Rapid Actuation of Thermo-Responsive Polymer Networks: Investigation of the Transition Kinetics

Fundamentals and Advances in Stimuli-Responsive Hydrogels and Their Applications: A Review

This review summarizes the fundamental concepts, recent advancements, and emerging trends in the field of stimuli-responsive hydrogels. While numerous reviews exist on this topic, the field continues to evolve dynamically, and certain research directions are often overlooked. To address this, we classify stimuli-responsive hydrogels based on their response mechanisms and provide an in-depth discussion of key properties and mechanisms, including swelling kinetics, mechanical properties, and biocompatibility/biodegradability. We then explore hydrogel design, synthesis, and structural engineering, followed by an overview of applications that are relatively well established from a scientific perspective, including biomedical uses (biosensing, drug delivery, wound healing, and tissue engineering), environmental applications (heavy metal and phosphate removal from the environment and polluted water), and soft robotics and actuation. Additionally, we highlight emerging and unconventional applications such as local micro-thermometers and cell mechanotransduction. This review concludes with a discussion of current challenges and future prospects in the field, aiming to inspire further innovations and advancements in stimuli-responsive hydrogel research and applications to bring them closer to the societal needs.

Plasmon-Enhanced Multiphoton Polymer Crosslinking for Selective Modification of Plasmonic Hotspots

A novel approach to selectively modify narrow subareas of metallic nanostructures adjacent to plasmonic hotspots, where strong electromagnetic field amplification occurs upon localized surface plasmon (LSP) excitation, is reported. In contrast to surface plasmon-triggered polymerization, it relies on plasmonically enhanced multiphoton crosslinking (MPC) of polymer chains carrying photoactive moieties. When they are contacted with metallic nanostructures and irradiated with a femtosecond near-infrared beam resonantly coupled with LSPs, the enhanced field intensity locally exceeds the threshold and initiates MPC only at plasmonic hotspots. This concept is demonstrated by using gold nanoparticle arrays coated with two specifically designed polymers. Local MPC of a poly(N,N-dimethylacrylamide)-based copolymer with an anthraquinone crosslinker is shown via atomic force microscopy. Additionally, MPC is tested with a thermoresponsive poly(N-isopropylacrylamide)-based terpolymer. The reversible thermally induced collapse and swelling of the MPC-formed hydrogel at specific nanoparticle locations are confirmed by polarization-resolved localized surface plasmon resonance (LSPR) spectroscopy. These hybrid metallic/hydrogel materials can be further postmodified, offering attractive characteristics for future spectroscopic/bioanalytical applications.

4D Multimaterial Printing of Soft Actuators with Spatial and Temporal Control

Soft actuators (SAs) are devices which can interact with delicate objects in a manner not achievable with traditional robotics. While it is possible to design a SA whose actuation is triggered via an external stimulus, the use of a single stimulus creates challenges in the spatial and temporal control of the actuation. Herein, a 4D printed multimaterial soft actuator design (MMSA) whose actuation is only initiated by a combination of triggers (i.e., pH and temperature) is presented. Using 3D printing, a multilayered soft actuator with a hydrophilic pH-sensitive layer, and a hydrophobic magnetic and temperature-responsive shape-memory polymer layer, is designed. The hydrogel responds to environmental pH conditions by swelling or shrinking, while the shape-memory polymer can resist the shape deformation of the hydrogel until triggered by temperature or light. The combination of these stimuli-responsive layers allows for a high level of spatiotemporal control of the actuation. The utility of the 4D MMSA is demonstrated via a series of cargo capture and release experiments, validating its ability to demonstrate active spatiotemporal control. The MMSA concept provides a promising research direction to develop multifunctional soft devices with potential applications in biomedical engineering and environmental engineering.

Thermoresponsive and Photocrosslinkable Poly(2-alkyl-2-oxazoline) Toolbox - Customizable Ultralow-Fouling Hydrogel Coatings for Blood Plasma Environments

This study focuses on developing surface coatings with excellent antifouling properties, crucial for applications in the medical, biological, and technical fields, for materials and devices in direct contact with living tissues and bodily fluids such as blood. This approach combines thermoresponsive poly(2-alkyl-2-oxazoline)s, known for their inherent protein-repellent characteristics, with established antifouling motifs based on betaines. The polymer framework is constructed from various monomer types, including a novel benzophenone-modified 2-oxazoline for photocrosslinking and an azide-functionalized 2-oxazoline, allowing subsequent modification with alkyne-substituted antifouling motifs through copper(I)-catalyzed azide-alkyne cycloaddition. From these polymers surface-attached networks are created on benzophenone-modified gold substrates via photocrosslinking, resulting in hydrogel coatings with several micrometers thickness when swollen with aqueous media. Given that poly(2-alkyl-2-oxazoline)s can exhibit a lower critical solution temperature in water, their temperature-dependent solubility is compared to the swelling behavior of the surface-attached hydrogels upon thermal stimulation. The antifouling performance of these hydrogel coatings in contact with human blood plasma is further evaluated by surface plasmon resonance and optical waveguide spectroscopy. All surfaces demonstrate extremely low retention of blood plasma components, even with undiluted plasma. Notably, hydrogel layers with sulfobetaine moieties allow efficient penetration by plasma components, which can then be easily removed by rinsing with buffer.

Photomotion of Hydrogels with Covalently Attached Azo Dye Moieties—Thermoresponsive and Non-Thermoresponsive Gels

The unique photomotion of azo materials under irradiation has been in the focus of research for decades and has been expanded to different classes of solids such as polymeric glasses, liquid crystalline materials, and elastomers. In this communication, azo dye-containing gels are obtained by photocrosslinking of non-thermoresponsive and lower critical solution temperature type thermoresponsive copolymers. These are analysed with light microscopy regarding their actuation behaviour under laser irradiation. The influences of the cloud-point temperature and of the laser power are investigated in a series of comparative experiments. The thermoresponsive hydrogels show more intense photoactuation when the cloud-point temperature of the non-crosslinked polymer is above, but closer to, room temperature, while higher laser powers lead to stronger motion, indicating a photothermal mechanism. In non-thermoresponsive gels, considerably weaker photoactuation occurs, signifying a secondary mechanism that is a direct consequence of the optical field-azo dye interaction.

Plasmonic nanomaterials with responsive polymer hydrogels for sensing and actuation

Plasmonic nanomaterials have become an integral part of numerous technologies, where they provide important functionalities spanning from extraction and harvesting of light in thin film optical devices to probing of molecular species and their interactions on biochip surfaces. More recently, we witness increasing research efforts devoted to a new class of plasmonic nanomaterials that allow for on-demand tuning of their properties by combining metallic nanostructures and responsive hydrogels. This review addresses this recently emerged vibrant field, which holds potential to expand the spectrum of possible applications and deliver functions that cannot be achieved by separate research in each of the respective fields. It aims at providing an overview of key principles, design rules, and current implementations of both responsive hydrogels and metallic nanostructures. We discuss important aspects that capitalize on the combination of responsive polymer networks with plasmonic nanostructures to perform rapid mechanical actuation and actively controlled nanoscale confinement of light associated with resonant amplification of its intensity. The latest advances towards the implementation of such responsive plasmonic nanomaterials are presented, particularly covering the field of plasmonic biosensing that utilizes refractometric measurements as well as plasmon-enhanced optical spectroscopy readout, optically driven miniature soft actuators, and light-fueled micromachines operating in an environment resembling biological systems.