Polarity Does Not Matter: Molecular Weight Reverses the Photoisomerization-Induced Phase Separation of an Azobenzene-Bearing Polymer.

The non-canonical photoisomerization-induced phase separation of an azobenzene-bearing polymer is found. The polymer composed of acrylate-based azobenzene (AzoAA) and N,N-dimethylacrylamide (DMA), namely poly(AzoAA-r-DMA), phase separates under visible light-induced cis-to-trans isomerization at high molecular weight, whereas the phase separation is realized under UV light-induced trans-to-cis isomerization at low molecular weight. Conventionally, the origin of photoisomerization-induced phase separation is believed to arise from the difference in polarity between the apolar trans and polar cis states; thereby the direction of phase changes, either to separate or dissolute, is uniquely determined by the polarity changes during the isomerization of azobenzene. Contrary to this common perception, the poly(AzoAA-r-DMA) in this study phase separates through both trans and cis isomerization, depending on the molecular weight. The non-canonical phase separation of poly(AzoAA-r-DMA) reported herein suggests that molecular weight plays a significant role in determining the phase behavior of azobenzene-bearing polymers. This study provides a platform for the development of spatial-temporally controlled delivery vehicles and microreactors.

Self-oscillating chemoelectrical interface of solution-gated ion-sensitive field-effect transistor based on Belousov-Zhabotinsky reaction.

The Belousov-Zhabotinsky (BZ) self-oscillation reaction is an important chemical model to elucidate nonequilibrium chemistry in an open system. However, there are only a few studies on the electrical behavior of pH oscillation induced by the BZ reaction, although numerous studies have been carried out to investigate the mechanisms by which the BZ reaction interacts with redox reactions, which results in potential changes. Needless to say, the electrical characteristic of a self-oscillating polymer gel driven by the BZ reaction has not been clarified. On the other hand, a solution-gated ion-sensitive field-effect transistor (ISFET) has a superior ability to detect ionic charges and includes capacitive membranes on the gate electrode. In this study, we carried out the electrical monitoring of self-oscillation behaviors at the chemoelectrical interface based on the BZ reaction using ISFET sensors, focusing on the pH oscillation and the electrical dynamics of the self-oscillating polymer brush. The pH oscillation induced by the BZ reaction is not only electrically observed using the ISFET sensor, the electrical signals of which results from the interfacial potential between the solution and the gate insulator, but also visualized using a large-scale and high-density ISFET sensor. Moreover, the N-isopropylacrylamide (NIPAAm)-based self-oscillating polymer brush with Ru(bpy)3 as a catalyst clearly shows a periodic electrical response based on the swelling-deswelling behavior caused by the BZ reaction on the gate insulator of the ISFET sensor. Thus, the elucidation of the electrical self-oscillation behaviors induced by the BZ reaction using the ISFET sensor provides a solution to the problems of nonequilibrium chemistry.

Viscoelastically tunable substrates elucidate the interface-relaxation-dependent adhesion and assembly behaviors of epithelial cells.

Recent progress in mechanobiology sheds light on the regulation of cellular phenotypes by dissipative property of matrices, i.e., viscosity, fluidity, and stress relaxation, in addition to extensively studied elasticity. However, most researches have focused on bulk mechanics, despite cells in 2D culture can only interact with matrix interface directly. Here, we studied the impact of interfacial viscosity as well as elasticity of substrates on the early stage of adhesion behaviors of epithelial cells through new material design and mechanical characterization. The materials are copolymers of ε-caprolactone and d,l-lactide photocrosslinked by benzophenone. The substrate viscoelasticity changes depending on the polymer molecular weight and irradiation time. The interfacial elasticity and relaxation were determined by atomic force microscopy with modes of nanoindentation and tip-dwelling, respectively. MDCK cells changed morphologically, ranging from loose beaded assembly to more compact spheroids and eventual spread monolayer clusters, in response to the interfacial viscoelasticity change. Such morphological changes were mainly determined by substrate interfacial relaxation, rather than interfacial elasticity. Single-cell tracking identified biphasic motility with the minimum speed at intermediate relaxation time (~350 ms), where cells showed transitional morphologies between epithelial and mesenchymal traits. In that relaxation level, partially deformed cells moved around to coalesce with surrounding cells, eventually assembling into compact cellular aggregates. These results highlight, unlike the conventional hanging-drop technique, an appropriate level of interfacial relaxation is critical for efficient cell aggregate maturation on adhesive viscoelastic matrices. This work not only elucidates that the interfacial relaxation as the essential mechanical parameter for epithelial cell adhesion and migration, but also gives useful tips for creating physiologically relevant drug screening platform.

Autonomous Nanoscale Chemomechanical Oscillation on the Self-Oscillating Polymer Brush Surface by Precise Control of Graft Density.

As a novel functional surface, a self-oscillating polymer brush that undergoes autonomous, periodic swelling/deswelling during the Belousov-Zhabotinsky (BZ) reaction has been developed. Although extensive research has revealed how the fundamental aspects of the BZ reaction can be regulated based on the surface design of the self-oscillating polymer brush, design strategies for the induction of mechanical oscillation remain unexplored. Herein, we investigated the graft density effects on the phase transition behavior, which is an important design parameter for the mechanical oscillation of the modified polymer. The self-oscillating polymer-modified substrates with controlled graft densities were prepared by immobilizing various compositions of an initiator and a noninitiator followed by surface-initiated atom transfer radical polymerization of the self-oscillating polymer chains. In addition to the characterization of each prepared substrate, atomic force microscopy (AFM) and digital holographic microscopy (DHM) were employed to evaluate the density effects on the static and dynamic surface structures. AFM revealed that equilibrium swelling as well as thermoresponsive behavior is profoundly affected by the graft density. Moreover, using DHM, autonomous mechanical oscillation was captured only on the self-oscillating polymer brush with adequate graft density. Notably, the oscillation amplitude (150 nm) and the period (20 s) in this study were superior to those in a previous report on the self-oscillating polymer modified through the grafting-to method by 10- and 3-fold, respectively. This study presents design guidelines for future applications, such as autonomous transport devices.

Design of azobenzene-bearing hydrogel with photoswitchable mechanics driven by photo-induced phase transition for in vitro disease modeling.

Mechanics of the extracellular matrix (ECM) exhibit changes during many biological events. During disease progression, such as cancer, matrix stiffening or softening occurs due to crosslinking of the collagen matrix or matrix degradation through cell-secreted enzymes. Engineered hydrogels have emerged as a prime in vitro model to mimic such dynamic mechanics during disease progression. Although there have been a variety of engineered hydrogels, few can offer both stiffening and softening properties under the same working principle. In addition, to model individual disease progression, it is desirable to control the kinetics of mechanical changes. To this end, we describe a photoresponsive hydrogel that undergoes stiffness changes by the photo-induced phase transition. The hydrogel was composed of a copolymer of azobenzene acrylate monomer (AzoAA) and N,N-dimethyl acrylamide (DMA). By tuning the amount of azobenzene, the phase transition behavior of this polymer occurs solely by light irradiation, because of the photoisomerization of azobenzene. This phase behavior was confirmed at 37 °C by turbidity measurements. In addition, the crosslinked poly(AzoAA-r-DMA) gel undergoes reversible swelling-deswelling upon photoisomerization by ultraviolet or visible light. Furthermore, the poly(AzoAA-r-DMA) sheet gels exhibited modulus changes at different isomerization states of azobenzene. When MCF-7 cells were cultured on the gels, stiffening at different timepoints induced varied responses in the gene expression levels of E-cadherin. Not only did this suggest an adaptive behavior of the cells against changes in mechanics during disease progression, this also demonstrated our material's potential towards in vitro disease modeling. STATEMENT OF SIGNIFICANCE: During disease progression such as cancer, cellular microenvironment called extracellular matrix (ECM) undergoes stiffness changes. Hydrogels, which are swollen network of crosslinked polymers, have been used to model such dynamic mechanical environment of the ECM. However, few could offer both stiffening and softening properties under the same working principle. Herein, we fabricated a novel photoresponsive hydrogel with switchable mechanics, activated by photo-induced structural change of the polymer chains within the hydrogel. When breast cancer cells were cultured on our dynamic hydrogels, gene expression and morphological observation suggested that cells react to changes in stiffness by a transient response, as opposed to a sustained one. The photoresponsive hydrogel offers possibility for use as a patient-specific model of diseases.

Crosslinked Poly(N-Isopropylacrylamide)-Based Microfibers as Cell Manipulation Materials with Prompt Cell Detachment.

Stimuli-responsive smart materials are a key to the realization of next-generation medical technologies. Among them, the temperature-responsive polymer poly(N-isopropylacrylamide) (PNIPAAm) is attracting particular attention because it is easy to use in physiological conditions. PNIPAAm-grafted surfaces can undergo temperature-modulated cell adhesion and detachment without proteolytic enzymes, and can be used as cell-separating materials through selective cell adhesion/detachment. However, cell detachment at reduced temperatures is problematic because it takes several hours. A novel thermoresponsive crosslinked microfiber system that can greatly reduce the cell detachment time is introduced in this study. The crosslinked fibers provide temperature-dependent volume change, and enable cell detachment within 10 min of reducing the temperature, which is one-sixth of the time required in previous studies. The prompt cell detachment is thought to arise from a completely new mechanism derived from fiber swelling. This system will make a significant contribution as a novel cell manipulating system for next-generation medical technology.

Stable and Prolonged Autonomous Oscillation in a Self-Oscillating Polymer Brush Prepared on a Porous Glass Substrate.

We developed an autonomous functional surface, named a "self-oscillating polymer brush surface", which exhibits swelling-deswelling of the modified polymer chains synchronized with the Belousov-Zhabotinsky (BZ) reaction. The grafted polymer chain is a random copolymer composed of thermoresponsive N-isopropylacrylamide, N-(3-aminopropyl)methacrylamide, and ruthenium tris(2,2'-bipyridine) [Ru(bpy)3]. To provide stable oscillations over a long period of time, suppression of the dilution of the BZ reactants inside the polymer surface and the increase in the amount of immobilized Ru(bpy)3 are important. Here, we modified the self-oscillating polymer brush on a porous glass substrate and characterized its dynamic behavior. The increased surface area of the porous glass allowed for an efficient introduction of the metal catalyst, which resulted in a stable BZ reaction observable by optical microscopy. Compared with an aqueous BZ solution and the self-oscillating polymer modified on a glass coverslip, the wave velocity and diffusion coefficient were significantly lower for the porous glass system, which suggested that the reaction-diffusion of the reactants was markedly different than those of the other two systems. Moreover, the wave velocity was unchanged on the porous glass system for 1 h, whereas that of the solution dropped by 30 µm s-1. Waveform analyses based on the Field-Körös-Noyes mechanism revealed that densely packed Ru(bpy)3 in the porous glass system affects the duration of the key processes in the BZ reaction. These findings can help with understanding the dynamic behavior of the self-oscillating polymer brush on a porous glass substrate. Stable self-oscillations on the polymer brush-grafted porous glass substrate will aid future applications such as mass transport systems.

Adsorption-Desorption Control of Fibronectin in Real Time at the Liquid/Polymer Interface on a Quartz Crystal Microbalance by Thermoresponsivity.

The cell manipulation technique using thermoresponsive polymers is currently attracting much attention for applications in the medical field. To achieve arbitrary and accurate cell control, it is necessary to intensely research fibronectin behavior. A smart surface, which has thermoresponsive wettability and which can adsorb or desorb fibronectin repeatedly without the presence of cells, was fabricated by an electrospinning method. The fabricated coating changed its structure as the temperature was changed, and this transformation could substitute for the pulling force generated by the cytoskeletal contraction of cells. Moreover, a coated quartz crystal microbalance was able to detect the fibronectin behavior as frequency shifts, which could be used in the estimation of the mass shift with the aid of suitable equations. This coating and measurement system can contribute greatly not only to the development in the medical field centered on biomaterial manipulation technologies, but also to the improvement of medical instruments.

Fabrication of Micropatterned Self-Oscillating Polymer Brush for Direction Control of Chemical Waves.

The propagation control of chemical waves via a pentagonal patterned structure in a self-oscillating polymer brush composed of N-isopropylacrylamide and a metal catalyst for the Belousov-Zhabotinsky (BZ) reaction is reported. The patterned self-oscillating polymer brush is prepared by combining surface-initiated atom transfer radical polymerization and maskless photolithography. Surface modification is confirmed by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, 3D measuring laser microscopy, and fluorescence microscopy. The polymer brush patterns are fabricated with gaps between the pentagonal regions, and investigations on the effect of the gap distance on the BZ reaction reveal that at the appropriate distance, chemical waves propagate across the array from the plane to the corner between the patterns. Unidirectional control is achieved not only in the 1D array, but also in a 2D curved array. This patterned self-oscillating polymer brush is a novel and advantageous approach for creating an autonomous dynamic soft interface.