross-linked nanogels for enzyme-triggered protein delivery

Many diseases are associated with the dysregulated activity of enzymes, such as matrix metalloproteinases (MMPs). This dysregulation can be leveraged in drug delivery to achieve disease- or site-specific cargo release. Self-assembled polymeric nanoparticles are versatile drug carrier materials due to the accessible diversity of polymer chemistry. However, efficient loading of sensitive cargo, such as proteins, and introducing functional enzyme-responsive behaviour remain challenging. Herein, peptide-crosslinked, temperature-sensitive nanogels for protein delivery were designed to respond to MMP-7, which is overexpressed in many pathologies including cancer and inflammatory diseases. The incorporation of N-cyclopropylacrylamide (NCPAM) into N-isopropylacrylamide (NIPAM)-based copolymers enabled us to tune the polymer lower critical solution temperature from 33 to 44 °C, allowing the encapsulation of protein cargo and nanogel-crosslinking at slightly elevated temperatures. This approach resulted in nanogels that were held together by MMP-sensitive peptides for enzyme-specific protein delivery. We employed a combination of cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), small angle neutron scattering (SANS), and fluorescence correlation spectroscopy (FCS) to precisely decipher the morphology, self-assembly mechanism, enzyme-responsiveness, and model protein loading/release properties of our nanogel platform. Simple variation of the peptide linker sequence and combining multiple different crosslinkers will enable us to adjust our platform to target specific diseases in the future.

Association and Internal Morphology of Self-Assembled HPPhOx/BSA Hybrid Nanoparticles in Aqueous Solutions

We investigate the formation of hybrid polyelectrolyte/protein nanoparticles by associations between aggregates of partially hydrolyzed poly(2-phenyl-2-oxazoline) (HPPhOx) and bovine serum albumin (BSA) in aqueous solutions. Light scattering experiments show that at conditions of low salt, BSA creates interaggregate bridges and increases the size of the HPPhOx nanoparticles. At high salt contents, breaking of aggregates leads to well-defined nanoparticles. The interior of the formed nanoparticles is probed by small-angle neutron scattering. At low salt, diffuse arrangements are observed, whereas at high salt concentration, scattering is dominated by well-defined hydrophobic domains enhanced by the incorporation of BSA. This system shows that the combination of hydrophobic and electrostatic interactions in random-amphiphilic-polyelectrolyte/protein complexes can be used to determine the properties of self-assembled hybrid multifunctional nanoparticles.

Tuning the solution organization of cationic polymers through interactions with bovine serum albumin

The interactions of bovine serum albumin (BSA) with aggregates of cationic polymers, i.e. quaternized poly(chloromethyl styrene) chains (QIm-PCMS), in aqueous solutions are investigated using small angle neutron scattering on length scales relevant to the size of BSA. The arrangement of the macromolecular chains within their aggregates is consistent with a blob description of overlapping chains that contain hydrophobic domains. The local conformations depend on the salt content as in typical linear polyelectrolytes. Although the hydrophobic content of the cationic polymers does not cause measurable local morphology differences, the interactions with BSA are enhanced in the case of the not fully quaternized polymer. The secondary structure of BSA is critically compromised by the interaction with the quaternized polymers as the signature of the alpha helix conformation is lost. The complexation with BSA and the resulting enhancement of interchain associations on higher length scales are verified using dynamic light scattering experiments. This study demonstrates the ability to tune the polyelectrolyte/protein interactions and polyelectrolyte chain-chain associations by modifying the hydrophobic content of the polyelectrolytes.

Thermoresponsive behavior of micellar aggregates from end-functionalized PnBA-b-PNIPAM-COOH block copolymers and their complexes with lysozyme

The temperature response of micellar aggregates of poly(n-butyl acrylate)-b-poly(N-isopropylacrylamide)-carboxylic acid (PnBA-b-PNIPAM-COOH) end-functionalized diblock copolymers in aqueous solutions is investigated by small angle neutron scattering and light scattering techniques. The particular micellar aggregates present -COOH groups at their surface due to the molecular architecture of the block copolymer chains. Above the critical solution temperature micellar aggregation depends on the initial solution concentration, while at the highest polymer content intermicellar correlations are observed as a hard-sphere interaction intensity peak. Addition of lysozyme induces this morphological transition even at low concentrations. The scattering profiles are consistent with lysozyme accumulating in the vicinity of the micellar cores, a finding that is supported by measurements in lysozyme contrast matched solvent. Upon temperature increase negatively charged units are exposed to the surface of the aggregates during thermal transition which is a stabilizing force against the phase separating coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPAM).

Internal Nanoparticle Structure of Temperature-Responsive Self-Assembled PNIPAM-b-PEG-b-PNIPAM Triblock Copolymers in Aqueous Solutions: NMR, SANS, and Light Scattering Studies

In this study, we report detailed information on the internal structure of PNIPAM-b-PEG-b-PNIPAM nanoparticles formed from self-assembly in aqueous solutions upon increase in temperature. NMR spectroscopy, light scattering, and small-angle neutron scattering (SANS) were used to monitor different stages of nanoparticle formation as a function of temperature, providing insight into the fundamental processes involved. The presence of PEG in a copolymer structure significantly affects the formation of nanoparticles, making their transition to occur over a broader temperature range. The crucial parameter that controls the transition is the ratio of PEG/PNIPAM. For pure PNIPAM, the transition is sharp; the higher the PEG/PNIPAM ratio results in a broader transition. This behavior is explained by different mechanisms of PNIPAM block incorporation during nanoparticle formation at different PEG/PNIPAM ratios. Contrast variation experiments using SANS show that the structure of nanoparticles above cloud point temperatures for PNIPAM-b-PEG-b-PNIPAM copolymers is drastically different from the structure of PNIPAM mesoglobules. In contrast with pure PNIPAM mesoglobules, where solidlike particles and chain network with a mesh size of 1-3 nm are present, nanoparticles formed from PNIPAM-b-PEG-b-PNIPAM copolymers have nonuniform structure with "frozen" areas interconnected by single chains in Gaussian conformation. SANS data with deuterated "invisible" PEG blocks imply that PEG is uniformly distributed inside of a nanoparticle. It is kinetically flexible PEG blocks which affect the nanoparticle formation by prevention of PNIPAM microphase separation.

Aqueous Solutions of Poly(ethylene oxide)-Poly(N-isopropylacrylamide): Thermosensitive Behavior and Distinct Multiple Assembly Processes

Detailed phase transition and conformational changes taking place as a function of temperature in poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) semidiluted aqueous solutions are elucidated in the present study. By the use of elaborate vibrational spectroscopy techniques in combination with two-dimensional correlation spectroscopy (2Dcos), three transition regions including respective rich domains (<29 °C), loose aggregations (30-36 °C), and dense sphere micelles (>37 °C) are depicted. In particular, subtle variations of hydrogen bonds are detected even under the lower critical solution temperature (LCST), and the respective rich domain regime is marked with strong participation from hydrogen bonding at different concentrations and compositions. Both the formation of intermolecular hydrogen bonds and the less hydration degrees of PNIPAM segments compared with PNIPAM homopolymer at elevated temperatures verify the evolution of PNIPAM from their own domains to loose aggregations with PEO shells. Dense micelles are formed beyond the LCST of PNIPAM, while the outmost PEOs act as buffer layers and postpone the shrinkage of PNIPAM chains. Due to the existence of a buffer layer, higher phase transition temperatures compared with PNIPAM homopolymer are observed.

Monitoring the internal structure of poly(N-vinylcaprolactam) microgels with variable cross-link concentration

The combination of a set of complementary techniques allows us to construct an unprecedented and comprehensive picture of the internal structure, temperature dependent swelling behavior, and the dependence of these properties on the cross-linker concentration of microgel particles based on N-vinylcaprolactam (VCL). The microgels were synthesized by precipitation polymerization using different amounts of cross-linking agent. Characterization was performed by small-angle neutron scattering (SANS) using two complementary neutron instruments to cover a uniquely broad Q-range with one probe. Additionally we used dynamic light scattering (DLS), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). Previously obtained nuclear magnetic resonance spectroscopy (NMR) results on the same PVCL particles are utilized to round the picture off. Our study shows that both the particle radius and the cross-link density and therefore also the stiffness of the microgels rises with increasing cross-linker content. Hence, more cross-linker reduces the swelling capability distinctly. These findings are supported by SANS and AFM measurements. Independent DLS experiments also found the increase in particle size but suggest an unchanged cross-link density. The reason for the apparent contradiction is the indirect extraction of the parameters via a model in the evaluation of DLS measurements. The more direct approach in AFM by evaluating the cross section profiles of observed microgel particles gives evidence of significantly softer and more deformable particles at lower cross-linker concentrations and therefore verifies the change in cross-link density. DSC data indicate a minor but unexpected shift of the volume phase transition temperature (VPTT) to higher temperatures and exposes a more heterogeneous internal structure of the microgels with increasing cross-link density. Moreover, a change in the total energy transfer during the VPT gives evidence that the strength of hydrogen bonds is significantly affected by the cross-link density. A strong and reproducible deviation of the material density of the cross-linked microgel polymer chains toward a higher value compared to the respective linear chains has yet to be explained.