Light-responsive Hydrogels Research Articles

Near-infrared light-responsive hydrogels for on-demand dual delivery of proangiogenic growth factors

Achieving precise spatiotemporal control over the release of proangiogenic factors is crucial for vasculogenesis, the process of de novo blood vessel formation. Although various strategies have been explored, there is still a need to develop cell-laden biomaterials with finely controlled release of proangiogenic factors at specific locations and time points. We report on the developed of a near-infrared (NIR) light-responsive collagen hydrogel comprised of gold nanorods (GNRs)-conjugated liposomes containing proangiogenic growth factors (GFs). We demonstrated that this system enables on-demand dual delivery of GFs at specific sites and over selected time intervals. Liposomes were strategically formulated to encapsulate either platelet-derived growth factor (PDGF) or vascular endothelial growth factor (VEGF), each conjugated to gold nanorods (GNRs) with distinct geometries and surface plasmon resonances at 710 nm (GNR710) and 1064 nm (GNR1064), respectively. Using near infrared (NIR) irradiation and two-photon (2P) luminescence imaging, we successfully demonstrated the independent release of PDGF from GNR710 conjugated liposomes and VEGF from GNR1064-conjugated liposomes. Our imaging data revealed rapid release kinetics, with localized PDGF released in approximately 4 min and VEGF in just 1 and a half minutes following NIR laser irradiation. Importantly, we demonstrated that the release of each GF could be independently triggered using NIR irradiation with the other GF formulation remaining retained within the liposomes. This light-responsive collagen hydrogels holds promise for various applications in regenerative medicine where the establishment of a guided vascular network is essential for the survival and integration of engineered tissues. Statement of significanceIn this study, we have developed a light-responsive system with gold nanorods (GNRs)-conjugated liposomes in a collagen hydrogel, enabling precise dual delivery of proangiogenic growth factors (GFs) at specific locations and timepoints. Liposomes, containing platelet-derived growth factor (PDGF) or vascular endothelial growth factor (VEGF), release independently under near- infrared irradiation. This approach allows external activation of desired GF release, ensuring high cell viability. Each GF can be triggered independently, retaining the other within the liposomes. Beyond its application in establishing functional vascular networks, this dual delivery system holds promise as a universal platform for delivering various combinations of two or more GFs.

Light- and Solvent-Responsive Bilayer Hydrogel Actuators with Reversible Bending Behaviors.

Light-responsive hydrogel systems have gained significant attention due to their unique ability to undergo controlled and reversible swelling behavior in response to light stimuli. Combining light-responsive hydrogels with nonresponsive polymers offers a unique self-folding feature that can be used in soft robotic actuator designs. However, simple formulation of such systems with rapid response time is still a challenging task. Herein, we demonstrate a simple but versatile bilayer polymeric design combining light-responsive spiropyran-polyacrylamide (SP-PAAm) with polyacrylamide (PAAm) hydrogels. The photochromic spiropyran in our polymer design is a closed-ring, hydrophobic compound and turns into an open-ring, hydrophilic merocyanine isomer under light irradiation. The swelling degree of SP-PAAm and PAAm hydrogels was evaluated using LED lights with different wavelengths and solvent media (e.g., water, ethanol, DMF, and DMSO). We observed that SP-PAAm hydrogels reached a swelling ratio of ∼370% with the illumination of the blue LED in the DMF medium. By combining light-responsive SP-PAAm hydrogels with nonresponsive PAAm, a proof-of-concept demonstration was performed to demonstrate the applicability of our fabricated platforms. Although fabricated one-armed bilayer hydrogels possessed self-folding ability with a folding angle of ∼40° in 30 min, the four-armed bilayer platforms demonstrated more efficient and rapid folding behavior and reached a folding angle of ∼75° in ∼15 min. Given their simplicity and efficiency, we believe that such polymeric designs may offer new avenues for the fields of polymeric actuators and soft robotic systems.

Mussel-inspired near-infrared light-responsive gelatin-based hydrogels for enhancing MRSA-infected wound healing

Infected wound management is a great challenge to healthcare, especially in emergencies such as accidents or battlefields. Hydrogels as wound dressings can replace or supplement traditional wound treatment strategies, such as bandages or sutures. It is significant to develop novel hydrogel-based wound dressings with simple operation, inexpensive, easy debridement, effective antibacterial, biocompatibility, etc. Here, we designed a novel gelatin-based hydrogel wound dressing Gel-TA-Fe3+. The hydrogels used tannic-modified gelatin as the main body and Fe3+ as the crosslinking agent to achieve a controllable rapid sol-gel transition. The hydrogels exhibited tough mechanical properties, excellent antibacterial ability, biocompatibility and an acceptable temperature response to near-infrared light (NIR). Moreover, the hydrogels could promote the healing process of MRSA-infected skin wound in rats. This multifunctional hydrogel was thought to have potential for emergency treatment of bacterial infected wound.

NIR-responsive carboxymethyl-cellulose hydrogels containing thioketal-linkages for on-demand drug delivery system

Near-infrared (NIR) light-responsive hydrogels have emerged as a highly promising strategy for effective anticancer therapy owing to the remotely controlled release of chemotherapeutic molecules with minimal invasive manner. In this study, novel NIR-responsive hydrogels were developed from reactive oxygen species (ROS)-cleavable thioketal cross-linkers which possessed terminal tetrazine groups to undergo a bio-orthogonal inverse electron demand Diels Alder click reaction with norbornene modified carboxymethyl cellulose. The hydrogels were rapidly formed under physiological conditions and generated N2 gas as a by-product, which led to the formation of porous structures within the hydrogel networks. A NIR dye, indocyanine green (ICG) and chemotherapeutic doxorubicin (DOX) were co-encapsulated in the porous network of the hydrogels. Upon NIR-irradiation, the hydrogels showed spatiotemporal release of encapsulated DOX (>96 %) owing to the cleavage of thioketal bonds by interacting with ROS generated from ICG, whereas minimal release of encapsulated DOX (<25 %) was observed in the absence of NIR-light. The in vitro cytotoxicity results revealed that the hydrogels were highly cytocompatible and did not induce any toxic effect on the HEK-293 cells. In contrast, the DOX + ICG-encapsulated hydrogels enhanced the chemotherapeutic effect and effectively inhibited the proliferation of Hela cancer cells when irradiated with NIR-light.

Additive-Free Method for Enhancing the Volume Phase Transition Rate in Light-Responsive Hydrogels: A Study of Micro-Nano Bubble Water on PNIPAM-co-AAc Hydrogels.

Light-responsive hydrogels containing light-thermal convertible pigments have received interest for their possible applications in light-responsive shutters, valves, drug delivery systems, etc. However, their utility is limited by the slow response time. In this study, we investigated the use of micro-nano bubble water as a preparation solvent to accelerate the volume phase transition kinetics of poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-AAc) hydrogels. The hydrogels were characterized by dynamic light scattering (DLS) and dissolved oxygen (DO) measurements. The mechanical properties, surface morphology, and chemical composition of the hydrogels were analyzed by Young's modulus measurements, scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy, respectively. The results showed that hydrogels prepared with bubble water changed the volume transition rate by more than two orders of magnitude by simply changing the standing time of the bubble water for only a few hours. The cooperative diffusion coefficients obtained from the light-induced volume transition kinetics correlated linearly with Young's modulus and metastable state swelling ratio. Our results suggest that bubbles act as efficient water channels, thereby modulating the response rate and providing a simple, additive-free method for preparing hydrogels with a wide range of response rates.

Near-Infrared Light-Responsive Hydrogels for Highly Flexible Bionic Photosensors.

Soft biological tissues perform various functions. Sensory nerves bring sensations of light, voice, touch, pain, or temperature variation to the central nervous system. Animal senses have inspired tremendous sensors for biomedical applications. Following the same principle as photosensitive nerves, we design flexible ionic hydrogels to achieve a biologic photosensor. The photosensor allows responding to near-infrared light, which is converted into a sensory electric signal that can communicate with nerve cells. Furthermore, with adjustable thermal and/or electrical signal outputs, it provides abundant tools for biological regulation. The tunable photosensitive performances, high flexibility, and low cost endow the photosensor with widespread applications ranging from neural prosthetics to human-machine interfacing systems.

Light responsive hydrogels for controlled drug delivery.

Light is an easy acquired, effective and non-invasive external stimulus with great flexibility and focusability. Thus, light responsive hydrogels are of particular interests to researchers in developing accurate and controlled drug delivery systems. Light responsive hydrogels are obtained by incorporating photosensitive moieties into their polymeric structures. Drug release can be realized through three major mechanisms: photoisomerization, photochemical reaction and photothermal reaction. Recent advances in material science have resulted in great development of photosensitizers, such as rare metal nanostructures and black phosphorus nanoparticles, in order to respond to a variety of light sources. Hydrogels incorporated with photosensitizers are crucial for clinical applications, and the use of ultraviolet and near-infrared light as well as up-conversion nanoparticles has greatly increased the therapeutic effects. Existing light responsive drug delivery systems have been utilized in delivering drugs, proteins and genes for chemotherapy, immunotherapy, photodynamic therapy, gene therapy, wound healing and other applications. Principles associated with site-specific targeting, metabolism, and toxicity are used to optimize efficacy and safety, and to improve patient compliance and convenience. In view of the importance of this field, we review current development, challenges and future perspectives of light responsive hydrogels for controlled drug delivery.

CaCO3-assistant synthesis of pH/near-infrared light-responsive and injectable sodium alginate hydrogels for melanoma synergistic treatment

Melanoma is an aggressive tumor located in skin with high rates of recurrence and metastasis. Due to the limited traditional therapies, the development of novel strategies against melanoma is urgently quested. To reduce the side effects of traditional administration ways and amplify the killing effect, an injectable sodium alginate (SA)-based hydrogels were developed, in which CaCO3/polydopamine nanoparticles (CaCO3/PDA NPs) were embedded for the synergistic photothermal/calcium ions interference therapy of melanoma. In the study, the formation conditions and mechanical properties of CaCO3/PDA-SA hydrogels were characterized, and their antitumor efficiency and mechanism against mouse melanoma cells were investigated. Wheninjectedintratumorally, CaCO3/PDA-SA fluid was converted into hydrogel in situ through the interaction of pH-sensitive released Ca2+ and alginate chains, which increased the retention time of photothermal agents (CaCO3/PDA NPs) at tumor sites and thereby was more conducive to produce hyperthermia via photothermal conversion to combat melanoma. Moreover, in acidic tumor microenvironment, the residual CaCO3/PDA NPs in hydrogels continuously decomposed and released Ca2+ to destroy the Ca2+ buffering capacity and evoke the mitochondrial Ca2+-overloading, resulting in the inhibition of adenosine triphosphate production to accelerate cell death. Notably, besides the heat elevation, the near-infrared light (NIR) irradiation would further enhance the release of Ca2+ to promote the Ca2+-involved cell death. Therefore, a pH/NIR-responsive and injectable SA-based hydrogels were successfully established and showed enhanced treatment efficacy of melanoma through the synergism of photothermal therapy and calcium ions interference therapy.

Near-infrared light-responsive and antibacterial injectable hydrogels with antioxidant activity based on a Dopamine-functionalized Gellan Gum for wound healing

The development of wound dressings with combined antioxidant, antibacterial and tissue adhesion functions has been a difficult medical task for the treatment of wound infections. We synthetized a dopamine and PEG functionalized Gellan Gum (GG) to produce an injectable hydrogel with radical scavenging activity having both specific and aspecific antibiotic/antimicrobial properties. Using starting GG with different molecular weights, we obtained two derivatives that have been used to prepare the gel precursor dispersion, that undergoes gelation in the presence of colistin and dried microparticles (MPs) functionalized on the surface with polydopamine (pDA). Both were used to dope the hydrogel, increase the radical scavenger activity and impart near-infrared light (NIR) responsiveness. Indeed, with an irradiation of 810 nm, the incorporated microparticles exhibit photothermal transformation properties and improve the release of antibiotics on demand. The combination of photothermal and antibiotic therapy with synergistic antibacterial action acts on Pseudomonas aeruginosa and leads to a bactericidal effect in a few hours, while on Staphylococcus aureus there is an effect of inhibition of growth over time due only to the hyperthermic effect. We believe this study provides a promising method for fabricating a multifunctional injectable hydrogel for the potential treatment of infected skin wounds.

Research progress and clinical application of stimuli-responsive hydrogels in cervical cancer

Cervical cancer is the fourth most common malignancy in women worldwide and poses a great risk to women's health. There is an urgent need to develop a range of effective and innovative therapeutic options to overcome the shortcomings of conventional treatments: poor efficacy and toxic side effects. As an alternative therapy, a number of advances have been made in hydrogel-based drug delivery systems that enable targeted and localized therapy, as well as controlled release of drugs at the tumor site. These advantages can effectively increase drug concentration and reduce damage to normal sites caused by chemical drug toxicity. This paper reviews the progress of research applications of stimuli-responsive hydrogels in cervical cancer. The response mechanisms of hydrogels and the principles of enhanced drug efficacy are discussed in focus. These include thermal-responsive hydrogels, pH-responsive hydrogels, light-responsive hydrogels, enzyme-responsive hydrogels, and dual-responsive hydrogels. It is also argued that through the increasing understanding of hydrogels, it can be used clinically as an effective and durable therapeutic tool.

Near-infrared responsive quaternized chitosan-coated MoS2/poly(vinyl alcohol) hydrogel with improved mechanical and rapid antibacterial properties

Hydrogel is an ideal biomedical material because of its 3D network structure. However, their applications are greatly limited due to their poor mechanical strength and antibacterial property. Herein, a physically cross-linked and light-responsive poly(vinyl alcohol) (PVA) hydrogel embedded with quaternized chitosan-coated molybdenum disulfide (QCS-MoS2) nanomaterials was successfully prepared. The photothermal performance of QCS-MoS2 was studied with the results that QCS-MoS2 could be employed as a superior photothermal agent for light responsive hydrogels. After being incorporated with QCS-MoS2, a hydrogen bonding network could be formed in the hydrogel matrix, significantly increasing the mechanical properties. Moreover, the hybrid hydrogels exhibited excellent antibacterial properties. 95.6 % of Staphylococcus aureus (S. aureus) and 98.8 % of Escherichia coli (E. coli) were killed in 15 min due to the synergistic effects of the high temperature and the reactive oxygen species (ROS) generated by the irradiation of 808 nm near-infrared (NIR) light. No cytotoxicity was found by the introduction of QCS-MoS2, suggesting a great potential in photothermal antibacterial application for biomedical materials.

A bamboo/PNIPAM composite hydrogel assembly for both programmable and remotely-controlled light-responsive biomimetic actuations

Near-infrared light-responsive hydrogels are one of the most important soft biomimetic actuators owing to their accuracy, remote control performance, and permeability. However, the complex deformations and materials of hydrogel actuators are still a serious challenge. Herein, we have developed a novel composite hydrogel actuator by combining natural bamboo sheet and nigrosine/poly(N-isopropylacrylamide) (PNIPAM) composite hydrogel. The high-efficient photothermal conversion of the nigrosine/PNIPAM composite hydrogel sheet generates light-responsive actuation that is remotely controllable. Furthermore, the bamboo sheet with fiber-oriented structure can be cut at different angles, which facilitates programmable 3D complex deformations in the actuator. Accordingly, we have designed a series of biomimetic actuating devices, including simulations of butterfly antenna, Dendrobium, smart switch, and even a multi-step deformational biomimetic claw. In addition, we also explored a biomimetic insect with precisely-controlled fast movements on the water surface based on Marangoni effect. This work provides a simple but general strategy for fabricating biomimetic actuators and will inspire new designs for intelligent soft actuators.

Green–Light–Driven Poly(N-isopropylacrylamide-acrylamide)/Fe3O4 Nanocomposite Hydrogel Actuators

Light-responsive hydrogel actuators show attractive biomedical applications for in vivo drug delivery tool, surgical tissue repair operation, and vascular cleaning due to its non-contact, rapid, precise, and remote spatial control of light. Conventional visible–light–responsive hydrogels contain special chemical structure or groups, and the difficulty in synthesis results in that few can be applied to fabricate visible–light–driven hydrogel actuators. In this study, based on photothermal effect, surface-modified Fe3O4 nanoparticles were incorporated into poly(N-isopropylacrylamide-acrylamide) hydrogel by UV photopolymerization, which revealed excellent green–light–responsive volume change. Under a laser irradiation of 200 mW at 520 nm, the bending angle deformation of hydrogel strips with 2.62 wt% Fe3O4 reached 107.8°. Strip-shaped hydrogel actuators could be applied to transport tiny objects. Furthermore, a boomerang-like hydrogel actuator was designed and fabricated to drive floating foam on water. By 12 cycles of continuous laser on–off irradiation to a hydrogel actuator underwater, a circular returning movement of the float was accomplished. The study on driving a float using visible–light–triggered hydrogel actuators provides a new idea for the design of light-driven biomedical devices and soft robots.

Contact-Free Remote Manipulation of Hydrogel Properties Using Light-Triggerable Nanoparticles: A Materials Science Perspective for Biomedical Applications.

Considerable progress has been made in synthesizing "intelligent", biodegradable hydrogels that undergo rapid changes in physicochemical properties once exposed to external stimuli. These advantageous properties of stimulus-triggered materials make them highly appealing to diverse biomedical applications. Of late, research on the incorporation of light-triggered nanoparticles (NPs) into polymeric hydrogel networks has gained momentum due to their ability to remotely tune hydrogel properties using facile, contact-free approaches, such as adjustment of wavelength and intensity of light source. These multi-functional NPs, in combination with tissue-mimicking hydrogels, are increasingly being used for on-demand drug release, preparing diagnostic kits, and fabricating smart scaffolds. Here, the authors discuss the atomic behavior of different NPs in the presence of light, and critically review the mechanisms by which NPs convert light stimuli into heat energy. Then, they explain how these NPs impact the mechanical properties and rheological behavior of NPs-impregnated hydrogels. Understanding the rheological behavior of nanocomposite hydrogels using different sophisticated strategies, including computer-assisted machine learning, is critical for designing the next generation of drug delivery systems. Next, they highlight the salient strategies that have been used to apply light-induced nanocomposites for diverse biomedical applications and provide an outlook for the further improvement of these NPs-driven light-responsive hydrogels.

Light-Responsive Dynamic Protein Hydrogels Based on LOVTRAP.

Protein-based hydrogels can mimic many aspects of native extracellular matrices (ECMs) and are promising biomedical materials that find various applications in cell proliferation, drug/cell delivery, and tissue engineering. To be adapted for different tasks, it is important that the mechanical and/or biochemical properties of protein-based hydrogels can be regulated by external stimuli. Light as a regulation stimulus is of advantage because it can be easily applied in demanded spatiotemporal manners. The noncovalent binding between the light-oxygen-voltage-sensing domain 2 (LOV2) and its binding partner ZDark1 (zdk1), named as LOVTRAP, is a light-responsive interaction. The binding affinity of LOVTRAP is much higher in dark than that under blue light irradiation. Taking advantage of these light-responsive interactions, herein we endeavored to use LOVTRAP as a crosslinking mechanism to engineer light-responsive protein hydrogels. Using LOV2-containing and zdk1-containing multifunctional protein building blocks, we successfully engineered a light-responsive protein hydrogel whose viscoelastic properties can change in response to light: in the dark, the hydrogel showed higher storage modulus; under blue light irradiation, the storage modulus decreased. Due to the noncovalent nature of the LOVTRAP, the engineered LOVTRAP protein hydrogels displayed shear-thinning and self-healing properties and served as an excellent injectable protein hydrogel. We anticipated that this new class of light-responsive protein hydrogels will broaden the scope of dynamic protein hydrogels and help develop other light-responsive protein hydrogels for biomedical applications.

Redox-responsive and light-responsive DNA-based hydrogels and their applications

The synthesis and applications of redox-responsive and photoresponsive hydrogels are introduced. One approach to synthesize redox-responsive hydrogels involves the crosslinking of polymers with nucleic acid duplexes and metal-carboxylate bridges. The second approach to assemble redox-responsive hydrogels includes the cooperative crosslinking of the polymers with duplex nucleic acids and donor-acceptor redox-active bridges. By reversible oxidation/reduction of the redox-active groups, switchable stiffness of the hydrogels is demonstrated. The use of redox-responsive hydrogels as shape-memory and self-healing materials is discussed. Photoresponsive hydrogels are introduced by the design of polymer matrices cooperatively stabilized by permanent crosslinking units, e.g. boronate ester/glucosamine, and trans/cis-photoisomerizable nucleic acid bridges. Alternatively, photoresponsive hydrogels crosslinked by nucleic acids and supramolecular complexes consisted of photoisomerizable host-guest or donor-acceptor complexes are introduced. Cyclic photoinduced formation/dissociation of the photoresponsive bridges lead to reversible stiffness of the hydrogels. In addition, Au nanoparticles (NPs) or nanorods (NRs) are immobilized in hydrogels. The light-induced thermoplasmonic melting of the nucleic acid bridges by Au NPs or NRs and switchable stiffness of the hydrogels are demonstrated. The stiffness-controlled hydrogels are applied as shape-memory and self-healing matrices and as controlled drug release materials. In addition, light-induced mechanical bending of bilayer hydrogels consisting of Au NPs/NRs is demonstrated.

Near-infrared light-responsive hybrid hydrogels for the synergistic chemo-photothermal therapy of oral cancer.

Light-stimulus-responsive therapies have been recognized as a promising strategy for the efficient and safe treatment of oral squamous cell carcinoma (OSCC). Hydrogels have emerged as a promising multifunctional platform combining localized drug delivery and sustained drug release with multimodal properties for combined OSCC therapy. However, inaccurate drug release and limited light-absorption efficiency have hindered their on-demand chemo-photothermal applications. To tackle these problems, an injectable and near-infrared (NIR) light-responsive hybrid system was developed by incorporating light-responsive mesoporous silica nanoparticles (MSNs) as doxorubicin (DOX) carriers into the IR820/methylcellulose hydrogel networks for chemophotothermal therapy. Under NIR radiation, the incorporated IR820, a new green cyanine dye, was excited to induce photothermal effects against tumor cells. Meanwhile, MSNs achieved self-degradation-controlled DOX release via the cleavage of diselenide bonds induced by reactive oxygen species. Through the combination of chemotherapy and phototherapy, a long-lasting synergistic anti-tumor effect was achieved in vitro and in vivo with less toxicity. These findings demonstrate the potential of light-responsive hydrogels as a multifunctional platform for accurate synergistic chemophotothermal treatment of OSCC.

On-demand and tunable dual wavelength release of antibody using light-responsive hydrogels.

There has been an increased interest in the use of protein therapeutics, especially antibodies, for the treatment of a variety of diseases due to their high specificity to tissues and biological pathways of interest. However, the use of antibodies can be hindered by physical aggregation, degradation, and diffusion when injected in vivo leading to the need for antibody-releasing depots for the controlled and localized delivery within tissues of interest. Here, we investigated photolabile hydrogel chemistries for creating on-demand and tunable antibody release profiles. Innovative, scalable synthetic procedures were established and applied for fabricating hydrogels with nitrobenzyl (NB) and coumarin (CMR) photolabile crosslinks that responded to clinically relevant doses of long-wavelength UV and short-wavelength visible light. This synthetic procedure includes a route to make a CMR linker possessing two functional handles at the same ring position with water-stable bonds. The photocleavage properties of NB and CMR crosslinked hydrogels were characterized, as well as their potential for translational studies by degradation through pig skin, a good human skin mimic. The mechanism of hydrogel degradation, bulk versus surface eroding, was determined to be dependent on the wavelength of light utilized and the molar absorptivity of the different photolabile linkers, providing a facile means for altering protein release upon hydrogel degradation. Further, the encapsulation and on-demand release of a model monoclonal antibody was demonstrated, highlighting the ability to control antibody release from these hydrogels through the application of light while retaining its bioactivity. In particular, the newly designed CMR hydrogels undergo surface erosion-based protein release using visible light, which is more commonly used clinically. Overall, this work establishes scalable syntheses and relevant pairings of formulation-irradiation conditions for designing on-demand and light-responsive material systems that provide controlled, tunable release of bioactive proteins toward addressing barriers to preclinical translation of light-based materials and ultimately improving therapeutic regimens.

Biofunctional hydrogels based on host–guest interactions

Biological systems involve the most complex materials in the world. Mimicking biological systems is not an easy task. Materials researchers are continuing to push themselves to prepare synthetic materials that can replicate biological systems. Hydrogels have attracted great interest from materials researchers for mimicking biological systems due to their biocompatibility. One approach to preparing hydrogels is using host–guest interactions. Host–guest interactions can be achieved by using cyclodextrins (CDs) as host units and suitable guest units. Hydrogels prepared based on host–guest interactions show several functionalities, such as self-healing ability, stimuli responsiveness, the ability to function as soft actuators for use in artificial muscles, and conductive responsiveness. These functions can be attributed to reversible bond formation between the CDs and guest units. Self-healing materials, which mimic the recovery of injured skin, can be achieved if the association constant between the CDs and guests is sufficiently high. Several specific guest units can also show external stimuli responsivity (redox, pH, temperature, and light) when paired with CDs, allowing them to mimic the responsiveness of the human body to external stimuli. Light-responsive hydrogels can be used to prepare soft actuators that can be employed as artificial muscles to mimic the sliding motion of human sarcomeres. Conductive hydrogels will be required to support the function of artificial muscles in the near future. This review summarizes the advancements made in biofunctional hydrogels based on host–guest interactions. Hydrogels as biocompatible polymer have been attracted materials researchers for mimicking biological systems. One efficient approach to preparing hydrogels is using host–guest interactions between cyclodextrins (CDs) as host units and suitable guest units. The hydrogels formed by CD and guest unit reversible bonds show several biofunctionalities, such as self-healing ability, stimuli responsiveness, the ability to function as soft actuators for use in artificial muscles, and conductive responsiveness.

Halloysite nanotubes@polydopamine reinforced polyacrylamide-gelatin hydrogels with NIR light triggered shape memory and self-healing capability

In this study, the polyacrylamide-gelatin composite hydrogels reinforced by halloysite nanotubes@polydopamine (HNTs@PDA) with NIR triggered shape memory performance and self-healing capacity were prepared through in-situ free radical polymerization of acrylamide in the mixture of Laponite-RD, HNTs@PDA and gelatin. HNTs@PDA was firstly synthesized through oxidative polymerization of dopamine on the surfaces of HNTs. HNTs@PDA was characterized by transmission electron microscopy, scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, Zeta potential, particle size analysis, and X-ray photoelectron spectroscopy. The photothermal performance of HNTs@PDA was then determined and the results reveal that HNTs@PDA can be employed as a superior photothermal agent to prepare light responsive hydrogels. The structure, morphology, mechanical properties, NIR triggered shape memory performance and self-healing capacity of the composite hydrogels were studied. The modified nanotubes act as cross-linking agent of polyacrylamide to form a rigid network in the hydrogel matrix, which leads to a significant increase in the mechanical properties. Moreover, the NIR triggered shape recovery process of the hydrogel is quite rapid, for example, it is only 63 s for recovering 720° shape change for the hydrogel of HNTs@PDA 40. The maximum NIR triggered healing efficiency of the hydrogels can reach 76%. The HNTs@PDA reinforced hydrogels with superior mechanical properties, NIR triggered shape memory, and self-healing ability exhibit promising applications in biomedical materials and smart engineering materials.