Recent Advances in Self-Oscillating Polymer Material Systems

Temperature-modulated separation of vascular cells using thermoresponsive-anionic block copolymer-modified glass

Introduction

Vascular tissue engineering is a key technology in the field of regenerative medicine. In tissue engineering, the separation of vascular cells without cell modification is required, as cell modifications affect the intrinsic properties of the cells. In this study, we have developed an effective method for separating vascular cells without cell modification, using a thermoresponsive anionic block copolymer.

Methods

A thermoresponsive anionic block copolymer, poly(acrylic acid)-b-poly(N-isopropylacryl-amide) (PAAc-b-PNIPAAm), with various PNIPAAm segment lengths, was prepared in two steps: atom transfer radical polymerization and subsequent deprotection. Normal human umbilical vein endothelial cells (HUVECs), normal human dermal fibroblasts, and human aortic smooth muscle cells (SMCs) were seeded onto the prepared thermoresponsive anionic block copolymer brush-modified glass. The adhesion behavior of cells on the copolymer brush was observed at 37 °C and 20 °C.

Results

A thermoresponsive anionic block copolymer, poly(acrylic acid)-b-poly(N-isopropylacrylamide) (PAAc-b-PNIPAAm), with various PNIPAAm segment lengths was prepared. The prepared copolymer-modified glass exhibited anionic properties attributed to the bottom PAAc segment of the copolymer brush. On the PAAc-b-PNIPAAm, which had a moderate PNIPAAm length, a high adhesion ratio of HUVECs and low adhesion ratio of SMCs were observed at 37 °C. By reducing temperature from 37 °C to 20 °C, the adhered HUVECs were detached, whereas the SMCs maintained adhesion, leading to the recovery of purified HUVECs by changing the temperature.

Conclusions

The prepared thermoresponsive anionic copolymer-modified glass could be used to separate HUVECs and SMCs by changing the temperature without modifying the cell surface. Therefore, the developed cell separation method will be useful for vascular tissue engineering.

Harvesting methods of umbilical cord-derived mesenchymal stem cells from culture modulate cell properties and functions

Introduction

Human umbilical cord-derived mesenchymal stem cells (UC-MSCs) are promising candidates for stem cell therapy. Various methods such as enzymatic treatment, cell scraping, and temperature reduction using temperature-responsive cell culture dishes have been employed to culture and harvest UC-MSCs. However, the effects of different harvesting methods on cell properties and functions in vitro remain unclear. In this study, we investigated the properties and functions of UC-MSC using various cell-harvesting methods.

Methods

UC-MSC suspensions were prepared using treatments with various enzymes, cell scraping, and temperature reduction in temperature-responsive cell culture dishes. UC-MSC sheets were prepared in a temperature-responsive cell culture dish. The properties and functions of the UC-MSC suspensions and sheets were assessed according to Annexin V staining, lactate dehydrogenase (LDH) assay, re-adhesion behavior, and cytokine secretion analysis via enzyme-linked immunosorbent assay.

Results

Annexin V staining revealed that accutase induced elevated UC-MSC apoptosis. Physical scraping using a cell scraper induced a relatively high LDH release due to damaged cell membranes. Dispase exhibited relatively low adhesion from initial incubation until 3 h. UC-MSC sheets exhibited rapid re-adhesion at 15 min and cell migration at 6 h. UC-MSC sheets expressed higher levels of cytokines such as HGF, TGF-β1, IL-10, and IL-6 than did UC-MSCs in suspension.

Conclusions

The choice of enzyme and physical scraping methods for harvesting UC-MSCs significantly influenced their activity and function. Thus, selecting appropriate cell-harvesting methods is important for successful stem cell therapy.

Bioanalytical technologies using temperature-responsive polymers

In recent decades, various bioanalytical technologies have been investigated for appropriate medical treatment and effective therapy. Temperature-responsive chromatography is a promising bioanalytical technology owing to its functional properties. Temperature-responsive chromatography uses a poly(N-isopropylacrylamide)(PNIPAAm) modified stationary phase as the column packing material. The hydrophobic interactions between PNIPAAm and the analyte could be modulated by changing the column temperature because of the temperature-responsive hydrophobicity of PNIPAAm. Thus, the chromatography system does not require organic solvents in the mobile phase, making it suitable for therapeutic drug monitoring in medical settings such as hospitals. This review summarizes recent developments in temperature-responsive chromatography systems for therapeutic drug monitoring applications. In addition, separation methods for antibody drugs using PNIPAAm are also summarized because these methods apply to the therapeutic drug monitoring of biopharmaceutics. The temperature-responsive chromatography systems can also be utilized for clinical diagnosis, as they can assess multiple medicines simultaneously. This highlights the significant potential of temperature-responsive chromatography in medicine and healthcare.

Evolution of the Microrobots: Stimuli-Responsive Materials and Additive Manufacturing Technologies Turn Small Structures into Microscale Robots

The development of functional microsystems and microrobots that have characterized the last decade is the result of a synergistic and effective interaction between the progress of fabrication techniques and the increased availability of smart and responsive materials to be employed in the latter. Functional structures on the microscale have been relevant for a vast plethora of technologies that find application in different sectors including automotive, sensing devices, and consumer electronics, but are now also entering medical clinics. Working on or inside the human body requires increasing complexity and functionality on an ever-smaller scale, which is becoming possible as a result of emerging technology and smart materials over the past decades. In recent years, additive manufacturing has risen to the forefront of this evolution as the most prominent method to fabricate complex 3D structures. In this review, we discuss the rapid 3D manufacturing techniques that have emerged and how they have enabled a great leap in microrobotic applications. The arrival of smart materials with inherent functionalities has propelled microrobots to great complexity and complex applications. We focus on which materials are important for actuation and what the possibilities are for supplying the required energy. Furthermore, we provide an updated view of a new generation of microrobots in terms of both materials and fabrication technology. While two-photon lithography may be the state-of-the-art technology at the moment, in terms of resolution and design freedom, new methods such as two-step are on the horizon. In the more distant future, innovations like molecular motors could make microscale robots redundant and bring about nanofabrication.

Enhancement of Piezoelectric Properties of Flexible Nanofibrous Membranes by Hierarchical Structures and Nanoparticles

Piezoelectric nanogenerators (PENGs) show superiority in self-powered energy converters and wearable electronics. However, the low power output and ineffective transformation of mechanical energy into electric energy l limit the role of PENGs in energy conversion and storage devices, especially in fiber-based wearable electronics. Here, a PAN-PVDF/ZnO PENG with a hierarchical structure was designed through electrospinning and a hydrothermal reaction. Compared with other polymer nanofibers, the PAN-PVDF/ZnO nanocomposites not only showed two distinctive diameter distributions of uniform nanofibers, but also the complete coverage and embedment of ZnO nanorods, which brought about major improvements in both mechanical and piezoelectric properties. Additionally, a simple but effective method to integrate the inorganic nanoparticles into different polymers and regulate the hierarchical structure by altering the types of polymers, concentrations of spinning solutions, and growth conditions of nanoparticles is presented. Further, the designed P-PVDF/ZnO PENG was demonstrated as an energy generator to successfully power nine commercial LEDs. Thus, this approach reveals the critical role of hierarchical structures and processing technology in the development of high-performance piezoelectric nanomaterials.

pH Oscillator-Driven Jellyfish-like Hydrogel Actuator with Dissipative Synergy between Deformation and Fluorescence Color Change

Development of soft actuators with complex practical functions is significant for imitating the behaviors of living organisms. However, it is still a challenge to fabricate artificial soft actuators with jellyfish-like synergistic deformation and fluorescence color change (SDFC) and autonomous dynamic behavior, but such a system could obviously endow the classic soft actuators with more functions. Herein, we proposed to utilize tetra(4-pyridylphenyl)ethylene (TPE-4N) luminogen with pH-responsive aggregation-induced emission (AIE) to fabricate the AIE active hydrogel, which could be further employed to obtain an anisotropic bilayer soft actuator based on strong interfacial adhesion with acrylic acid (AA) gels. Furthermore, artificial flower-shape actuators showing SDFC behaviors were demonstrated. On the basis of these findings, jellyfish-inspired autonomous gel actuators driven by a pH oscillator have been fabricated, in which periodical SDFC behaviors completely regulated by the system itself without repetitive on/off switches of external stimuli were well synchronized with the pH oscillator. The described combination of nonlinear chemistry and responsive hydrogels actuator opens pathways toward out-of-equilibrium SDFC devices with autonomous behavior useful for biomimetic fields.

Design of biointerfaces composed of soft materials using controlled radical polymerizations

Soft interface materials have an immense potential for the improvement of biointerfaces, which are the interface of biological and artificially designed materials. Controlling the chemical and physical structures of the interfaces at the nanometer level plays an important role in understanding the mechanism of the functioning and its applications. Controlled radical polymerization (CRP) techniques, including atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain-transfer (RAFT) polymerization, have been developed in the field of precision polymer chemistry. It allows the formation of well-defined surfaces such as densely packed polymer brushes and self-assembled nanostructures of block copolymers. More recently, a novel technique to prepare polymers containing biomolecules, called biohybrids, has also been developed, which is a consequence of the advancement of CRP so as to proceed in an aqueous media with oxygen. This review article summarizes recent advances in CRP for the design of biointerfaces.

Moisture-induced autonomous surface potential oscillations for energy harvesting

A variety of autonomous oscillations in nature such as heartbeats and some biochemical reactions have been widely studied and utilized for applications in the fields of bioscience and engineering. Here, we report a unique phenomenon of moisture-induced electrical potential oscillations on polymers, poly([2-(methacryloyloxy)ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide-co-acrylic acid), during the diffusion of water molecules. Chemical reactions are modeled by kinetic simulations while system dynamic equations and the stability matrix are analyzed to show the chaotic nature of the system which oscillates with hidden attractors to induce the autonomous surface potential oscillation. Using moisture in the ambient environment as the activation source, this self-excited chemoelectrical reaction could have broad influences and usages in surface-reaction based devices and systems. As a proof-of-concept demonstration, an energy harvester is constructed and achieved the continuous energy production for more than 15,000 seconds with an energy density of 16.8 mJ/cm2. A 2-Volts output voltage has been produced to power a liquid crystal display toward practical applications with five energy harvesters connected in series.

Good reactions for low-power shape-memory microactuators

Microscale programmable shape-memory actuators based on reversible electrochemical reactions can provide exciting opportunities for microrobotics.

Towards synergistic oscillations in enzymatically active hydrogel spheres

Stimuli-responsive polymers are capable of reacting to an external trigger. We report self-regulated, enzymatically active, and pH-responsive hydrogels that show dynamic behavior without an external trigger. This is enabled by a feedback loop between the enzymatic conversion of glucose into gluconic acid and the pH-induced volume phase transition that leads to a modulation in glucose permeability. The synthesized hydrogel spheres combine all required properties for sustained oscillation including enzymatic activity, switchable reactivity, hysteresis in volume phase transition and feedback between the reaction and permeation. A simple model of the system identified possible operating points where sustained oscillations are possible. Experiments at these operating points revealed that the system is able to perform a self-regulated oscillation cycle under a constant nutrient supply. A sensitivity analysis showed that the system is especially sensitive around the point of oscillation, so that precise control of the process parameters is essential to achieve sustained oscillations.

Recent advances in soft functional materials: preparation, functions and applications

Synthetic materials and biomaterials with elastic moduli lower than 10 MPa are generally considered as soft materials. Research studies on soft materials have been boosted due to their intriguing features such as light-weight, low modulus, stretchability, and a diverse range of functions including sensing, actuating, insulating and transporting. They are ideal materials for applications in smart textiles, flexible devices and wearable electronics. On the other hand, benefiting from the advances in materials science and chemistry, novel soft materials with tailored properties and functions could be prepared to fulfil the specific requirements. In this review, the current progress of soft materials, ranging from materials design, preparation and application are critically summarized based on three categories, namely gels, foams and elastomers. The chemical, physical and electrical properties and the applications are elaborated. This review aims to provide a comprehensive overview of soft materials to researchers in different disciplines.

Synthesis of Non-Uniform Functionalized Amphiphilic Block Copolymers and Giant Vesicles in the Presence of the Belousov-Zhabotinsky Reaction

Giant vesicles with several-micrometer diameters were prepared by the self-assembly of an amphiphilic block copolymer in the presence of the Belousov-Zhabotinsky (BZ) reaction. The vesicle is composed of a non-uniform triblock copolymer synthesized by multi-step reactions in the presence of air at room temperature. The triblock copolymer contains poly(glycerol monomethacrylate) (PGMA) as the hydrophilic block copolymerized with tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)3), which catalyzes the BZ reaction, and 2-hydroxypropyl methacrylate (HPMA) as the hydrophobic block. In this new approach, the radicals generated in the BZ reaction can activate a reversible addition-fragmentation chain transfer (RAFT) polymerization to self-assemble the polymer into vesicles with diameters of approximately 3 µm. X-ray photoelectron spectroscopy (XPS) measurements demonstrated that the PGMA-b-Ru(bpy)3-b-PHPMA triblock copolymer is brominated and increases the osmotic pressure inside the vesicle, leading to micrometer-sized features. The effect of solvent on the morphological transitions are also discussed briefly. This BZ strategy, offers a new perspective to prepare giant vesicles as a platform for promising applications in the areas of microencapsulation and catalyst support, due to their significant sizes and large microcavities.

Chemomechanical Motion of a Self-Oscillating Gel in a Protic Ionic Liquid

An autonomous swelling-deswelling oscillation of polymer gels in a hydrated protic ionic liquid (PIL) as a proton source for the Belousov-Zhabotinsky (BZ) reaction is presented. Methylammonium hydrogen sulfate ([maH⁺ ][HSO₄ ^- ]) was employed as the PIL because it provides stable redox oscillation in the BZ reaction. Due to the significantly higher pK_a for [maH⁺ ][HSO₄ ^- ] than those for conventional proton sources for the BZ reaction, chemomechanical oscillation can be expected under weaker acidic conditions. The self-oscillating polymer was designed as a ternary random copolymer of N-isopropylacrylamide, N-(3-aminopropyl)methacrylamide, and the Ru(bpy)₃ moiety as a catalyst for the BZ reaction. The copolymer exhibited spontaneous soluble-insoluble oscillation in hydrated [maH⁺ ][HSO₄ ^- ] containing NaBrO₃ and malonic acid. Macroscopic swelling-deswelling oscillation of the porous bulk gel prepared by covalently connecting microgel particles was also observed.

Pulsatile release from a flat self-oscillating chitosan macrogel

Coupling oscillatory chemical reactions to smart materials which can respond to external stimuli is considered an answer to the long-standing issue of pulsatile drug delivery. Although a number of coupled architectures exist, there are no systems reporting pH-controlled pulsed drug release based on chemical oscillators. In this paper, we report for the first time a proof-of-concept self-oscillatory chitosan macrogel, employing the palladium-catalysed oxidative carbonylation reaction as the driving force of its oscillations. The reported hydrogel is composed of highly biocompatible components and a novel imine-functionalised chitosan-palladium catalyst with zero leaching rates. This macrogel was shown to rhythmically release not only the products of the reaction, but also fluorescein, which is used as an FDA-approved model drug. The step-wise release pattern corresponded to the step-wise dynamics of pH decrease in methanol:water, while in pure methanol, the changes in pH had an oscillatory mode, accompanied by mirrored oscillations in fluorescein concentration. This proof-of-concept system significantly expands the horizons of pulsatile delivery materials for future research.

Aspects of the Belousov-Zhabotinsky Reaction inside a Self-Oscillating Polymer Brush

We have developed a novel polymer brush surface exhibiting autonomous swelling-deswelling changes driven by the Belousov-Zhabotinsky (BZ) reaction, that is, the self-oscillating polymer brush. In this system, the ruthenium tris(2,2'-bipyridine) [Ru(bpy)₃] catalyst-conjugated polymer chains are densely packed on the solid substrate. It is expected that the BZ reaction in the polymer brush would be influenced by the immobilization effect of the catalyst. To clarify the effect of the immobilization of the catalyst on the self-oscillating polymer brush, the self-oscillating behavior of the polymer brush was investigated by comparing it with that of other self-oscillating polymer materials, the free polymer, and the gel particle under various initial substrate concentrations. The initial substrate dependency of the oscillating period for the polymer brush was found to be different from those for the free polymer and the gel particle. Furthermore, the oscillatory waveform was analyzed on the basis of the Field-Körös-Noyes model. These investigations revealed that the dense immobilization of the self-oscillating polymer on the surface restricted accessibility for the Ru(bpy)₃ moiety. These findings would be helpful in understanding the reaction-diffusion mechanism in the polymer brush, which is a novel reaction medium for the BZ reaction.

From small molecules to polymeric catalysts in the oscillatory carbonylation reaction: multiple effects of adding HI

A step closer to all polymer self-oscillating systems: pH oscillations were achieved in the phenylacetylene carbonylation reaction, catalysed by polymer-bound palladium acetate.

Microfluidics Fabrication of Self-Oscillating Microgel Clusters with Tailored Temperature-Responsive Properties Using Polymersomes as "Microreactors"

Poly(N-isopropylacrylamide)-based microgel clusters were successfully prepared using polymersomes as "microreactors", which were fabricated through microfluidics. The clusters were formed from the cross-linking reaction between ruthenium/amino group dual functionalized poly(N-isopropylacrylamide) microgels and linear poly(N-isopropylacrylamide)-r-(N-acryloxysuccinimide)-based polymer linkers under neutral pH conditions. By simply adjusting the ratio of N-isopropylacrylamide to N-acryloxysuccinimide in the polymer cross-linkers, the internal structures of the clusters can be controlled; hence, the temperature response of the clusters can be regulated. It was demonstrated that these different microgel clusters showed various degrees of chemomechanical oscillations when the clusters were exposed to a catalyst-free solution containing Belousov-Zhabotinsky reaction substrates.

Thermoresponsive Surface-Grafted Gels: Controlling the Bulk Volume Change Properties by Surface-Localized Polymer Grafting with Various Densities

We prepared poly(N-isopropylacrylamide-r-N-3-(aminopropyl)methacrylamide) (poly(NIPAAm-r-NAPMAm)) gels with poly NIPAAm (PNIPAAm) grafted only in the surface region (so-called thermoresponsive surface-grafted gels) with various graft densities and investigated the effect of the graft density on the bulk volume change properties, shrinking and swelling, in response to temperature changes. Initiators for atom-transfer radical polymerization (ATRP) and structurally analogous compounds were introduced at certain ratios onto the surface regions of the gels, and a subsequent activator regeneration by electron transfer ATRP of NIPAAm was conducted in aqueous media. The graft densities and molecular weights of the grafted polymers were evaluated from the increment in the dry mass of the gels and the amount of introduced ATRP initiators, which was measured by elemental analyses. Three-dimensional measuring laser microscopy revealed that the prepared gels had graft-density-dependent fine wrinkle structures on their surfaces. The surface-grafted gels induced the formation of skin layers during the shrinking process in response to a temperature increase, and their permeability strongly depended on the graft density. The graft density also controlled the kinetics of the swelling behavior in response to a temperature decrease. These physical properties were discussed on the basis of Young's modulus of the surface determined by an atomic force microscopy force curve measurement and the homogeneity of the surface polymer network observed by cryo-scanning electron microscopy. This makes it possible to arbitrarily control the characteristics of gels as open or semiclosed systems, which was uniquely determined by the designs of the surface gel networks.

A new function for thermal phase transition-based polymer actuators: autonomous motion on a surface of constant temperature

It is very challenging to make materials capable of autonomous oscillation known in many living systems (such as the heartbeat). Herein, we describe an approach to creating a thermo-mechano-thermal feedback loop for thermal phase transition-based polymer actuators, which leads to hour-long, autonomous motion on a substrate surface of constant temperature. We investigated the variables that determine the amplitude and period of the motion, and demonstrated exemplary physical work powered by direct thermomechanical energy conversion. Such continuous motion of a solid polymer driven by thermal energy without the need for temperature up/down switching is unprecedented, and the validated feedback loop can be implemented into other thermal phase transition-based polymer actuators.

Dissipative Assembly of Aqueous Carboxylic Acid Anhydrides Fueled by Carbodiimides

Biochemical systems make extensive use of chemically fueled processes (e.g., using ATP), but analogous abiotic systems remain rare. A key challenge is the identification of transformations that can be adapted to a range of applications and make use of readily available chemical fuels. In this context, the generation of transient covalent bonds is a fundamental tool for nonequilibrium systems chemistry. Here, we show that carbodiimides constitute a simple class of chemical fuels for dissipative assembly, taking advantage of their known reactivity to produce (hydrolytically unstable) anhydrides from carboxylic acids in water. Both aliphatic and aromatic anhydrides are formed on convenient time scales using the common, commercially available peptide coupling agent 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC). An important feature of this reaction is that no part of the carbodiimide is incorporated into the transient species; that is, the fuel is decoupled from the structure-and thus function-of the assembled state. We show that intramolecular anhydride formation of oligo(ethylene glycol) diacids gives macrocycles analogous to crown ethers, representing minimal examples of out-of-equilibrium supramolecular hosts. The kinetics and yields of macrocycle formation respond to cation guests, with the presence of matched cations decreasing their overall production.

Amoeba-like self-oscillating polymeric fluids with autonomous sol-gel transition

In the field of polymer science, many kinds of polymeric material systems that show a sol-gel transition have been created. However, most systems are unidirectional stimuli-responsive systems that require physical signals such as a change in temperature. Here, we report on the design of a block copolymer solution that undergoes autonomous and periodic sol-gel transition under constant conditions without any on–off switching through external stimuli. The amplitude of this self-oscillation of the viscosity is about 2,000 mPa s. We also demonstrate an intermittent forward motion of a droplet of the polymer solution synchronized with the autonomous sol-gel transition. This polymer solution bears the potential to become the base for a type of slime-like soft robot that can transform its shape kaleidoscopically and move autonomously, which is associated with the living amoeba that moves forward by a repeated sol-gel transition.

Most polymeric materials that show sol-gel transitions are unidirectional and stimuli-responsive systems. Here the authors show a block copolymer solution that undergoes autonomous and periodic sol-gel transitions under constant conditions.

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.

Mastering the Photothermal Effect in Liquid Crystal Networks: A General Approach for Self-Sustained Mechanical Oscillators

Chemical networks and molecular switches dominate the area of research geared toward macroscopic motion of materials. A counter-intuitive approach to create self-sustained oscillation by light irradiation of ordinary photostabilizers in splay-aligned liquid-crystalline networks made from commercial mesogens is developed. Photostabilizers or any molecules that are able to quickly dissipate the absorbed light through heat, by vibrational and/or rotational modes, can reach self-oscillating macroscopic motion where self-shadowing plays a critical role. The mechanical self-oscillation is linked to temperature oscillations and the asymmetric response over the film thickness. Only a localized responsive zone, acting as hinge, activates the oscillation of a beam-shaped device. The outcome of this research is extended from UV to near-IR actuation, making bulk applications to convert sunlight into mechanical work within reach.

Designing self-powered materials systems that perform pattern recognition

Inspired by the advances in both materials and computer science, we describe efforts to design “materials that compute” where the material and the computer are the same entity.