Adhesion and Mechanical Properties of PNIPAM Microgel Films and Their Potential Use as Switchable Cell Culture Substrates

Thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) microgel films are shown to allow controlled detachment of adsorbed cells via temperature stimuli. Cell response occurs on the timescale of several minutes, is reversible, and allows for harvesting of cells in a mild fashion. The fact that microgels are attached non‐covalently allows using them on a broad variety of (charged) surfaces and is a major advantage as compared to approaches relying on covalent attachment of active films. In the following, the microgels’ physico‐chemical parameters in the adsorbed state and their changes upon temperature variation are studied in order to gain a deeper understanding of the involved phenomena. By means of atomic force microscopy (AFM), the water content, mechanical properties, and adhesion forces of the microgel films are studied as a function of temperature. The analysis shows that these properties change drastically when crossing the critical temperature of the polymer film, which is the basis of the fast cell response upon temperature changes. Furthermore, nanoscale mechanical analysis shows that the films posses a nanoscopic gradient in mechanical properties.     

Responsive Hybrid Polymeric/Metallic Nanoparticles for Catalytic Applications

The aims of the current study were to synthesize new responsive polymeric microgels with embedded silver nanoparticles and then to employ these particles as catalyst for reduction reactions. To these ends, stimuli‐responsive microgels from PNIPAAm and the chitosan derivative were firstly synthesized by free radical precipitation polymerization. Then, silver nanoparticles were synthesized inside these microgel networks by in situ reduction of AgNO₃. These microgels were temperature/pH sensitive with a phase transition temperature of 32–35 °C in water at pH = 3 and 8, respectively. The catalytic activity of the Ag nanoparticles for the reduction of 4‐nitrophenol can be tuned through the swelling or collapse of the responsive microgel network hosting the active nanoparticles.

     

Effects of subphase pH and temperature on the interfacial behavior of double hydrophilic diblock copolymer PDEGMA-b-PDIPAEMA

The aggregation behavior of two pH- and temperature-responsive diblock copolymers of poly[di-(ethylene glycol) methyl ether methacrylate]-block-poly[2-(diisopropylamino) ethyl methacrylate] (PDEGMA-b-PDIPAEMA) at the air/water interface and the structures of their Langmuir–Blodgett (LB) films were studied by the Langmuir monolayer technique and atomic force microscopy, respectively. At the air/water interface, PDEGMA-b-PDIPAEMA tends to form the core-shell-corona micellar structure composed of a PDIPAEMA main chain core, an amino ethyl ester shell, and a PDEGMA corona. Under acidic, neutral, and alkaline conditions, PDIPAEMA blocks are completely protonated, partially protonated, and completely non-protonated, respectively, and the protonated amino ethyl ester groups are immersed in water before monolayer compression, whereas PDEGMA coronas are adsorbed at the interface. At pH 3, 7, and 10, the limiting areas (A₀) for PDEGMA42%-PDIPAEMA58% (weight percents) and PDEGMA55%-PDIPAEMA45% are 8.2/10.2/14.0 and 6.7/8.3/8.4 nm², respectively. The A₀ values of the former copolymer are larger than those of the latter. This is because the shells in the former copolymer are denser due to the higher polymerization degree of PDIPAEMA blocks, providing greater steric hindrance for PDEGMA coronas and making the latter more extended at the interface. In contrast to other copolymer systems, the effect of temperature on the isotherms of PDEGMA-b-PDIPAEMA is less obvious.     

ATRP with Alkyl Pseudohalides Acting as Initiators and Chain Transfer Agents: When ATRP and RAFT Polymerization Become One

All controlled radical polymerization (CRP) procedures rely on a dynamic and rapid equilibrium between dormant and active species. This equilibrium can be established through different mechanisms and all the resulting CRP processes have their own advantages and limitations. Therefore, it becomes interesting to investigate the possibility of combining CRP techniques to eliminate specific drawbacks of each individual procedure. Atom transfer radical polymerization (ATRP) with alkyl pseudohalides acting as initiators and chain transfer agents was developed for that purpose. The process relies on a dual mechanism involving both activation deactivation and reversible addition fragmentation chain transfer (RAFT). This peculiar characteristic of ATRP with alkyl pseudohalides acting as chain transfer agents made it possible to overcome some of the limitations typically associated with conventional ATRP and RAFT polymerization as well as to prepare new responsive materials.     

Nanostructures from Single Amino Acid‐Based Molecules: Stability,Fibrillation, Encapsulation,and Fabrication of Silver Nanoparticles

The small‐sized molecules that have been developed from single hydrophobic amino acids (Phe, Trp, Tyr and Leu) by suitably protecting the –NH₂ and –CO₂H groups generate diverse nanoscopic structures – such as nanorods, nanofibrils, nanotubes, and nanovesicles – depending upon the protection parameters and solvent polarity. The vesicular structures get disrupted in the presence of various salts, such as KCl, CaCl₂, (NH₄)₂SO₄ and N(n‐Bu)₄Br. Insertion of unnatural (o/m/p)‐aminobenzoic acids as a protecting group and the lack of conventional peptide bonds in the molecules give the nanostructures proteolytic stability. The nanostructures also show significant thermal stability along with a morphological transformation upon heat treatment. Our in vitro studies reveal that the addition of micromolar concentration “curcumin” significantly reduces the formation of amyloid‐like fibrils. These diverse nanostructures are used as a template for fabricating silver nanoparticles on their outer surfaces as well as in the inner part, followed by calcination in air which helps to obtain a 1D silver nanostructure. Furthermore, the nanovesicles are observed to encapsulate a potent drug (curcumin) and other biologically important molecules, which could be released through salt‐triggered disruption of vesicles.     

Synthesis and self‐assembly behavior of pH‐responsive amphiphilic copolymers containing ketal functional groups

Polymeric micelles that are responsive to pH are particularly attractive for application in drug delivery systems. In this study, one type of amphiphilic block copolymers with hydrophobic building blocks bearing pH‐sensitive ketal groups was designed. In an acidic environment, the polarity transfer from amphiphile to double hydrophile for this copolymer destroyed the driving force of micelle formation, which triggered the release of encapsulated hydrophobic molecules. The amphiphilic block copolymers monomethoxy‐poly(ethylene glycol)‐block‐poly(2,2‐dimethyl‐1,3‐dioxolane‐4‐yl)methyl acrylate (MPEG‐block‐PDMDMA) was fabricated by atom transfer radical polymerization using MPEG‐Br as macroinitiator. The critical micelle concentration of various compositions of this copolymer in aqueous solution ranged from 4.0 to 10.0 mg L^?1, and the partition equilibrium constant (K_v) of pyrene in micellar solutions of the copolymers varied from 1.61 × 10⁵ to 4.86 × 10⁵. Their overall effective hydrodynamic diameters from dynamic light scattering measurements were between 80 and 400 nm, and the micellar morphology showed spherical geometry as investigated using transmission electron microscopy. At pH = 1.0, all of these polymeric micelles presented 100% payload release in 24 h of incubation, while at pH = 3.0, nearly 70 and 25% of pyrene was released for MPEG‐block‐PDMDMA (44/18) and MPEG‐block‐PDMDMA (44/25) in 260 h, respectively. The pH‐responsive MPEG‐block‐PDMDMA polymeric micelles having good encapsulation efficiency for hydrophobic drugs are potential candidates for biomedical and drug delivery applications. Copyright © 2010 Society of Chemical Industry     

The emerging professional adhocracy: A general framework of responsive organizing (a republication).

This reprinted article originally appeared in Consulting Psychology Journal: Practice and Research, 1997, Vol 49(1), 25-34. (The following abstract of the original article appeared in record 1997-30041-003.) Offers a framework of the characteristics of the responsive organization. A responsive organization is one in which structures and procedures enhance the organization's ability to take advantage of the changes in the environment. Responsiveness, in this case, is more than just being reactive; it means that an organization is able to anticipate change and maintain a proactive orientation to the external environment. Within the framework discussed, the authors also include some of the problems that might be expected in this new organizational form. It is suggested that these tools will allow for the systematic examination of organizational performance and assist change agents in facilitating efforts intended to result in more responsive organizing. (PsycINFO Database Record (c) 2010 APA, all rights reserved)