Stimuli-responsive Brushes Research Articles

Adsorption Active Diblock Copolymers as Universal Agents for Unusual Barrier-Free Transitions in Stimuli-Responsive Brushes

We reconsider a recently proposed design for smart responsive brushes, which is based on a conformational transition in very dilutely embedded block copolymers with a surface active block (Qi et al., Macromolecules 53, 5326, 2020). Under certain conditions, the transition acquires an unusual character: it remains very sharp, but the barrier separating the adsorbed and desorbed states disappears completely. We show that these features are very robust with respect to changing almost all system parameters: the lengths of the inert and active blocks of the minority chain, the brush length, its density, and its polydispersity. The only relevant condition is that the inert block of the minority chain is long enough to extend outside the brush density profile. We develop an analytical theory that predicts the relevant characteristics of the transition and verify it with Monte Carlo simulations. We also show that the surprising universality of the transition properties is rooted in an underlying connection to the force-induced desorption transition, which is known to combine the features of the first- and second-order transitions with a pretransition fluctuation growth accompanied by phase coexistence.

Stress in a Stimuli-Responsive Polymer Brush

The application of a polymer brush in sensing, actuation, self-folding, among others acutely depends on the tuneable bending of a brush-grafted substrate caused by the stress in the brush. However, the stress in a stimuli-responsive brush has not been investigated. In this work, we study the stress in the stimuli-responsive planar polymer brushes of neutral water-soluble polymers with low to very high graft densities using strong stretching theory (SST). First, SST with the Langevin force-extension relation for a polymer chain is extended to the study of stimuli-responsive brushes. Stress profile and other properties of a Poly(N-isopropylacrylamide) (PNIPAm) brush are then obtained using the extended SST and an empirical Flory-Huggins parameter. The model predicts that the stress in a PNIPAm brush is inhomogeneous and compressive at all temperatures and graft densities. The resultant stress is predicted to increase in magnitude with increasing graft density. Moreover, it decreases in magnitude with an increase in temperature before plateauing in low graft density brushes. In contrast, its magnitude increases weakly with increasing temperature in high density brushes. This contrasting behavior is traced to the minimum in interaction free energy density \emph{vs} polymer volume fraction curve for PNIPAm solution at a large volume fraction, and stiffening of chains due to finite extensibility. Furthermore, our results indicate that the ability to tune the resultant stress by changing temperature diminishes with increasing graft density.

Fabrication of a smart and biocompatible brush copolymer decorated on magnetic graphene oxide hybrid nanostructure for drug delivery application

This study aimed to improve the therapeutic efficacy of doxorubicin (DOX) as an anticancer drug via loading it in the nanostructure. Therefore, for the first time, a new stimuli-responsive brushes copolymer decorated on the magnetic graphene oxide surface (MG-PB) fabricated, as well as, its capability as a targeted and stimuli-responsive drug delivery nanostructure was evaluated as in vitro. In this regard, at first, the magnetic graphene oxide nanohybrid (MG) was synthesized. Modification of the MG with triethoxyvinylsilane (TEVS) introduced the vinylic groups on the MG surface (MG-TEVS) and offered the polymerization possibility of the acrylic monomers from the MG surface. The copolymer brushes of N-isopropylacrylamide (NIPAM) and acrylated β-cyclodextrin (Ac-β-CD) were grown from the MG-TEVS surface. The Fourier transform infrared (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) analysis used to explore the successful growth of the PB brushes. The lower critical solution temperature (LCST) of the prepared nanosystem was determined at about 35 °C. DOX was loaded about 50.15% in MG-PB. The drug release results displayed the controlled release profile with ~ 65% of DOX release in pH 5.0, 40 °C. The obtained drug release kinetic results demonstrated that the first-order kinetic model is more fitted with experimental data and DOX release follows from the Fickian mechanism. Cell viability confirmed the biocompatibility of the MG-PB. Additionally, the quantitative and qualitative cellular uptake, 4′,6-diamidino-2-phenylindole (DAPI) staining, and cell cycle tests revealed the high anticancer performance of drug-loaded nanocarrier through the penetration within the cells and their killing as a result of DOX release. The targeted and stimuli-responsive drug release capability of the MG-BB could be the valuable advantages, hence, obtained results suggest the great potential of the ternary MG-PB hybrid nanostructure as a controlled anticancer drug delivery vehicle.

Stimuli-Responsive Brushes with Active Minority Components: Monte Carlo Study and Analytical Theory

Using a combination of analytical theory, Monte Carlo simulations, and three dimensional self-consistent field calculations, we study the equilibrium properties and the switching behavior of adsorption-active polymer chains included in a homopolymer brush. The switching transition is driven by a conformational change of a small fraction of minority chains, which are attracted by the substrate. Depending on the strength of the attractive interaction, the minority chains assume one of two states: An exposed state characterized by a stem-crown-like conformation, and an adsorbed state characterized by a flat two-dimensional structure. Comparing the Monte Carlo simulations, which use an Edwards-type Hamiltonian with density dependent interactions, with the predictions from self-consistent-field theory based on the same Hamiltonian, we find that thermal density fluctuations affect the system in two different ways. First, they renormalize the excluded volume interaction parameter $v_\mathrm{\tiny bare}$ inside the brush. The properties of the brushes can be reproduced by self-consistent field theory if one replaces $v_\mathrm{\tiny bare}$ by an effective parameter $v_{\mathrm{\tiny eff}}$, where the ratio of second virial coefficients $B_{\mathrm{\tiny eff}}/B_\mathrm{\tiny bare}$ depends on the range of monomer interactions, but not on the grafting density, the chain length, and $v_\mathrm{\tiny bare}$. Second, density fluctuations affect the conformations of chains at the brush surface and have a favorable effect on the characteristics of the switching transition: In the interesting regime where the transition is sharp, they reduce the free energy barrier between the two states significantly. The scaling behavior of various quantities is also analyzed and compared with analytical predictions.

Stimuli-sensitive intrinsically disordered protein brushes.

Grafting polymers onto surfaces at high density to yield polymer brush coatings is a widely employed strategy to reduce biofouling and interfacial friction. These brushes almost universally feature synthetic polymers, which are often heterogeneous and do not readily allow incorporation of chemical functionalities at precise sites along the constituent chains. To complement these synthetic systems, we introduce a biomimetic, recombinant intrinsically disordered protein that can assemble into an environment-sensitive brush. This macromolecule adopts an extended conformation and can be grafted to solid supports to form oriented protein brushes that swell and collapse dramatically with changes in solution pH and ionic strength. We illustrate the value of sequence specificity by using proteases with mutually orthogonal recognition sites to modulate brush height in situ to predictable values. This study demonstrates that stimuli-responsive brushes can be fabricated from proteins and introduces them as a new class of smart biomaterial building blocks.

Dual-responsive polymer brushes as an electrochemical logic gate

Stimuli-responsive polymer brushes modified surfaces and binary polymer surfaces can be created on an initiator terminated substrate via surface initiated atom-transfer radical polymerization (SI-ATRP), in which the growth kinetics can be modulated by Cu(II) ratio. As stimuli-responsive brushes, many environmental triggers can drive their conformation change such as pH and salt concentration. The external-stimuli modulated reversible conformational changes of polymer brushes can influence the charge transport behavior investigated by electrochemistry method, which have potential applications in in situ switching ion permeability and charge transport. The charge transport of binary polymer brush can be switched on or off by different ion flexibly which is an electrochemical logic switch.

Development of Intrinsically Disordered Protein Brushes as Smart Biomaterials

The grafting of chains onto surfaces at high density to yield polymer coatings is a widely employed strategy to reduce biofouling and interfacial friction. Here we report the design, development, and characterization of synthetic intrinsically disordered proteins (IDPs) based on the C-terminal sidearm domain of the human neurofilament heavy chain protein. Dynamic light scattering measurements indicate that the recombinant wild type protein adopts an extended conformation with a hydrodynamic radius of ∼10 nm, much larger than the expected value for a folded protein. This macromolecule adopts an extended, disordered conformation in solution and can be grafted at high density in an oriented fashion to solid supports, as measured by quartz crystal microbalance with dissipation studies. The resulting IDP brushes are responsive to multiple stimuli such as solution pH and ionic strength. The swelling and collapsing behavior of the brush exhibits similar salt-dependence and comparable dynamic range to a synthetic, weak polyelectrolyte brush. This study provides evidence that stimuli-responsive brushes can be fabricated from proteins and introduces these molecules as a new class of “smart” biomaterial building blocks.

Surface Modification of Hydroxy Carbonate Apatite Nanoparticles with PDMAEMA via Surface-initiated ATRP

Abstract A facile strategy for grafting poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) on the surface of hydroxy carbonate apatite (HCA) nanoparticles via surface-initiated ATRP has been presented. SEM, FT-IR, TGA, N2 adsorption–desorption analysis, and DLS were employed for their characterization. The results demonstrated that the stimuli-responsive polymer had been successfully grafted onto the surface of HCA. The stimuli-responsive brushes attached to HCA could act as a switch to control the opening and closing of the pore on the surface of HCA. In this paper, the pH-responsive property was mainly focused on.

Synthetic Proton-Gated Ion Channels via Single Solid-State Nanochannels Modified with Responsive Polymer Brushes

The creation of switchable and tunable nanodevices displaying transport properties similar to those observed in biological pores poses a major challenge in molecular nanotechnology. Here, we describe the construction of a fully "abiotic" nanodevice whose transport properties can be accurately controlled by manipulating the proton concentration in the surrounding environment. The ionic current switching characteristics displayed by the nanochannels resemble the typical behavior observed in many biological channels that fulfill key pH-dependent transport functions in living organisms, that is, the nanochannel can be switched from an "off" state to an "on" state in response to a pH drop. The construction of such a chemical nanoarchitecture required the integration of stable and ductile macromolecular building blocks constituted of pH-responsive poly(4-vinyl pyridine) brushes into solid state nanopores that could act as gate-keepers managing and constraining the flow of ionic species through the confined environment. In this context, we envision that the integration of environmental stimuli-responsive brushes into solid-state nanochannels would provide a plethora of new chemical alternatives for molecularly design robust signal-responsive "abiotic" devices mimicking the function of proton-gated ion channels commonly encountered in biological membranes.