Mechanical properties of PNIPAM based hydrogels: A review

Toward Designing of Anti-infective Hydrogels for Orthopedic Implants: From Lab to Clinic

An alarming increase in implant failure incidence due to microbial colonization on the administered orthopedic implants has become a horrifying threat to replacement surgeries and related health concerns. In essence, microbial adhesion and its subsequent biofilm formation, antibiotic resistance, and the host immune system's deficiency are the main culprits. An advanced class of biomaterials termed anti-infective hydrogel implant coatings are evolving to subdue these complications. On this account, this review provides an insight into the significance of anti-infective hydrogels for preventing orthopedic implant associated infections to improve the bone healing process. We briefly discuss the clinical course of implant failure, with a prime focus on orthopedic implants. We identify the different anti-infective coating strategies and hence several anti-infective agents which could be incorporated in the hydrogel matrix. The fundamental design criteria to be considered while fabricating anti-infective hydrogels for orthopedic implants will be discussed. We highlight the different hydrogel coatings based on the origin of the polymers involved in light of their antimicrobial efficacy. We summarize the relevant patents reported in the prevention of implant infections, including orthopedics. Finally, the challenges concerning the clinical translation of the aforesaid hydrogels are described, and considerable solutions for improved clinical practice and better future prospects are proposed.

Role of mass effect and trehalose on early erythrolysis after experimental intracerebral hemorrhage

The mechanisms of brain injury after intracerebral hemorrhage (ICH) involve mass effect-induced primary injury and secondary injury caused by a pathologic response to the hematoma. Considerable attentions have recently been paid to the mechanisms and therapeutic strategy for secondary brain injury due to no overall benefit from early surgery compared with initial conservative treatment. However, it is unclear whether there is a causal relationship between mass effect and secondary brain injury. Here, the role of mass effect on early erythrolysis after experimental ICH was investigated based on the poly(N-isopropylacrylamide) (PNIPAM) ICH model. Autologous blood and PNIPAM hydrogel were co-injected into the right basal ganglia of rats to induce different degrees of mass effect, but with a constant hematoma. The influences of different mass effect and time courses on erythrolysis and brain damages after ICH were investigated. Furthermore, the protective effect of trehalose against erythrolysis after ICH was evaluated. The results showed that mass effect caused erythrocyte morphological change at 24 hr after ICH. The released hemoglobin was quantitatively evaluated by a polynomial concerning with the mass effect, the volume of hematoma, and the time of ICH. An obvious increase in heme oxygenase-1 (HO-1) and ionized calcium binding adaptor molecule-1 (Iba-1) expression, iron deposition, cell death, and neurological deficits was observed with increasing mass effect. Moreover, trehalose alleviated brain injury by inhibiting erythrolysis after ICH. These data demonstrated that mass effect accelerated the erythrolysis and brain damages after ICH, which could be relieved through trehalose therapy.

Equilibrium Swelling of Biocompatible Thermo-Responsive Copolymer Gels

Biomedical applications of thermo-responsive (TR) hydrogels require these materials to be biocompatible, non-cytotoxic, and non-immunogenic. Due to serious concerns regarding potential toxicity of poly(N-isopropylacrylamide) (PNIPAm), design of alternative homo- and copolymer gels with controllable swelling properties has recently become a hot topic. This study focuses on equilibrium swelling of five potential candidates to replace PNIPAm in biomedical and biotechnological applications: poly(N-vinylcaprolactam), poly(vinyl methyl ether), poly(N,N-dimethyl amino ethyl methacrylate), and two families of poly(2-oxazoline)s, and poly(oligo(ethylene glycol) methacrylates). To evaluate their water uptake properties and to compare them with those of substituted acrylamide gels, a unified model is developed for equilibrium swelling of TR copolymer gels with various types of swelling diagrams. Depending on the strength of hydrophobic interactions (high, intermediate, and low), the (co)polymers under consideration are split into three groups that reveal different responses at and above the volume phase transition temperature.

Stimuli-responsive nanoparticles based on poly acrylic derivatives for tumor therapy

Serve side effects caused by discriminate damage of chemotherapeutic drugs to normal cell and cancer cells remain a main obstacle in clinic. Hence, continuous efforts have been made to find ways to effectively enhance drug delivery and reduce side effects. Recent decades have witnessed impressive progresses in fighting against cancer, with improved understanding of tumor microenvironment and rapid development in nanoscale drug delivery system (DDS). Nanocarriers based on biocompatible materials provide possibilities to improve antitumor efficiency and minimize off-target effects. Among all kinds of biocompatible materials applied in DDS, polymeric acrylic derivatives such as poly(acrylamide), poly(acrylic acid), poly(N-isopropylacrylamide) present inherent biocompatibility and stimuli-responsivity, and relatively easy to be functionalized. Furthermore, nanocarrier based on polymeric acrylic derivatives have demonstrated high drug encapsulation, improved uptake efficiency, prolonged circulation time and satisfactory therapeutic outcome in tumor. In this review, we aim to discuss recent progress in design and development of stimulus-responsive poly acrylic polymer based nanocarriers for tumor targeting drug delivery.

4D Printable Tough and Thermoresponsive Hydrogels

Hydrogels with attractive stimuli-responsive volume changing abilities are seeing emerging applications as soft actuators and robots. However, many hydrogels are intrinsically soft and fragile for tolerating mechanical damage in real world applications and could not deliver high actuation force because of the mechanical weakness of the porous polymer network. Conventional tough hydrogels, fabricated by forming double networks, dual cross-linking, and compositing, could not satisfy both high toughness and high stimuli responsiveness. Herein, we present a material design of combining responsive and tough components in a single hydrogel network, which enables the synergistic realization of high toughness and actuation performance. We showcased this material design in an exemplary tough and thermally responsive hydrogel based on PVA/(PVA-MA)-g-PNIPAM, which achieved 100 times higher toughness (∼10 MJ/m³) and 20 times higher actuation stress (∼10 kPa) compared to conventional PNIPAM hydrogels, and a contraction ratio of up to 50% simultaneously. The effects of salt concentration, polymer ratio, and structural design on the mechanical and actuation properties have been systematically investigated. Utilizing 4D printing, actuators of various geometries were fabricated, as well as lattice-architected hydrogels with macro-voids, presenting 4 times faster actuation speed compared to bulk hydrogel, in addition to the high toughness, actuation force, and contraction ratio.

Rational Design of Smart Hydrogels for Biomedical Applications

Hydrogels are polymeric three-dimensional network structures with high water content. Due to their superior biocompatibility and low toxicity, hydrogels play a significant role in the biomedical fields. Hydrogels are categorized by the composition from natural polymers to synthetic polymers. To meet the complicated situation in the biomedical applications, suitable host–guest supramolecular interactions are rationally selected. This review will have an introduction of hydrogel classification based on the formulation molecules, and then a discussion over the rational design of the intelligent hydrogel to the environmental stimuli such as temperature, irradiation, pH, and targeted biomolecules. Further, the applications of rationally designed smart hydrogels in the biomedical field will be presented, such as tissue repair, drug delivery, and cancer therapy. Finally, the perspectives and the challenges of smart hydrogels will be outlined.

A dendronised polymer architecture breaks the conventional inverse relationship between porosity and mechanical properties of hydrogels

We present a series of synthetic polymer hydrogels which break the traditional correlation between pore size and mechanical properties. The hydrogels are prepared from a dendronised polymer architecture based on a methacrylate copolymer to which poly(amido amine) dendrons are attached. Our approach will be useful in tailoring hydrogels for tissue engineering, controlled drug release, and flexible electronics.

Injectable thermoresponsive hydrogels as drug delivery system for the treatment of central nervous system disorders: A review

The central nervous system (CNS), consisting of the brain, spinal cord, and retina, superintends to the acquisition, integration and processing of peripheral information to properly coordinate the activities of the whole body. Neurodegenerative and neurodevelopmental disorders, trauma, stroke, and brain tumors can dramatically affect CNS functions resulting in serious and life-long disabilities. Globally, the societal and economic burden associated with CNS disorders continues to grow with the ageing of the population thus demanding for more effective and definitive treatments. Despite the variety of clinically available therapeutic molecules, medical interventions on CNS disorders are mostly limited to treat symptoms rather than halting or reversing disease progression. This is attributed to the complexity of the underlying disease mechanisms as well as to the unique biological microenvironment. Given its central importance, multiple barriers, including the blood brain barrier and the blood cerebrospinal fluid barrier, protect the CNS from external agents. This limits the access of drug molecules to the CNS thus contributing to the modest therapeutic successes. Loco-regional therapies based on the deposition of thermoresponsive hydrogels loaded with therapeutic agents and cells are receiving much attention as an alternative and potentially more effective approach to manage CNS disorders. In this work, the current understanding and challenges in the design of thermoresponsive hydrogels for CNS therapy are reviewed. First, the biological barriers that hinder mass and drug transport to the CNS are described, highlighting the distinct features of each barrier. Then, the realization, characterization and biomedical application of natural and synthetic thermoresponsive hydrogels are critically presented. Advantages and limitations of each design and application are discussed with the objective of identifying general rules that could enhance the effective translation of thermoresponsive hydrogel-based therapies for the treatment of CNS disorders.

Temperature and Photo Dual-Stimuli Responsive Block Copolymer Self-Assembly Micelles for Cellular Controlled Drug Release

To well adapt to the complicated physiological environments, it is necessary to engineer dual- and/or multi-stimuli responsive drug carriers for more effective drug release. For this, a novel temperature responsive lateral chain photosensitive block copolymer, poly[(N-isopropylacrylamide-co-N,N-dimethylacrylamide) -block-propyleneacylalkyl-4-azobenzoate] (P(NIPAM-co-DMAA)-b-PAzoHPA), is synthesized by atom transfer radical polymerization. The structure is characterized by ¹ H nuclear magnetic resonance spectrometry and laser light scattering gel chromatography system. The self-assembly behavior, morphology, and sizes of micelles are investigated by fluorescence spectroscopy, transmission electron microscope, and laser particle analyzer. Dual responsiveness to light and temperature is explored by ultraviolet-visible absorption spectroscopy. The results show that the copolymer micelles take on apparent light and temperature dual responsiveness, and its lower critical solution temperature (LCST) is above 37 °C, and changes with the trans-/cis- isomerization of azobenzene structure under UV irradiation. The blank copolymers are nontoxic, whereas the paclitaxel (PTX)-loaded counterparts possessed comparable anticancer activities to free PTX, with entrapment efficiency of 83.7%. The PTX release from the PTX-loaded micelles can be mediated by changing temperature and/or light stimuli. The developed block copolymers can potentially be used for cancer therapy as drug controlled release carriers.

Therapeutic effect of the injectable thermosensitive hydrogel loaded with SHP099 on intervertebral disc degeneration

AIMS

Intervertebral disc (IVD) degeneration (IDD), a common musculoskeletal disease with limited self-healing ability, is challenging to treat. The development of innovative therapies to reverse IDD depends on the elucidation of its regulatory mechanisms. Therefore, the role of Src homology region 2-containing protein tyrosine phosphatase 2 (SHP2) in the pathogenesis of IDD and the therapeutic effect of its small-molecule inhibitor, SHP099, were investigated.

MATERIALS AND METHODS

The expression of SHP2 by nucleus pulposus (NP) cells in IVD was investigated in vitro and in vivo, and its molecular mechanism in IDD was explored using transfection technology. Injectable N-isopropylacrylamide-based thermosensitive hydrogels were synthesized for SHP099 delivery.

KEY FINDINGS

SHP2 was highly expressed in degenerated IVDs, where its overexpression in NP cells inhibited the expression of Sry-related HMG box-9 (Sox9), leading to the decreased expression of key proteins (collagen II and aggrecan) and consequently to IDD. SHP099 reversed the degeneration of NP cells in vitro. Moreover, its administration in rats via the injectable thermosensitive hydrogel had a therapeutic effect on IDD.

SIGNIFICANCE

Our results suggest that SHP2 is a key factor in IDD progression, and SHP099 inhibits both its expression and NP cell degeneration. Therefore, SHP099 delivery via injectable thermosensitive hydrogels is a potential treatment strategy for IDD.

Multifunctional Platform Based on Electrospun Nanofibers and Plasmonic Hydrogel: A Smart Nanostructured Pillow for Near-Infrared Light-Driven Biomedical Applications

Multifunctional nanomaterials with the ability to respond to near-infrared (NIR) light stimulation are vital for the development of highly efficient biomedical nanoplatforms with a polytherapeutic approach. Inspired by the mesoglea structure of jellyfish bells, a biomimetic multifunctional nanostructured pillow with fast photothermal responsiveness for NIR light-controlled on-demand drug delivery is developed. We fabricate a nanoplatform with several hierarchical levels designed to generate a series of controlled, rapid, and reversible cascade-like structural changes upon NIR light irradiation. The mechanical contraction of the nanostructured platform, resulting from the increase of temperature to 42 °C due to plasmonic hydrogel-light interaction, causes a rapid expulsion of water from the inner structure, passing through an electrospun membrane anchored onto the hydrogel core. The mutual effects of the rise in temperature and water flow stimulate the release of molecules from the nanofibers. To expand the potential applications of the biomimetic platform, the photothermal responsiveness to reach the typical temperature level for performing photothermal therapy (PTT) is designed. The on-demand drug model penetration into pig tissue demonstrates the efficiency of the nanostructured platform in the rapid and controlled release of molecules, while the high biocompatibility confirms the pillow potential for biomedical applications based on the NIR light-driven multitherapy strategy.

Ion-Conducting Thermoresponsive Films Based on Polymer-Grafted Cellulose Nanocrystals

Mechanically robust, thermoresponsive, ion-conducting nanocomposite films are prepared from poly(2-phenylethyl methacrylate)-grafted cellulose nanocrystals (MxG-CNC-g-PPMA). One-component nanocomposite films of the polymer-grafted nanoparticle (PGN) MxG-CNC-g-PPMA are imbibed with 30 wt % imidazolium-based ionic liquid to produce flexible ion-conducting films. These films with 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (MxG-CNC-g-PPMA/[H]) not only display remarkable improvements in toughness (>25 times) and tensile strength (>70 times) relative to the corresponding films consisting of the ionic liquid imbibed in the two-component CNC/PPMA nanocomposite but also show higher ionic conductivity than the corresponding neat PPMA with the same weight percent of ionic liquid. Notably, the one-component film containing 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (MxG-CNC-g-PPMA/[E]) exhibits temperature-responsive ionic conduction. The ionic conductivity decreases at around 60 °C as a consequence of the lower critical solution temperature phase transition of the grafted polymer in the ionic liquid, which leads to phase separation. Moreover, holding the MxG-CNC-g-PPMA/[E] film at room temperature for 24 h returns the film to its original homogenous state. These materials exhibit properties relevant to thermal cutoff safety devices (e.g., thermal fuse) where a reduction in conductivity above a critical temperature is needed.

Application of stimuli-responsive materials for extraction purposes

Stimuli-responsive materials, frequently designated as "smart/intelligent materials", can modify their structure or properties by either a biological, physical, or chemical stimulus which, if properly controlled, could be used for specific applications. Such materials have been studied and exploited in several fields, like electronics, photonics, controlled drugs administration, imaging and medical diagnosis, among others, as well as in Analytical Chemistry where they have been used as chromatographic stationary phases, as part of sensors and for extraction purposes. This review article pretends to provide an overview of the most recent applications of these materials (mostly polymeric materials) in sample preparation for extraction purposes, as well as to provide a general vision of the current state-of-the-art of this field, their potential use and future applications.

Rational design of hydrogels to enhance osteogenic potential

Bone tissue engineering (BTE) encompasses the field of biomaterials, cells, and bioactive molecules to successfully guide the growth and repair of bone tissue. Current BTE strategies rely on delivering osteogenic molecules or cells via scaffolding materials. However, growth factor- and stem cell-based treatments have several limitations, such as source restriction, low stability, difficulties in predicting long-term efficacy, and high costs, among others. These issues have promoted the development of material-based therapy with properties of accessibility, high stability, tunable efficacy, and low-cost production. Hydrogels are widely used in BTE applications because of their unique hydrophilic nature and tunable physicochemical properties to mimic the native bone environment. However, current hydrogel materials are not ideal candidates due to minimal osteogenic capability on their own. Therefore, recent studies of BTE hydrogels attempt to counterbalance these issues by modifying their biophysical properties. In this article, we review recent progress in the design of hydrogels to instruct osteogenic potential, and present strategies developed to precisely control its bone healing properties.

Equilibrium swelling of thermo-responsive copolymer microgels

Thermo-responsive (TR) hydrogels with a lower critical solution temperature swell strongly at temperatures below their volume phase transition temperature T_c and collapse above T_c. Biomedical application of these materials requires tuning the critical temperature in a rather wide interval. A facile method for modulation of T_c is to polymerize the basic monomers with hydrophilic or hydrophobic comonomers. Although the effectiveness of this method has been confirmed by experimental data, molar fractions of comonomers necessary for fine tuning of T_c in macroscopic gels and microgels are unknown. A simple model is developed for the equilibrium swelling of TR copolymer gels. Its adjustable parameters are found by fitting swelling diagrams on several macro- and microgels with N-isopropylacrylamide as a basic monomer. Good agreement is demonstrated between the experimental swelling curves and results of numerical analysis. An explicit expression is derived for the volume phase transition temperature as a function of molar fraction of comonomers. The ability of this relation to predict the critical temperature is confirmed by comparison with observations.

A model is developed for equilibrium swelling of thermo-responsive copolymer gels and is applied to predict the effect of molar fraction of comonomers on the volume phase transition temperature of macroscopic gels and microgels.

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.

Ibuprofen-Loaded Heparin Modified Thermosensitive Hydrogel for Inhibiting Excessive Inflammation and Promoting Wound Healing

Hydrogels have been investigated as ideal biomaterials for wound treatment owing to their ability to form a highly moist environment which accelerates cell migration and tissue regeneration for prompt wound healing. They can also be used as a drug carrier for local delivery, and are able to activate immune cells to enhance wound healing. Here, we developed heparin-conjugated poly(N-isopropylacrylamide), an injectable, in situ gel-forming polymer, and evaluated its use in wound healing. Ibuprofen was encapsulated into the hydrogel to help reduce pain and excessive inflammation during healing. In addition to in vitro studies, a BALB/c mice model was used to evaluate its effect on would healing and the secretion of inflammatory mediators. The in vitro assay confirmed that the ibuprofen released from the hydrogel dramatically reduced lipopolysaccharide-induced inflammation by suppressing the production of NO, PGE2 and TNF-α in RAW264.7 macrophages. Moreover, an in vivo wound healing assay was conducted by applying hydrogels to wounds on the backs of mice. The results showed that the ibuprofen-loaded hydrogel improved healing relative to the phosphate buffered saline group. This study indicates that ibuprofen loaded in an injectable hydrogel is a promising candidate for wound healing therapy.

Dual drug delivery system based on pH-sensitive silk fibroin/alginate nanoparticles entrapped in PNIPAM hydrogel for treating severe infected burn wound

Herein, the pH-sensitive vancomycin (VANCO) loaded silk fibroin-sodium alginate nanoparticles (NPs) embedded in poly(N-isopropylacrylamide) (PNIPAM) hydrogel containing epidermal growth factor (EGF) are introduced for treating chronic burn wound infections. The hybrid system was developed to control the release rates of an antibiotic and growth factor for optimal treatment of burn infections. VANCO had a pH responsive release behavior from the nanoparticle (NP) and showed higher release rate in an alkaline pH compared to the neutral pH during 10 d. About 30% of EGF was also released from the hydrogel within 20 d. The released VANCO and EGF preserved their bioactivity more than ∼ 80%. The suitable physico-chemical properties and cellular behaviors of PNIPAM hydrogel supported the proliferation and growth of the fibroblast cells. Furthermore, the higher re-epithelialization with good wound contraction rate, neovascular formation, and expression of transforming growth factor-beta were observed in S. aureus infected rat burn wound by using the hydrogel containing VANCO and EGF compared with untreated wounds and hydrogel alone. The wound infection was also significantly reduced in the groups treated with the hydrogels containing VANCO. Overall, in vitro and in vivo results suggested that developed hybrid system would be a promising construct to treat severe wound infection.

Stereocomplexation of Poly(Lactic acid) and Chemical Crosslinking of Ethylene Glycol Dimethacrylate (EGDMA) Double-Crosslinked Temperature/pH Dual Responsive Hydrogels

Physical crosslinking and chemical crosslinking were used to further improve the mechanical properties and stability of the gel. A temperature/pH dual sensitive and double-crosslinked gel was prepared by the stereo-complex of HEMA-PLLA20 and HEMA-PDLA20 as a physical crosslinking agent, ethylene glycol dimethacrylate (EGDMA) as a chemical crosslinking agent, and azodiisobutyronitrile (AIBN) as an initiator for free radical polymerization. This paper focused on the performance comparison of chemical crosslinked gel, a physical crosslinked gel, and a dual crosslinked gel. The water absorption, temperature, and pH sensitivity of the three hydrogels were studied by a scanning electron microscope (SEM) and swelling performance research. We used a thermal analysis system (TGA) and dynamic viscoelastic spectrometer to study thermal properties and mechanical properties of these gels. Lastly, the in vitro drug release behavior of double-crosslinked hydrogel loaded with doxorubicin under different conditions was studied. The results show that the double-crosslinked and temperature/pH dual responsive hydrogels has great mechanical properties and good stability.

Cutting-edge progress and challenges in stimuli responsive hydrogel microenvironment for success in tissue engineering today

The field of tissue engineering has numerous potential for modified therapeutic results and has been inspired by enhancements in bioengineering at the recent decades. The techniques of regenerating tissues and assembling functional paradigms that are responsible for repairing, maintaining, and revitalizing lost organs and tissues have affected the entire spectrum of health care studies. Strategies to combine bioactive molecules, biocompatible materials and cells are important for progressing the renewal of damaged tissues. Hydrogels have been utilized as one of the most popular cell substrate/carrier in tissue engineering since previous decades, respect to their potential to retain a 3D structure, to protect the embedded cells, and to mimic the native ECM. The hydrophilic nature of hydrogels can provide an ideal milieu for cell viability and structure, which simulate the native tissues. Hydrogel systems have been applied as a favorable matrix for growth factor delivery and cell immobilization. This study reviews a brief explanation of the structure, characters, applications, fabrication methods, and future outlooks of stimuli responsive hydrogels in tissue engineering and, in particular, 3D bioprinting.

Synthesis, characterization and in vitro cytotoxicity studies of poly-N-isopropyl acrylamide gel nanoparticles and films

In this work, we show synthesis that leads to thermoreponsive poly-N-isopropyl acrylamide (pNIPAM) nanogels with sizes below 100 nm, irrespectively of the surfactant to crosslinker ratio. We also show that in many environments the temperature induced pNIPAM collapse at Lower Critical Solution Temperature (LCST) of 32.5 °C is accompanied by gel nanoparticles' aggregation. Thus, the proper information on the nanoparticle (NP) structure and deswelling can be obtained only if the routinely measured hydrodynamic radius is supplemented by information on the molecular weight, which can be obtained from the intensity of scattered light. We measured the dynamics and reversibility of the deswelling and subsequent aggregation processes. Furthermore, we show that the highly concentrated pNIPAM gel NPs reversibly form bulk hydrogel networks of varied interconnected porous structure. We show, that in case of drying pNIPAM gel NPs above the LCST, it is possible to obtain films with 20-fold increase in storage modulus (G') compared to hydrogel networks measured at room temperature. They exhibit temperature hysteresis behavior around LCST of 32.5 °C similar to pNIPAM films. Finally, we show that these hydrogel films, lead to extended proliferation of cells across three different types: fibroblast, endothelial and cancer cells. Additionally, none of the films exhibited any cytotoxic effects. Our study brings new insights into physicochemical characterization of pNIPAM gel NPs and networks behavior in realistic conditions of in vitro measurements, especially by means of dynamic light scattering as well as final unique properties of both gel NPs and formed porous films for possible tissue engineering applications.

Design of Bio-Conjugated Hydrogels for Regenerative Medicine Applications: From Polymer Scaffold to Biomolecule Choice

Bio-conjugated hydrogels merge the functionality of a synthetic network with the activity of a biomolecule, becoming thus an interesting class of materials for a variety of biomedical applications. This combination allows the fine tuning of their functionality and activity, whilst retaining biocompatibility, responsivity and displaying tunable chemical and mechanical properties. A complex scenario of molecular factors and conditions have to be taken into account to ensure the correct functionality of the bio-hydrogel as a scaffold or a delivery system, including the polymer backbone and biomolecule choice, polymerization conditions, architecture and biocompatibility. In this review, we present these key factors and conditions that have to match together to ensure the correct functionality of the bio-conjugated hydrogel. We then present recent examples of bio-conjugated hydrogel systems paving the way for regenerative medicine applications.

A programmable epidermal microfluidic valving system for wearable biofluid management and contextual biomarker analysis

Active biofluid management is central to the realization of wearable bioanalytical platforms that are poised to autonomously provide frequent, real-time, and accurate measures of biomarkers in epidermally-retrievable biofluids (e.g., sweat). Accordingly, here, a programmable epidermal microfluidic valving system is devised, which is capable of biofluid sampling, routing, and compartmentalization for biomarker analysis. At its core, the system is a network of individually-addressable microheater-controlled thermo-responsive hydrogel valves, augmented with a pressure regulation mechanism to accommodate pressure built-up, when interfacing sweat glands. The active biofluid control achieved by this system is harnessed to create unprecedented wearable bioanalytical capabilities at both the sensor level (decoupling the confounding influence of flow rate variability on sensor response) and the system level (facilitating context-based sensor selection/protection). Through integration with a wireless flexible printed circuit board and seamless bilateral communication with consumer electronics (e.g., smartwatch), contextually-relevant (scheduled/on-demand) on-body biomarker data acquisition/display was achieved.

Wearable biosensors have been used successfully for biomarker analysis, however, a lack of control over sampling limits applications. Here, the authors report a programmable microfluidic valve to control flow rate, sampling times and allow for biofluid routing and compartmentalisation.

Injectable hydrogels for tendon and ligament tissue engineering

The problem of tendon and ligament (T/L) regeneration in musculoskeletal diseases has long constituted a major challenge. In situ injection of formable biodegradable hydrogels, however, has been demonstrated to treat T/L injury and reduce patient suffering in a minimally invasive manner. An injectable hydrogel is more suitable than other biological materials due to the special physiological structure of T/L. Most other materials utilized to repair T/L are cell-based, growth factor-based materials, with few material properties. In addition, the mechanical property of the gel cannot reach the normal T/L level. This review summarizes advances in natural and synthetic polymeric injectable hydrogels for tissue engineering in T/L and presents prospects for injectable and biodegradable hydrogels for its treatment. In future T/L applications, it is necessary develop an injectable hydrogel with mechanics, tissue damage-specific binding, and disease response. Simultaneously, the advantages of various biological materials must be combined in order to achieve personalized precision therapy.

Responsive polymers for medical diagnostics

Stimulus-responsive polymers have been used in improving the efficacy of medical diagnostics through different approaches including enhancing the contrast in imaging techniques and promoting the molecular recognition in diagnostic assays. This review overviews the mechanisms of stimulus-responsive polymers in response to external stimuli including temperature, pH, ion, light, etc. The applications of responsive polymers in magnetic resonance imaging, capture and purification of biomolecules through protein-ligand recognition and lab-on-a-chip technology are discussed.

Intravitreal hydrogels for sustained release of therapeutic proteins

This review highlights how hydrogel formulations can improve intravitreal protein delivery to the posterior segment of the eye in order to increase therapeutic outcome and patient compliance. Several therapeutic proteins have shown excellent clinical successes for the treatment of various intraocular diseases. However, drug delivery to the posterior segment of the eye faces significant challenges due to multiple physiological barriers preventing drugs from reaching the retina, among which intravitreal protein instability and rapid clearance from the site of injection. Hence, frequent injections are required to maintain therapeutic levels. Moreover, because the world population ages, the number of patients suffering from ocular diseases, such as age-related macular degeneration (AMD) and diabetic retinopathy (DR) is increasing and causing increased health care costs. Therefore, there is a growing need for suitable delivery systems able to tackle the current limitations in retinal protein delivery, which also may reduce costs. Hydrogels have shown to be promising delivery systems capable of sustaining release of therapeutic proteins and thus extending their local presence. Here, an extensive overview of preclinically developed intravitreal hydrogels is provided with attention to the rational design of clinically useful intravitreal systems. The currently used polymers, crosslinking mechanisms, in vitro/in vivo models and advancements are discussed together with the limitations and future perspective of these biomaterials.

Robust Multiscale-Oriented Thermoresponsive Fibrous Hydrogels with Rapid Self-Recovery and Ultrafast Response Underwater

Hydrogels with ultrafast response to environmental stimuli, possessing robust structural integrity and rapid self-recovery, have been considered as promising platforms for numerous applications, for example, in biomimetic materials and nanomedicine. Inspired by the bundled fibrous structure of actin, we developed a robust and ultrafast thermoresponsive fibrous hydrogel (TFH) by fully utilizing the weak noncovalent bonds and strong covalently cross-linked semiflexible electrospun fibrous nets. The TFH exhibits an ultrafast response (within 10 s), rapid self-recovery rate (74% within 10 s), tunable tensile strength (3-380 kPa), and high toughness (∼1560 J/m²) toward temperature. A multiscale orientation is considered to play a key role in the excellent mechanical properties at the fibrous mesh, fiber, and molecular scales. Furthermore, to take advantage of this TFH adequately, a novel kind of noodle-like hydrogel for thermo-controlled protein sorption based on the TFH is prepared, which exhibits high stability and ultrafast sorption properties. The bioinspired platforms hold promise as artificial skins and "smart" sorption membrane carriers, which provide a unique bioactive environment for tissue engineering and nanomedicine.

Design Rationale for Stimuli-Responsive, Semi-interpenetrating Polymer Network Hydrogels-A Quantitative Approach

Stimuli-responsive semi-interpenetrating polymer network (semi-IPN) hydrogels form an important class of polymers for their tunable properties via molecular design. They are widely investigated for a diverse range of applications including drug delivery, sensors, actuators, and osmotic agents. However, in-depth studies on some of the critical design principles affecting diffusion/leaching of linear polymer from semi-IPN hydrogels are lacking. Herein, for the first time, by preparing a series of model semi-IPN hydrogels based on thermally responsive poly (N-isopropyl acrylamide) (PNIPAM) network and linear poly(sodium acrylate) (PSA), a systematic and quantitative study concerning linear polymer chain retention and swelling/deswelling kinetics is reported. The study shows that PSA retention is significantly affected not only by PSA molecular weight and concentration, but also by polymerization temperature, which could be linked to homogeneity and internal morphology of the hydrogel. Surprisingly, there is no obvious influence of crosslinking density of PNIPAM network toward PSA retention, while faster swelling and deswelling at higher crosslinking density are observed in terms of swelling rate constant and deswelling activation energy. These findings offer new insights on the factors affecting structural and physicochemical properties of such semi-IPN hydrogels, which should in turn serve as a general guideline for materials design.

Azobenzene-Based Ionic Liquid Switches Phase Separation of Poly(N-isopropylacrylamide) Aqueous Solutions as a Molecular Trigger, Leading to UV Shutdown of Ionic Transport

This paper presents a light-switchable phase separation of poly(N-isopropylacrylamide) (PNIPAm) in aqueous solutions using an azobenzene-based ionic liquid as a molecular trigger. A different UV response was obtained with use of an azobenzene molecular trigger. A UV-induced aggregation of PNIPAm was observed in the current system, which is opposite to that reported for azobenzene-functionalized PNIPAm systems. According to in situ FT-IR, cis-isomers destabilized the hydrogen bonding between water and PNIPAm, contributing to a light-induced aggregation of PNIPAm. This polarity effect was quite similar to salting effects of hofmeister anions on clouding points. Furthermore, a UV shutdown of ion transport of the azobenzene-based ionic liquid was demonstrated with a significant reduction factor of 0.03 at ambient temperature, which can be potentially applied in biotechnology, optoelectronics, and ionic circuits.

Thermal responsive poly(N-isopropylacrylamide) grafted chicken feather keratin prepared via surface initiated aqueous Cu(0)-mediated RDRP: Synthesis and properties

Poultry chicken feather keratin was extracted and then modified for the fabrication of keratin-graft-PNIPAM copolymers. The keratin was well extracted from feather fiber and powdered. Subsequently, it underwent the surficial functionalization process with initiator groups. After the study conducted full disproportionation of Cu(I)Br/Me₆Tren into Cu(0) and Cu(II)Br₂ in the solvent, surface initiated aqueous Cu(0)-mediated reversible-deactivation radical polymerization (RDRP) of N-isopropylacrylamide (NIPAM) was performed in a methanol/water mixture solvent. The reaction was performed rapidly and efficiently, during which over 100% graft rate was achieved at 60 min. After 6 h reaction, 200% graft rate could be achieved. High graft rate (up to 287%) was achieved, and graft rate could be regulated by controlling the reaction time and the addition of monomer. The fabricated keratin-g-PNIPAM exhibited a rough surface. As revealed from the results of thermal analysis, the thermal stability of keratin-g-PNIPAM was enhanced noticeably compared with the original keratin. Besides, grafted PNIPAM chains exhibited a higher glass transition temperature. The grafted keratin particles displayed enhanced hydrophilicity. Keratin-g-PNIPAMs exhibit a lower LCST comparing to homopolymer and the flocculation in hot water behavior could be controlled by regulating graft rate.

Formulation and Characterization of Gelatin-Based Hydrogels for the Encapsulation of Kluyveromyces lactis-Applications in Packed-Bed Reactors and Probiotics Delivery in Humans

One of the main issues when orally administering microorganism-based probiotics is the significant loss of bioactivity as they pass through the gastrointestinal (GI) tract. To overcome these issues, here, we propose to encapsulate the probiotic yeast Kluyveromyces lactis on chemically crosslinked gelatin hydrogels as a means to protect the bioactive agents in different environments. Hydrogels were prepared by the chemical crosslinking of gelatin, which is commercially available and inexpensive. This is crucial to ensure scalability and cost-effectiveness. To explore changes in key physicochemical parameters and their impact on cell viability, we varied the concentration of the crosslinking agent (glutaraldehyde) and the gelatin. The synthesized hydrogels were characterized in terms of morphological, physical-chemical, mechanical, thermal and rheological properties. This comprehensive characterization allowed us to identify critical parameters to facilitate encapsulation and enhance cell survival. Mainly due to pore size in the range of 5-10 μm, sufficient rigidity (breaking forces of about 1 N), low brittleness and structural stability under swelling and relatively high shear conditions, we selected hydrogels with a high concentration of gelatin (7.5% (w/v)) and concentrations of the crosslinking agent of 3.0% and 5.0% (w/w) for cell encapsulation. Yeasts were encapsulated with an efficiency of about 10% and subsequently tested in bioreactor operation and GI tract simulated media, thereby leading to cell viability levels that approached 95% and 50%, respectively. After testing, the hydrogels' firmness was only reduced to half of the initial value and maintained resistance to shear even under extreme pH conditions. The mechanisms underlying the observed mechanical response will require further investigation. These encouraging results, added to the superior structural stability after the treatments, indicate that the proposed encapsulates are suitable to overcome most of the major issues of oral administration of probiotics and open the possibility to explore additional biotech applications further.

Synthesis and Evaluation of a Thermoresponsive Degradable Chitosan-Grafted PNIPAAm Hydrogel as a "Smart" Gene Delivery System

Thermoresponsive hydrogels demonstrate tremendous potential as sustained drug delivery systems. However, progress has been limited as formulation of a stable biodegradable thermosensitive hydrogel remains a significant challenge. In this study, free radical polymerization was exploited to formulate a biodegradable thermosensitive hydrogel characterized by sustained drug release. Highly deacetylated chitosan and N-isopropylacrylamide with distinctive physical properties were employed to achieve a stable, hydrogel network at body temperature. The percentage of chitosan was altered within the copolymer formulations and the subsequent physical properties were characterized using ¹H-NMR, FTIR, and TGA. Viscoelastic, swelling, and degradation properties were also interrogated. The thermoresponsive hydrogels were loaded with RALA/pEGFP-N1 nanoparticles and release was examined. There was sustained release of nanoparticles over three weeks and, more importantly, the nucleic acid cargo remained functional and this was confirmed by successful transfection of the NCTC-929 fibroblast cell line. This tailored thermoresponsive hydrogel offers an option for sustained delivery of macromolecules over a prolonged considerable period.

Formulation of Sugar/Hydrogel Inks for Rapid Thermal Response 4D Architectures with Sugar-derived Macropores

Programmed, reshaping hydrogel architectures were fabricated from sugar/hydrogel inks via a three-dimensional printing method involving a stimuli-responsive polymer. We developed a new hydrogel ink composed of monomers (acrylamide [AAm]) and N-isopropylacrylamide [NIPAAm]), and sugar (mixture of glucose and sucrose) as a pore-generator, enabling to improve printability by increasing the ink's viscoelastic properties and induce the formation of macropores in the hydrogel architectures. This study demonstrated that creating macropores in such architectures enables rapid responses to stimuli that can facilitate four-dimensional printing. We printed bilayer structures from monomer inks to which we had added sugar, and we exposed them to processes that cross-linked the monomers and leached out the sugar to create macropores. In comparison with a conventional poly(N-isopropylacrylamide) hydrogel, the macroporous hydrogels prepared using polymerization in the presence of a high concentration of sugar showed higher swelling ratios and exhibited much faster response rates to temperature changes. We used rheometry and scanning electron microscopy to characterize the properties of these inks and hydrogels. The results suggest that this method may provide a readily available route to the rapid design and fabrication of shape-morphing hydrogel architectures with potential application in soft robotics, hydrogel actuators, and tissue engineering.