Collagen-Poly(N-isopropylacrylamide) Hydrogels with Tunable Properties

Hydrogels with Reversible Crosslinks for Improved Localised Stem Cell Retention: A Review

Successful stem cell applications could have a significant impact on the medical field, where many lives are at stake. However, the translation of stem cells to the clinic could be improved by overcoming challenges in stem cell transplantation and in vivo retention at the site of tissue damage. This review aims to showcase the most recent insights into developing hydrogels that can deliver, retain, and accommodate stem cells for tissue repair. Hydrogels can be used for tissue engineering, as their flexibility and water content makes them excellent substitutes for the native extracellular matrix. Moreover, the mechanical properties of hydrogels are highly tuneable, and recognition moieties to control cell behaviour and fate can quickly be introduced. This review covers the parameters necessary for the physicochemical design of adaptable hydrogels, the variety of (bio)materials that can be used in such hydrogels, their application in stem cell delivery and some recently developed chemistries for reversible crosslinking. Implementing physical and dynamic covalent chemistry has resulted in adaptable hydrogels that can mimic the dynamic nature of the extracellular matrix.

Phase transition and potential biomedical applications of thermoresponsive compositions based on polysaccharides, proteins and DNA: A review

Smart thermoresponsive polymers have long attracted attention as materials of a great potential for biomedical applications, mainly for drug delivery, tissue engineering and wound dressing, with a special interest to injectable hydrogels. Poly-N-isopropylacrylamide (PNIPAM) is the most important synthetic thermoresponsive polymer due to its physiologically relevant transition temperature. However, the use of unmodified PNIPAM encounters such problems as low biodegradability, low drug loading capacity, slow response to thermal stimuli, and insufficient mechanical robustness. The use of natural polysaccharides and proteins in combinations with PNIPAM, in the form of grafted copolymers, IPNs, microgels and physical mixtures, is aimed at overcoming these drawbacks and creating dual-functional materials with both synthetic and natural polymers' properties. When developing such compositions, special attention should be paid to preserving their key property, thermoresponsiveness. Addition of hydrophobic and hydrophilic fragments to PNIPAM is known to affect its transition temperature. This review covers various classes of natural polymers - polysaccharides, fibrous and non-fibrous proteins, DNA - used in combination with PNIPAM for the prospective biomedical purposes, with a focus on their phase transition temperatures and its relation to the natural polymer's structure.

Network architecture dependent mechanical response in temperature responsive collagen-PNIPAM composites

Collagen is the most abundant protein in the mammalian extracellular matrix. In-vitro collagen-based materials with specific mechanical properties are important for various bio-medical and tissue-engineering applications. Here, we study the reversible mechanical switching behaviour of a bio-compatible composite formed by collagen networks seeded with thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) microgel particles, by exploiting the swelling/de-swelling of the particles across the lower critical solution temperature (LCST). Interestingly, we find that the shear modulus of the system reversibly enhances whenever the diameter of the microgel particles is changed from that corresponding to the polymerization temperature of the composite, irrespective of swelling or, de-swelling. However, the degree of such enhancement significantly depends on the temperature-dependent collagen network architecture quantified by the mesh size of the network. Furthermore, confocal imaging of the composite during the temperature switching reveals that the reversible clustering of microgel particles above LCST plays a crucial role in the observed switching response.

NIR-activated multi-hit therapeutic Ag2S quantum dot-based hydrogel for healing of bacteria-infected wounds

Hydrogel dressings are highly biocompatible and can maintain a moist wound environment, suggesting constructing an efficient multi-modal antibacterial hydrogel platform is a promising strategy for treating bacterial wound infections. In this work, a composite Ag₂S quantum dot/mSiO₂ NPs hydrogel (NP hydrogel) with antibacterial ability was constructed by incorporating Ag₂S quantum dots (QDs) modified by mesoporous silica (mSiO₂) into the network structure of 3-(trimethoxylmethosilyl) propyl methacrylate based on free radical polymerization. The NP hydrogel showed outstanding controllable photothermal and photodynamic characteristics under 808 nm near infrared (NIR) light irradiation, with a photothermal conversion efficiency of 57.3%. Additionally, the release of Ag⁺ could be controlled by the inherent volume change of the NP hydrogel made of N-isopropylacrylamide (NIPAAm) and acrylamide (AAm) during NIR laser exposure, with the embedded Ag₂S QDs working as a reservoir to release Ag⁺ continuously from the hydrogel matrix to achieve bactericidal activity. The synergetic effects between hyperthermia, radical oxygen species, and Ag⁺ released under NIR radiation endowed the NP hydrogel with prominent antibacterial properties against Escherichia coli (E. coli) and methicillin-resistant Staphylococcus aureus (MRSA), with an inhibition rate of 99.7% and 99.8%, respectively. In vivo wound healing experiments indicated that the NP hydrogel could enhance bacterial clearance, increase collagen coverage area and up-regulate VEGF expression, exhibiting high biocompatibility. Overall, this study proposed an efficient and highly biocompatible multi-modal therapeutic nanohydrogel, opening up a new way for developing broad-spectrum antibacterial wound dressings to treat bacterial wound infections. STATEMENT OF SIGNIFICANCE: Bacterial wound infection is still one of the most difficult medical problems. In this work, a stimulating NIR-responsive hydrogel encapsulating functional Ag₂S QDs was prepared, which showed high photothermal conversion efficiency (57.3%) and outstanding antibacterial ability under 808 nm NIR laser, killing 99.7% and 99.8% of E. coli and MRSA in 4 min, respectively. During NIR light irradiation, the release rate of Ag⁺ could be regulated by the intrinsic volume transition of the hydrogel, leading to remarkable antibacterial properties in vitro and in vivo under the combined action of hyperthermia, radical oxygen species and Ag⁺ released. This study proposed a novel multi-modal therapeutic nanohydrogel, opening up a new way for developing broad-spectrum antibacterial wound dressings to treat bacterial wound infections.

A Novel Temperature-Dependent Hydrogel Emulsion with Sol/Gel Reversible Phase Transition Behavior Based on Polystyrene-co-poly(N-isopropylacrylamide)/Poly(N-isopropylacrylamide) Core-Shell Nanoparticle

As a kind of temperature-responsive hydrogel, polystyrene-co-poly(N-isopropylacrylamide)/poly(N-isopropylacrylamide) (PS-co-PNIPAM/PNIPAM) core-shell nanoparticles prepared by two-step copolymerization are widely studied and used because of their specific structures and properties. Unlike most reports about the steady stability of PS-co-PNIPAM/PNIPAM core-shell nanoparticle hydrogel emulsion, in this work, the PS-co-PNIPAM/PNIPAM core-shell nanoparticle hydrogel emulsion (symbolized as PS/PNIPAM hydrogel emulsion), which is prepared after the second step of synthesis and without washing out a large number of PNIPAM polymer segments, shows a reversible temperature-dependent sol-gel transition characteristic during the temperature range of 34-80 °C. The PS/PNIPAM hydrogel emulsion is a normal solution at room temperature, and it changes from a sol to a gel statue when the temperature approaches up to low critical solution temperature (LCST). As the temperature continues to increase, the gel (core-shell nanoparticles as the crosslinkers and the linear PNIPAM chain as the 3D gel network) of the PS/PNIPAM hydrogel emulsion gradually shrinks and drains linearly. Compared with most crosslinked hydrogels, the hydrogel here can be arbitrarily changed in shape according to use needs, which is convenient for use, transportation, and storage. Here a new route is provided for the preparation of a PS/PNIPAM core-shell hydrogel nanoparticle system, as well as a new supramolecular crosslinking sol-gel system for application in biomedical materials, sensors, biological separation, drug release, macromolecular adsorption, and purification.

Extracellular matrix-based biomaterials for cardiac regeneration and repair

The spectrum of ischemic heart diseases, encompassing acute myocardial infarction to heart failure, represents the leading cause of death worldwide. Although extensive progress in cardiovascular diagnoses and therapy has been made, the prevalence of the disease continues to increase. Cardiac regeneration has a promising perspective for the therapy of heart failure. Recently, extracellular matrix (ECM) has been shown to play an important role in cardiac regeneration and repair after cardiac injury. There is also evidence that the ECM could be directly used as a drug to promote cardiomyocyte proliferation and cardiac regeneration. Increasing evidence supports that applying ECM biomaterials to maintain heart function recovery is an important approach to apply the concept of cardiac regenerative medicine to clinical practice in the future. Here, we will introduce the essential role of cardiac ECM in cardiac regeneration and summarize the approaches of delivering ECM biomaterials to promote cardiac repair in this review.

Synthesis of Injectable, Thermally Responsive, Chondroitin Sulfate-Cross-Linked Poly(N-isopropylacrylamide) Hydrogels

In this study, we describe the synthesis and characterization of a biosynthetic hydrogel system that consists of a thermally responsive macromer and biological cross-linkers. By combining a poly(N-isopropylacrylamide)-based thermogelling macromer with epoxy pendant groups and chondroitin sulfate cross-linkers that are modified to contain either hydrazide or N-hydroxysuccinimide pendant groups, we successfully fabricated a system that undergoes gelation when the temperature is raised from room temperature to 37 °C and is further stabilized via covalent links between the macromers. The anionic charge on chondroitin sulfate contributed to a high degree of gel swelling, while the cross-linking reaction between the macromers prevented post-formation syneresis. The rate of degradation of CS-cross-linked hydrogels was dependent on the degree of substitution of hydrazide-modified chondroitin sulfate cross-linkers. A higher molar content of chondroitin sulfate led to a greater osmotic pressure within the hydrogel and thus a higher compressive modulus. On the other hand, excessive amounts of chondroitin sulfate caused time-dependent cytotoxicity, as confirmed by a leachables cytocompatibility study. Overall, the system described in this study provides a versatile platform to synthesize hydrogels with differing combinations of compressive moduli and rates of degradation, which is achievable by varying the degree of substitution of hydrazide groups on CS-based cross-linkers.

Injectable PNIPAM/Hyaluronic acid hydrogels containing multipurpose modified particles for cartilage tissue engineering: Synthesis, characterization, drug release and cell culture study

Novel injectable thermosensitive PNIPAM/hyaluronic acid hydrogels containing various amounts of chitosan-g-acrylic acid coated PLGA (ACH-PLGA) micro/nanoparticles were synthesized and designed to facilitate the regeneration of cartilage tissue. The ACH-PLGA particles were used in the hydrogels to play a triple role: first, the allyl groups on the chitosan-g-acrylic acid shell act as crosslinkers for PNIPAM and improved the mechanical properties of the hydrogel to mimic the natural cartilage tissue. Second, PLGA core acts as a carrier for the controlled release of chondrogenic small molecule melatonin. Third, they could reduce the syneresis of the thermosensitive hydrogel during gelation. The optimum hydrogel with the minimum syneresis and the maximum compression modulus was chosen for further evaluations. This hydrogel showed a great integration with the natural cartilage during the adhesion test, and also, presented an interconnected porous structure in scanning electron microscopy images. Eventually, to evaluate the cytotoxicity, mesenchymal stem cells were encapsulated inside the hydrogel. MTT and Live/Dead assay showed that the hydrogel improved the cells growth and proliferation as compared to the tissue culture polystyrene. Histological study of glycosaminoglycan (GAG) showed that melatonin treatment has the ability to increase the GAG synthesis. Overall, due to the improved mechanical properties, low syneresis, the ability of sustained drug release and also high bioactivity, this injectable hydrogel is a promising material system for cartilage tissue engineering.

3D Cell-Based Assays for Drug Screens: Challenges in Imaging, Image Analysis, and High-Content Analysis

The introduction of more relevant cell models in early preclinical drug discovery, combined with high-content imaging and automated analysis, is expected to increase the quality of compounds progressing to preclinical stages in the drug development pipeline. In this review we discuss the current switch to more relevant 3D cell culture models and associated challenges for high-throughput screening and high-content analysis. We propose that overcoming these challenges will enable front-loading the drug discovery pipeline with better biology, extracting the most from that biology, and, in general, improving translation between in vitro and in vivo models. This is expected to reduce the proportion of compounds that fail in vivo testing due to a lack of efficacy or to toxicity.

Cytoskeletal stiffening in synthetic hydrogels

Although common in biology, controlled stiffening of hydrogels in vitro is difficult to achieve; the required stimuli are commonly large and/or the stiffening amplitudes small. Here, we describe the hierarchical mechanics of ultra-responsive hybrid hydrogels composed of two synthetic networks, one semi-flexible and stress-responsive, the other flexible and thermoresponsive. Heating collapses the flexible network, which generates internal stress that causes the hybrid gel to stiffen up to 50 times its original modulus; an effect that is instantaneous and fully reversible. The average generated forces amount to ~1 pN per network fibre, which are similar to values found for stiffening resulting from myosin molecular motors in actin. The excellent control, reversible nature and large response gives access to many biological and bio-like applications, including tissue engineering with truly dynamic mechanics and life-like matter.

Although common in biology, controlled stiffening of hydrogels in vitro is difficult to achieve. Here the authors show how a biomimetic hybrid hydrogel can be stiffened instantaneously and reversibly up to 50 times.

VEGF delivery by smart polymeric PNIPAM nanoparticles affects both osteogenic and angiogenic capacities of human bone marrow stem cells

OBJECTIVE

Bone tissue engineering (BTE) faces a major challenge with cell viability after implantation of a construct due to lack of functional vasculature within the implant. Human bone marrow derived mesenchymal stem cells (hBMSCs) have the potential to undergo transdifferentiation towards an endothelial cell phenotype, which may be appropriate for BTE in conjunction with the appropriate scaffolds and microenvironment.

HYPOTHESIS AND METHODS

We hypothesized that slow delivery of vascular endothelial growth factor (VEGF) by using nanoparticles in combination with osteogenic stimuli might enhance both osteogenic and angiogenic differentiation of angiogenic primed hBMSCs cultured in an osteogenic microenvironment. Therefore, we developed a new strategy to enhance vascularization in BTE in vitro by synthesis of smart temperature sensitive poly(N‑isopropylacrylamide) (PNIPAM) nanoparticles. We used PNIPAM nanoparticles loaded with collagen to investigate their ability to deliver VEGF for both angiogenic and osteogenic differentiation.

RESULTS

We used the free radical polymerization technique to synthesize PNIPAM nanoparticles, which had particle sizes of approximately 100 nm at 37 °C and LCST of 30-32 °C. The cumulative VEGF release after 72 h for VEGF loaded PNIPAM (VEGF-PNIPAM) nanoparticles was 70%; for VEGF-PNIPAM loaded collagen hydrogels, it was 23%, which indicated slower release of VEGF in the VEGF-PNIPAM loaded collagen system. Immunocytochemistry (ICC) and inverted microscope visualization confirmed endothelial differentiation and capillary-like tube formation in the osteogenic culture medium after 14 days. Quantitative real-time polymerase chain reaction (QRT-PCR) also confirmed expressions of collagen type I (Col I), runt-related transcription factor 2 (RUNX2), and osteocalcin (OCN) osteogenic markers along with expressions of platelet-endothelial cell adhesion molecule-1 (CD31), von Willebrand factor (vWF), and kinase insert domain receptor (KDR) angiogenic markers. Our data clearly showed that VEGF released from PNIPAM nanoparticles and VEGF-PNIPAM loaded collagen hydrogel could significantly contribute to the quality of engineered bone tissue.

High-Speed Single-Molecule Tracking of CXCL13 in the B-Follicle

Soluble factors are an essential means of communication between cells and their environment. However, many molecules readily interact with extracellular matrix components, giving rise to multiple modes of diffusion. The molecular quantification of diffusion in situ is thus a challenging imaging frontier, requiring very high spatial and temporal resolution. Overcoming this methodological barrier is key to understanding the precise spatial patterning of the extracellular factors that regulate immune function. To address this, we have developed a high-speed light microscopy system capable of millisecond sampling in ex vivo tissue samples and submillisecond sampling in controlled in vitro samples to characterize molecular diffusion in a range of complex microenvironments. We demonstrate that this method outperforms competing tools for determining molecular mobility of fluorescence correlation spectroscopy (FCS) and fluorescence recovery after photobleaching (FRAP) for evaluation of diffusion. We then apply this approach to study the chemokine CXCL13, a key determinant of lymphoid tissue architecture, and B-cell-mediated immunity. Super-resolution single-molecule tracking of fluorescently labeled CCL19 and CXCL13 in collagen matrix was used to assess the heterogeneity of chemokine mobility behaviors, with results indicating an immobile fraction and a mobile fraction for both molecules, with distinct diffusion rates of 8.4 ± 0.2 and 6.2 ± 0.3 µm²s⁻¹, respectively. To better understand mobility behaviors in situ, we analyzed CXCL13-AF647 diffusion in murine lymph node tissue sections and observed both an immobile fraction and a mobile fraction with an example diffusion coefficient of 6.6 ± 0.4 µm²s⁻¹, suggesting that mobility within the follicle is also multimodal. In quantitatively studying mobility behaviors at the molecular level, we have obtained an increased understanding of CXCL13 bioavailability within the follicle. Our high-speed single-molecule tracking approach affords a novel perspective from which to understand the mobility of soluble factors relevant to the immune system.

Enhanced multi-lineage differentiation of human mesenchymal stem/stromal cells within poly(N-isopropylacrylamide-acrylic acid) microgel-formed three-dimensional constructs

Human mesenchymal stem/stromal cells (hMSCs) are a potential cell source of stem cell therapy for many serious diseases and hMSC spheroids have emerged to replace single cell suspensions for cell therapy. Three-dimensional (3D) scaffolds or hydrogels which can mimic properties of the extracellular matrix (ECM) have been widely explored for their application in tissue regeneration. However, there are considerably less studies on inducing differentiation of hMSC spheroids using 3D scaffolds or hydrogels. This study is the first to explore multi-lineage differentiation of a stem cell line and primary stem cells within poly(N-isopropylacrylamide) (p(NIPAAm))-based thermosensitive microgel-formed constructs. We first demonstrated that poly(N-isopropylacrylamide-co-acrylic acid) (p(NIPAAm-AA)) was not toxic to hMSCs and the microgel-formed constructs facilitated formation of uniform stem cell spheroids. Due to functional enhancement of cell spheroids, hMSCs within the 3D microgel-formed constructs were induced for multi-lineage differentiation as evidenced by significant up-regulation of messenger RNA (mRNA) expression of chondrogenic and osteogenic genes even in the absence of induction media on day 9. When induction media were in situ supplied on day 9, mRNA expression of chondrogenic, osteogenic and adipogenic genes within the microgel-formed constructs were significantly higher than that in the pellet and 2D cultures, respectively, on day 37. In addition, histological and immunofluorescent images also confirmed successful multi-lineage differentiation of hMSCs within the 3D microgel-formed constructs. Hence, the thermosensitive p(NIPAAm-AA) microgel can be potentially used in an in vitro model for cell differentiation or in vivo transplantation of pre-differentiated human mesenchymal stromal cells into patients for specific lineage differentiation.

Multi-Responsive Behaviors of Copolymers Bearing N-Isopropylacrylamide with or without Phenylboronic Acid in Aqueous Solution

Continuing efforts to develop novel smart materials are anticipated to upgrade the quality of life of humans. Thermo-responsive poly(N-isopropylacrylamide) and glucose-responsive phenylboronic acid-typical representatives-are often integrated as multi-stimuli-sensitive materials, but few are available for side-by-side comparisons with their properties. In this study, both copolymers bearing N-isopropylacrylamide (NIPAAm), with or without 3-acrylamidophenylboronic acid (AAPBA), were synthesized by free radical polymerization, and characterized by Fourier transform infrared spectrometry, nuclear magnetic resonance hydrogen spectroscopy and gel permeation chromatography. Dynamic light scattering was used to analyze and compare the responsive behaviors of the copolymers in different aqueous solutions. Atomic force microscopy was also employed to investigate the apparent morphology changes with particle sizes. The results demonstrated that the introduction of NIPAAm endowed the composite materials with thermosensitivity, whereas the addition of AAPBA lowered the molecular weight of the copolymers, intensified the intermolecular aggregation of the nanoparticles, reduced the lower critical solution temperature (LCST) of the composites, and accordingly allowed the copolymers to respond to glucose. It was also concluded that the responding of smart copolymers to operating parameters can be activated only under special conditions, and copolymer dimension and conformation were affected by inter/intramolecular interactions.

Development of hydrogels for regenerative engineering

The aim of regenerative engineering is to restore complex tissues and biological systems through convergence in the fields of advanced biomaterials, stem cell science, and developmental biology. Hydrogels are one of the most attractive biomaterials for regenerative engineering, since they can be engineered into tissue mimetic 3D scaffolds to support cell growth due to their similarity to native extracellular matrix. Advanced nano- and micro-technologies have dramatically increased the ability to control properties and functionalities of hydrogel materials by facilitating biomimetic fabrication of more sophisticated compositions and architectures, thus extending our understanding of cell-matrix interactions at the nanoscale. With this perspective, this review discusses the most commonly used hydrogel materials and their fabrication strategies for regenerative engineering. We highlight the physical, chemical, and functional modulation of hydrogels to design and engineer biomimetic tissues based on recent achievements in nano- and micro-technologies. In addition, current hydrogel-based regenerative engineering strategies for treating multiple tissues, such as musculoskeletal, nervous and cardiac tissue, are also covered in this review. The interaction of multiple disciplines including materials science, cell biology, and chemistry, will further play an important role in the design of functional hydrogels for the regeneration of complex tissues.

Reactive Oxygen Species Shielding Hydrogel for the Delivery of Adherent and Nonadherent Therapeutic Cell Types<sup/>

Cell therapies suffer from poor survival post-transplant due to placement into hostile implant sites characterized by host immune response and innate production of high levels of reactive oxygen species (ROS). We hypothesized that cellular encapsulation within an injectable, antioxidant hydrogel would improve viability of cells exposed to high oxidative stress. To test this hypothesis, we applied a dual thermo- and ROS-responsive hydrogel comprising the ABC triblock polymer poly[(propylene sulfide)-block-(N,N-dimethyl acrylamide)-block-(N-isopropylacrylamide)] (PPS₁₃₅-b-PDMA₁₅₂-b-PNIPAAM₂₂₅, PDN). The PPS chemistry reacts irreversibly with ROS such as hydrogen peroxide (H₂O₂), imparting inherent antioxidant properties to the system. Here, PDN hydrogels were successfully integrated with type 1 collagen to form ROS-protective, composite hydrogels amenable to spreading and growth of adherent cell types such as mesenchymal stem cells (MSCs). It was also shown that, using a control hydrogel substituting nonreactive polycaprolactone in place of PPS, the ROS-reactive PPS chemistry is directly responsible for PDN hydrogel cytoprotection of both MSCs and insulin-producing β-cell pseudo-islets against H₂O₂ toxicity. In sum, these results establish the potential of cytoprotective, thermogelling PDN biomaterials for injectable delivery of cell therapies.

Cellularizing hydrogel-based scaffolds to repair bone tissue: How to create a physiologically relevant micro-environment?

Tissue engineering is a promising alternative to autografts or allografts for the regeneration of large bone defects. Cell-free biomaterials with different degrees of sophistication can be used for several therapeutic indications, to stimulate bone repair by the host tissue. However, when osteoprogenitors are not available in the damaged tissue, exogenous cells with an osteoblast differentiation potential must be provided. These cells should have the capacity to colonize the defect and to participate in the building of new bone tissue. To achieve this goal, cells must survive, remain in the defect site, eventually proliferate, and differentiate into mature osteoblasts. A critical issue for these engrafted cells is to be fed by oxygen and nutrients: the transient absence of a vascular network upon implantation is a major challenge for cells to survive in the site of implantation, and different strategies can be followed to promote cell survival under poor oxygen and nutrient supply and to promote rapid vascularization of the defect area. These strategies involve the use of scaffolds designed to create the appropriate micro-environment for cells to survive, proliferate, and differentiate in vitro and in vivo. Hydrogels are an eclectic class of materials that can be easily cellularized and provide effective, minimally invasive approaches to fill bone defects and favor bone tissue regeneration. Furthermore, by playing on their composition and processing, it is possible to obtain biocompatible systems with adequate chemical, biological, and mechanical properties. However, only a good combination of scaffold and cells, possibly with the aid of incorporated growth factors, can lead to successful results in bone regeneration. This review presents the strategies used to design cellularized hydrogel-based systems for bone regeneration, identifying the key parameters of the many different micro-environments created within hydrogels.

Mechanical properties of PNIPAM based hydrogels: A review

Materials which adjust their properties in response to environmental factors such as temperature, pH and ionic strength are rapidly evolving and known as smart materials. Hydrogels formed by smart polymers have various applications. Among the smart polymers, thermoresponsive polymer poly(N-isopropylacrylamide)(PNIPAM) is very important because of its well defined structure and property specially its temperature response is closed to human body and can be finetuned as well. Mechanical properties are critical for the performance of stimuli responsive hydrogels in diverse applications. However, native PNIPAM hydrogels are very fragile and hardly useful for any practical purpose. Intense researches have been done in recent decade to enhance the mechanical features of PNIPAM hydrogel. In this review, several strategies including interpenetrating polymer network (IPN), double network (DN), nanocomposite (NC) and slide ring (SR) hydrogels are discussed in the context of PNIPAM hydrogel.