The potential of poly(N-isopropylacrylamide) (PNIPAM)-grafted hyaluronan and PNIPAM-grafted gelatin in the control of post-surgical tissue adhesions

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.

Biomimetic and Multifunctional Hemostatic Hydrogel with Rapid Thermoresponsive Gelation and Robust Wet Adhesion for Emergency Hemostasis: A Rational Design Based on Photo-Cross-Linking Coordinated Hydrophilic-Hydrophobic Balance Strategies

Uncontrolled bleeding in emergency situations is a great threat to both military and civilian lives, and an ideal hemostat for effectively controlling prehospital hemorrhage is urgently needed but still lacking. Although hemostatic hydrogels are promising for emergency hemostasis, they are currently challenged by either the mutual exclusion between a short gelation time and strong adhesive network or the insufficient functionality of ingredients and complicated operations for in situ curing. Herein, an extracellular matrix biopolymer-based and multifunctional hemostatic hydrogel that simultaneously integrates rapid thermoresponsive gelation, robust wet adhesion, and ease of use in emergencies is rationally engineered. This hydrogel can be conveniently used via simple injection and achieves instant sol-gel phase transition at body temperature. Its comprehensive performance could be facilely regulated by tuning the proportions of components, and the optimal performance (gelation time 6-8 s, adhesion strength 125 ± 3.6 kPa, burst pressure 282 ± 4.1 mmHg) is established due to the coordinated enhancement of the photo-cross-linking pretreatment and the hydrophilic-hydrophobic balance among various interactions in the hydrogel system. Additionally, it exhibits significant coagulation effect in vitro and enables effective hemostasis and wound healing in vivo. This work provides a promising platform for versatile applications of hydrogel-based materials, including emergency hemostasis.

Investigation of the Effect of Molecular Weight, Density, and Initiator Structure Size on the Repulsive Force between a PNIPAM Polymer Brush and Protein

This paper focuses on the effect of degree of polymerization (N), density ( $\sigma$ ), and pattern size ( $x$ ) on the interaction force between a periodically patterned Poly(N-isopropylacrylamide) (PNIPAM) brush and protein. The hydrophobic interaction, the Van der Waals attractive force, and the steric repulsive force were expressed in terms of $N$ , $\sigma$ , and $x$ . The osmotic constant (k1) and the entropic constant (k2) were determined from the fit of the steric repulsive force to an experimentally obtained force distance curve. The osmotic constant was 0.105, and the entropic constant was 0.255. Using these constants, the steric repulsive force was plotted as a function of the separation distance(s) between the substrate and the protein. The forces were determined at a separation distance equal to 0.3 nm, where L0 is the equilibrium thickness of the PNIPAM brush. At this separation distance, the value of the steric repulsive force was much higher than the value of the sum of the hydrophobic interaction and the Van der Waals attractive force for large degree of polymerization ( $N>100$ ) and density ( $\sigma >0.2$ chains/nm2). However, the repulsive force was comparable to the sum of the hydrophobic interaction and the Van der Waals attractive force for a small degree of polymerization ( $N<100$ ) and density ( $\sigma =0.2$ ). Furthermore, the steric repulsive force was plotted as a function of pattern size $x$ . The plot indicated that the steric repulsive force becomes nearly zero for all degrees of polymerization and density when the value of the initiator structure size was less than 200 nm. In addition to the steric repulsive force, the lateral extension of the chains in the periodically patterned PNIPAM brush was calculated by scaling low and compared with the experimental data taken from previously published literatures. The polymer brush structure was modelled as if the immediate bare substrate is so wide that even a stretched polymer segment cannot reach to the next polymer brush structure. In such models, the value of the lateral extension was equal to the thickness of the homogenous brush. It was independent of the pattern size. However, when the polymer brush structure was modelled as if there is another polymer brush structure at a distance half of the size of the period, the lateral extension was found to be dependent on the size of the initiator structure size due to chain bridging. This was witnessed by the patterning of polymer brushes using the interferometric patterning of PNIPAM brushes and an atomic force microscopy imaging of the polymer brush structures both in air and in water. The polymer brush structure resolution in water was much lower than the resolution in air, which indicates the lateral extension of the polymer chains in water. For such kind of periodic polymer brush structures, the gap between them was calculated, and it was found dependent on the degree of polymerization, density, and initiator structure size.

In Vivo Biocompatible Self-Assembled Nanogel Based on Hyaluronic Acid for Aqueous Solubility and Stability Enhancement of Asiatic Acid

Asiatic acid (AA), a natural triterpene found in Centalla asiatica, possesses polypharmacological properties that can contribute to the treatment and prophylaxis of various diseases. However, its hydrophobic nature and rapid metabolic rate lead to poor bioavailability. The aim of this research was to develop a thermoresponsive nanogel from hyaluronic acid (HA) for solubility and stability enhancement of AA. Poly(N-isopropylacrylamide) (pNIPAM) was conjugated onto HA using a carbodiimide reaction followed by ¹H NMR characterization. pNIPAM-grafted HA (HA-pNIPAM) nanogels were prepared with three concentrations of polymer, 0.1, 0.15 and 0.25% w/v, in water by the sonication method. AA was loaded into the nanogel by the incubation method. Size, morphology, AA loading capacity and encapsulation efficiency (EE) were analyzed. In vitro cytocompatibility was evaluated in fibroblast L-929 cells using the PrestoBlue assay. Single-dose toxicity was studied using rats. HA-pNIPAM nanogels at a 4.88% grafting degree showed reversible thermo-responsive behavior. All nanogel formulations could significantly increase AA water solubility and the stability was higher in nanogels prepared with high polymer concentrations over 180 days. The cell culture study showed that 12.5 µM AA in nanogel formulations was considered non-toxic to the L-929 cells; however, a dose-dependent cytotoxic effect was observed at higher AA-loaded concentrations. In vivo study proved the non-toxic effect of AA loaded in HA-pNIPAM nanogels compared with the control. Taken together, HA-pNIPAM nanogel is a promising biocompatible delivery system both in vitro and in vivo for hydrophobic AA molecules.

Advancement of Biomaterial-Based Postoperative Adhesion Barriers

Postoperative peritoneal adhesion (PPA) is a prevalent incidence that generally happens during the healing process of traumatized tissues. It causes multiple severe complications such as intestinal obstruction, chronic abdominal pain, and female infertility. To prevent PPA, several antiadhesion materials and drug delivery systems composed of biomaterials are used clinically, and clinical antiadhesive is one of the important applications nowadays. In addition to several commercially available materials, like film, spray, injectable hydrogel, powder, or solution type have been energetically studied based on natural and synthetic biomaterials such as alginate, hyaluronan, cellulose, starch, chondroitin sulfate, polyethylene glycol, polylactic acid, etc. Moreover, many kinds of animal adhesion models, such as cecum abrasion models and unitary horn models, are developed to evaluate new materials' efficacy. A new animal adhesion model based on hepatectomy and conventional animal adhesion models is recently developed and a new adhesion barrier by this new model is also developed. In summary, many kinds of materials and animal models are studied; thus, it is quite important to overview this field's current progress. Here, PPA is reviewed in terms of the species of biomaterials and animal models and several problems to be solved to develop better antiadhesion materials in the future are discussed.

Four-Dimensional (Bio-)printing: A Review on Stimuli-Responsive Mechanisms and Their Biomedical Suitability

The applications of tissue engineered constructs have witnessed great advances in the last few years, as advanced fabrication techniques have enabled promising approaches to develop structures and devices for biomedical uses. (Bio-)printing, including both plain material and cell/material printing, offers remarkable advantages and versatility to produce multilateral and cell-laden tissue constructs; however, it has often revealed to be insufficient to fulfill clinical needs. Indeed, three-dimensional (3D) (bio-)printing does not provide one critical element, fundamental to mimic native live tissues, i.e., the ability to change shape/properties with time to respond to microenvironmental stimuli in a personalized manner. This capability is in charge of the so-called “smart materials”; thus, 3D (bio-)printing these biomaterials is a possible way to reach four-dimensional (4D) (bio-)printing. We present a comprehensive review on stimuli-responsive materials to produce scaffolds and constructs via additive manufacturing techniques, aiming to obtain constructs that closely mimic the dynamics of native tissues. Our work deploys the advantages and drawbacks of the mechanisms used to produce stimuli-responsive constructs, using a classification based on the target stimulus: humidity, temperature, electricity, magnetism, light, pH, among others. A deep understanding of biomaterial properties, the scaffolding technologies, and the implant site microenvironment would help the design of innovative devices suitable and valuable for many biomedical applications.

Research status and development potential of composite hemostatic materials

Through the discussion of the coagulation mechanism of compositehemostatic materials, the future development potential of hemostatic materials is proposed.

Recent Advances in Hemostasis at the Nanoscale

Rapid and effective hemostatic materials have received wide attention not only in the battlefield but also in hospitals and clinics. Traditional hemostasis relies on materials with little designability which has many limitations. Nanohemostasis has been proposed since the use of peptides in hemostasis. Nanomaterials exhibit excellent adhesion, versatility, and designability compared to traditional materials, laying a good foundation for future hemostatic materials. This review first summarizes current hemostatic methods and materials, and then introduces several cutting-edge designs and applications of nanohemostatic materials such as polypeptide assembly, electrospinning of cyanoacrylate, and nanochitosan. Particularly, their advantages and working mechanisms are introduced. Finally, the challenges and prospects of nanohemostasis are discussed.

A glimpse of biodegradable polymers and their biomedical applications

Over the past two decades, biodegradable polymers (BPs) have been widely used in biomedical applications such as drug carrier, gene delivery, tissue engineering, diagnosis, medical devices, and antibacterial/antifouling biomaterials. This can be attributed to numerous factors such as chemical, mechanical and physiochemical properties of BPs, their improved processibility, functionality and sensitivity towards stimuli. The present review intended to highlight main results of research on advances and improvements in terms of synthesis, physical properties, stimuli response, and/or applicability of biodegradable plastics (BPs) during last two decades, and its biomedical applications. Recent literature relevant to this study has been cited and their developing trends and challenges of BPs have also been discussed.

Local Toxicity of Topically Administrated Thermoresponsive Systems: In Vitro Studies with In Vivo Correlation

Thermoresponsive materials have the ability to respond to a small change in temperature-a property that makes them useful in a wide range of applications and medical devices. Although very promising, there is only little conclusive data about the cytotoxicity and tissue toxicity of these materials. This work studied the biocompatibility of three Food and Drug Administration approved thermoresponsive polymers: poly( N-isopropyl acrylamide), poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) tri-block copolymer, and poly(lactic acid-co-glycolic acid) and poly(ethylene glycol) tri-block copolymer. Fibroblast NIH 3T3 and HaCaT keratinocyte cells were used for the cytotoxicity testing and a mouse model for the in vivo evaluation. In vivo results generally showed similar trends as the results seen in vitro, with all tested materials presenting a satisfactory biocompatibility in vivo. pNIPAM, however, showed the highest toxicity both in vitro and in vivo, which was explained by the release of harmful monomers and impurities. More data focusing on the biocompatibility of novel thermoresponsive biomaterials will facilitate the use of existing and future medical devices.

Thermoreversible Hyaluronic Acid-PNIPAAm Hydrogel Systems for 3D Stem Cell Culture

Human pluripotent stem cells (hPSCs) offer considerable potential for biomedical applications including drug screening and cell replacement therapies. Clinical translation of hPSCs requires large quantities of high quality cells, so scalable methods for cell culture are needed. However, current methods are limited by scalability, the use of animal-derived components, and/or low expansion rates. A thermoresponsive 3D hydrogel for scalable hPSC expansion and differentiation into several defined lineages is recently reported. This system would benefit from increased control over material properties to further tune hPSC behavior, and here a scalable 3D biomaterial with the capacity to tune both the chemical and the mechanical properties is demonstrated to promote hPSC expansion under defined conditions. This 3D biomaterial, comprised of hyaluronic acid and poly(N-isopropolyacrylamide), has thermoresponsive properties that readily enable mixing with cells at low temperatures, physical encapsulation within the hydrogel upon elevation at 37 °C, and cell recovery upon cooling and reliquefaction. After optimization, the resulting biomaterial supports hPSC expansion over long cell culture periods while maintaining cell pluripotency. The capacity to modulate the mechanical and chemical properties of the hydrogel provides a new avenue to expand hPSCs for future therapeutic application.

Thermo-responsive in-situ forming hydrogels as barriers to prevent post-operative peritendinous adhesion

In this study, we aimed to assess whether thermo-responsive in-situ forming hydrogels based on poly(N-isopropylacrylamide) (PNIPAM) could prevent post-operative peritendinous adhesion. The clinical advantages of the thermo-responsive hydrogels are acting as barrier material to block penetration of fibroblasts, providing mobility and flexibility during application and enabling injection through a small opening to fill spaces of any shape after surgery. The thermo-responsiveness of hydrogels was determined to ensure their clinic uses. By grafting hydrophilic biopolymers chitosan (CS) and hyaluronic acid (HA) to PNIPAM, the copolymer hydrogels show enhanced water retention and lubrication, while reduced volume shrinkage during phase transition. In cell culture experiments, the thermo-responsive hydrogel has good biocompatibility and reduces fibroblast penetration. In animal experiments, the effectiveness of preventing post-operative peritendinous adhesion was studied in a rabbit deep flexor tendon model. From gross examination, histology, bending angles of joints, tendon gliding excursion and pull-out force, HA-CS-PNIPAM (HACPN) was confirmed to be the best barrier material to prevent post-operative peritendinous adhesion compared to PNIPAM and CS-PNIPAM (CPN) hydrogels and a commercial barrier film Seprafilm®. There was no significant difference in the breaking strength of HACPN-treated tendons and spontaneously healed ones, indicating HACPN hydrogel application did not interfere with normal tendon healing. We conclude that HACPN hydrogel can provide the best functional outcomes to significantly prevent post-operative tendon adhesion in vivo.

STATEMENT OF SIGNIFICANCE

We prepared thermo-responsive in-situ forming hydrogels based on poly(N-isopropylacrylamide) (PNIPAM) to prevent post-operative peritendinous adhesion. The injectable barrier hydrogel could have better anti-adhesive properties than current commercial products by acting as barrier material to block penetration of fibroblasts, providing mobility and flexibility during application and enabling injection through a small opening to fill spaces of any shape after surgery. The effectiveness of preventing post-operative peritendinous adhesion was studied in a rabbit deep flexor tendon model. From gross examination, histology, bending angles of joints, tendon gliding excursion and pull-out force, HA-CS-PNIPAM (HACPN) was confirmed to be the best barrier material to prevent post-operative peritendinous adhesion compared to PNIPAM and CS-PNIPAM (CPN) hydrogels and a commercial barrier film Seprafilm®.

Injectable thermosensitive hydrogel containing hyaluronic acid and chitosan as a barrier for prevention of postoperative peritoneal adhesion

Peritoneal adhesion is one of the common complications after abdominal surgery. Injectable thermosensitive hydrogel could serve as an ideal barrier to prevent this postoperative tissue adhesion. In this study, poly(N-isopropylacrylamide) (PNIPAm) was grafted to chitosan (CS) and the polymer was further conjugated with hyaluronic acid (HA) to form thermosensitive HA-CS-PNIPAm hydrogel. Aqueous solutions of PNIPAm and HA-CS-PNIPAm at 10%(w/v) are both free-flowing and injectable at room temperature and exhibit sol-gel phase transition around 31°C; however, HA-CS-PNIPAm shows less volume shrinkage after gelation and higher complex modulus than PNIPAm. Cell culture studies indicate both injectable hydrogel show barrier effects to reduce fibroblasts penetration while induce little cytotoxicity in vitro. From a sidewall defect-bowel abrasion model in rats, significant reduction of postoperative peritoneal adhesion was found for peritoneal defects treated with HA-CS-PNIPAm compared with those treated with PNIPAm and untreated controls from gross and histological evaluation. Furthermore, HA-CS-PNIPAm did not interfere with normal peritoneal tissue healing and did not elicit acute toxicity from blood analysis and tissue biopsy examination. By taking advantage of the easy handling and placement properties of HA-CS-PNIPAm during application, this copolymer hydrogel would be a potentially ideal injectable anti-adhesion barrier after abdominal surgeries.

Stimuli-Regulated Smart Polymeric Systems for Gene Therapy

The physiological condition of the human body is a composite of different environments, each with its own parameters that may differ under normal, as well as diseased conditions. These environmental conditions include factors, such as pH, temperature and enzymes that are specific to a type of cell, tissue or organ or a pathological state, such as inflammation, cancer or infection. These conditions can act as specific triggers or stimuli for the efficient release of therapeutics at their destination by overcoming many physiological and biological barriers. The efficacy of conventional treatment modalities can be enhanced, side effects decreased and patient compliance improved by using stimuli-responsive material that respond to these triggers at the target site. These stimuli or triggers can be physical, chemical or biological and can be internal or external in nature. Many smart/intelligent stimuli-responsive therapeutic gene carriers have been developed that can respond to either internal stimuli, which may be normally present, overexpressed or present in decreased levels, owing to a disease, or to stimuli that are applied externally, such as magnetic fields. This review focuses on the effects of various internal stimuli, such as temperature, pH, redox potential, enzymes, osmotic activity and other biomolecules that are present in the body, on modulating gene expression by using stimuli-regulated smart polymeric carriers.

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.

Copper catalyst efficiency for the CuAAC synthesis of a poly(N-isopropylacrylamide) conjugated hyaluronan

BACKGROUND

Poly(N-isopropylacrylamide) conjugated hyaluronan (HA-pN), a brush-like copolymer system which serves as a polymer vehicle for cellular and drug delivery, has been previously synthesized via the copper catalyzed azide-alkyne reaction (CuAAC) using a combination of copper sulfate and ascorbic acid (CuAsc) as the catalytic system of choice. Bromotris(triphenylphosphine) copper(I) (CuBr(PPh3)3) is an alternative catalytic compound containing a phosphorous ligand which stabilizes copper in the +1 oxidative state in aqueous solvents and can be employed at true catalyst concentrations.

OBJECTIVE

CuAsc and CuBr(PPh3)3 were compared for their efficiency; 1) in the synthesis of HA-pN via CuAAC; 2) in producing thermoresponsive compositions and 3) in being extracted from the polymeric compositions.

METHODS

The synthesis of the brush copolymer was carried out under strict Schlenk conditions, then characterized by ATR-FTIR, 1H NMR, ICP-SFMS, and rheological measurements.

RESULTS

CuBr(PPh3)3 catalyzed CuAAC leads to better grafting in water, at a true catalyst concentration, compared to CuAsc. Polymeric solutions exhibited similar traits of increasing mechanical stiffness with rising temperature. Despite purification via chelation and/or dialysis, residual copper was present in similar concentrations in the final polymers.

CONCLUSIONS

In the CuAAC driven copolymer synthesis of the HA-pN, CuBr(PPh3)3 is a better catalyst than CuAsc.

Adhesive and robust multilayered poly(lactic acid) nanosheets for hemostatic dressing in liver injury model

Freestanding biodegradable nanosheets composed of poly(l-lactic acid) (PLLA) have been developed for various biomedical applications. These nanosheets exhibit unique properties such as high adhesiveness and exquisite flexibility; however, they burst easily due to their nanometer thickness. We herein describe a freestanding, multilayered nanosheet composed of PLLA fabricated using a simple combination procedure: (i) multilayering of PLLA and alginate, (ii) gelation of the alginate layers, (iii) fusion-cut sealing, and (iv) elution of the alginate layers. The multilayered nanosheets not only reinforced the bursting strength but also provided a high level of adhesive strength. In fact, they were found to show potential as a hemostatic dressing, and they tended to show reduced tissue adhesion that accompanies liver injury. Therefore, we propose this biomaterial as a candidate for an alternative to conventional therapy in hemorrhage. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1747-1757, 2017.

Evaluation of an injectable thermoresponsive hyaluronan hydrogel in a rabbit osteochondral defect model

Articular cartilage displays very little self-healing capabilities, generating a major clinical need. Here, we introduce a thermoresponsive hyaluronan hydrogel for cartilage repair obtained by covalently grafting poly(N-isopropylacrylamide) to hyaluronan, to give a brush co-polymer HpN. The gel is fluid at room temperature and becomes gel at body temperature. In this pilot study HpN safety and repair response were evaluated in an osteochondral defect model in rabbit. Follow-up was of 1 week and 12 weeks and the empty defect served as a control, for a total of four experimental groups. At 12 weeks the defect sites were evaluated macroscopically and histologically. Local lymph nodes, spleen, liver, and kidneys were analyzed for histopathological evaluation. HpN could be easily injected and remained into the defect throughout the study. The macroscopic score was statistically superior for HpN versus empty. Histological score gave opposite trend but not statistically significant. A slight tissue reaction was observed around HpN, however, vascularization and subchondral bone formation were not impeded. An upper proteoglycans rich fibro-cartilaginous tissue with fairly good continuity and lateral integration into the existing articular cartilage was observed in all cases. No signs of local or systemic acute or subacute toxicity were observed. In conclusion, HpN is easily injectable, remains into an osteochondral defect within a moving synovial joint, is biocompatible and does not interfere with the intrinsic healing response of osteochondral defects in a rabbit model. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 1469-1478, 2016.

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

There is a lack of hydrogel materials whose properties can be tuned at the point of use. Biological hydrogels, such as collagen, gelate at physiological temperatures; however, they are not always ideal as scaffolds because of their low mechanical strength. Their mechanics can be improved through cross-linking and chemical modification, but these methods still require further synthesis. We have demonstrated that by combining collagen with a thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM), the mechanical properties can be improved while maintaining cytocompatibility. Furthermore, different concentrations of this polymer led to a range of hydrogels with shear moduli ranging from 10(5) Pa down to less than 10(2) Pa, similar to the soft tissues in the body. In addition to variable mechanical properties, the hydrogel blends have a range of micron-scale structures and porosities, which caused adipose-derived stromal cells (ADSCs) to adopt different morphologies when encapsulated within and may therefore be able to direct cell fate.

Multi-Functional Macromers for Hydrogel Design in Biomedical Engineering and Regenerative Medicine

Contemporary biomaterials are expected to provide tailored mechanical, biological and structural cues to encapsulated or invading cells in regenerative applications. In addition, the degradative properties of the material also have to be adjustable to the desired application. Oligo- or polymeric building blocks that can be further cross-linked into hydrogel networks, here addressed as macromers, appear as the prime option to assemble gels with the necessary degrees of freedom in the adjustment of the mentioned key parameters. Recent developments in the design of multi-functional macromers with two or more chemically different types of functionalities are summarized and discussed in this review illustrating recent trends in the development of advanced hydrogel building blocks for regenerative applications.

A thermoresponsive biodegradable polymer with intrinsic antioxidant properties

Oxidative stress in tissue can contribute to chronic inflammation that impairs wound healing and the efficacy of cell-based therapies and medical devices. We describe the synthesis and characterization of a biodegradable, thermoresponsive gel with intrinsic antioxidant properties suitable for the delivery of therapeutics. Citric acid, poly(ethylene glycol) (PEG), and poly-N-isopropylacrylamide (PNIPAAm) were copolymerized by sequential polycondensation and radical polymerization to produce poly(polyethylene glycol citrate-co-N-isopropylacrylamide) (PPCN). PPCN was chemically characterized, and the thermoresponsive behavior, antioxidant properties, morphology, potential for protein and cell delivery, and tissue compatibility in vivo were evaluated. The PPCN gel has a lower critical solution temperature (LCST) of 26 °C and exhibits intrinsic antioxidant properties based on its ability to scavenge free radicals, chelate metal ions, and inhibit lipid peroxidation. PPCN displays a hierarchical architecture of micropores and nanofibers, and contrary to typical thermoresponsive polymers, such as PNIPAAm, PPCN gel maintains its volume upon formation. PPCN efficiently entrapped and slowly released the chemokine SDF-1α and supported the viability and proliferation of vascular cells. Subcutaneous injections in rats showed that PPCN gels are resorbed over time and new connective tissue formation takes place without signs of significant inflammation. Ultimately, this intrinsically antioxidant, biodegradable, thermoresponsive gel could potentially be used as an injectable biomaterial for applications where oxidative stress in tissue is a concern.

Anomalous temperature dependence of speed of sound of bulk poly(N-isopropylacrylamide) hydrogels near the phase transition

Bulk Poly(N-isopropylacrylamide) (PNIPAm) hydrogels are thermally responsive polymers that undergo a sharp volumetric phase transition around its lower critical solution temperature of 33 °C. The physical characteristics of bulk, micro-, and nano-form PNIPAm hydrogel have been well-studied, and have applications ranging from biomedical devices to mechanical actuators. An important physical characteristics which reveals lack of available information is speed of sound. Prior studies have utilized Brillouin scattering, multi-echo reflection ultrasound spectroscopy, the sing-around method, and others in measuring the speed of sound. We use a planar resonant cavity with bulk PNIPAm hydrogel in aqueous solution to determine the temperature dependent speed of sound around the lower critical solution temperature. The results show sharp nonmonotonic behavior of the sound velocity in vicinity of the phase transition.

Smart functional polymers – a new route towards creating a sustainable environment

Smart functional polymers have gained a huge amount of interest in recent times due to their innumerable applications in areas including sensors, actuators, switchable wettability, bio-medical and environmental applications.

Hemostatic strategies for traumatic and surgical bleeding

Wide interest in new hemostatic approaches has stemmed from unmet needs in the hospital and on the battlefield. Many current commercial hemostatic agents fail to fulfill the design requirements of safety, efficacy, cost, and storage. Academic focus has led to the improvement of existing strategies as well as new developments. This review will identify and discuss the three major classes of hemostatic approaches: biologically derived materials, synthetically derived materials, and intravenously administered hemostatic agents. The general class is first discussed, then specific approaches discussed in detail, including the hemostatic mechanisms and the advancement of the method. As hemostatic strategies evolve and synthetic-biologic interactions are more fully understood, current clinical methodologies will be replaced.

Thermoresponsive hydrogels from phosphorylated ABA triblock copolymers: a potential scaffold for bone tissue engineering

A series of thermoresponsive and biocompatible ABA triblock copolymers in which the outer A blocks comprise poly(N-isopropylacrylamide) and the central B block consists of O-phosphoethanolamine (PEA) grafted poly(acrylic acid) (PAA(PEA)) are achieved by atom transfer radical polymerization (ATRP) and subsequent modification. At a relatively low concentration (2 w/v% in phosphate buffered saline), the triblock copolymers can form free-standing gels at 37 °C. Using a combination of variable-temperature 1H NMR, dynamic light scattering, and rheological measurements, it is demonstrated that the gelation behavior is highly dependent on both the length of A blocks and the substitution degree of phosphate group. To examine the potential application as scaffold for bone tissue engineering, the physical gels are incubated in the simulated body fluid (SBF) for 2 weeks. Obvious nucleation and growth of hydroxyapatite are found in the gels, as indicated by the scanning electron microscope, energy dispersive spectroscopy, and X-ray diffraction measurements. The triblock copolymers also exhibit low cytotoxicity in cell viability test. Thus the triblock copolymers have great potential for bone tissue engineering.

Poly (N-isopropylacrylamide) (PNIPAM)-grafted gelatin as thermoresponsive three-dimensional artificial extracellular matrix: molecular and formulation parameters vs. cell proliferation potential

A series of poly(N-isopropylacrylamide)-grafted gelatins (PNIPAM gelatins) of three different graft densities (approx. 11, 22 and 34 graft chains per gelatin molecule) and three different molecular weights of their graft chains (molecular weight approximately 1.2 x 10(4), 5.0 x 10(4) and 1.3 x 10(5) g/mol) were prepared by multiple derivatization of dithiocarbamyl (DC) group in a gelatin molecule and subsequent iniferter (acts as an initiator, transfer-agent and terminator)-based photopolymerization of NIPAM. The weight ratio of PNIPAM graft chains to gelatin (P/G) varied from 1.4 to 49. Aqueous solutions of PNIPAM-gelatins showed thermo-responsiveness, depended on the graft density and the molecular weight of PNIPAM graft chain or P/G. Aqueous solutions (10 or 20%, w/v) of PNIPAM-gelatins with P/G of more than 5.8 were converted to gels at 37 degrees C. Focal plane images of PNIPAM-gelatin gels by confocal laser scanning microscopy revealed that the size of hydrophobically clustered aggregates increased with P/G, whereas the space of microvoids decreased with concentration. Compressive strain-stress measurements revealed that compressive strength of PNIPAM-gelatin increased with P/G. Bovine smooth muscle cells (SMCs)-entrapped gels were produced from PNIPAM-gelatin-containing cell-suspended medium solutions at 37 degrees C. The entrapped cells proliferated in the gel with P/G of more than 12. A higher cell proliferativity was obtained at low concentration (5%, w/v) and higher P/G (>18). Tissue formation composed of proliferative SMCs and cell-secreted extracellular matrices (collagen) was obtained at 14 days incubation. The inter-relationship between the molecular parameters of PNIPAM-gelatin, internal structural features and cell proliferation potential was discussed.

Stimuli-responsive polymers and their applications in nanomedicine

This review focuses on smart nano-materials built of stimuli-responsive (SR) polymers and will discuss their numerous applications in the biomedical field. The authors will first provide an overview of different stimuli and their corresponding, responsive polymers. By introducing myriad functionalities, SR polymers present a wide range of possibilities in the design of stimuli-responsive devices, making use of virtually all types of polymer constructs, from self-assembled structures (micelles, vesicles) to surfaces (polymer brushes, films) as described in the second section of the review. In the last section of this review the authors report on some of the most promising applications of stimuli-responsive polymers in nanomedicine. In particular, we will discuss applications pertaining to diagnosis, where SR polymers are used to construct sensors capable of selective recognition and quantification of analytes and physical variables, as well as imaging devices. We will also highlight some examples of responsive systems used for therapeutic applications, including smart drug delivery systems (micelles, vesicles, dendrimers...) and surfaces for regenerative medicine.

Genetically-manipulated adult stem cells as therapeutic agents and gene delivery vehicle for wound repair and regeneration

Wound therapy remains a clinical challenge and much effort has been focused on the development of novel therapeutic approaches for wound management. New knowledge about the way in which signals control wound cellular and molecular behavior has promoted the topical application of multipotent stem cells and bioactive molecules to injured tissue, for skin regeneration with less scar formation. However, limited clinical success indicates that the effective delivery of polypeptides and therapeutic cells, with controlled releasing profile, is a major challenge which is yet to be overcome. Recently, a technique in which the genetically-manipulated stem cells were used both as the therapeutic agents and the vehicle for gene delivery for wound treatment - a method which serves to provide regenerative cells and bioactive genes within an optimal environment of regulatory molecular expression for wound sites - has emerged as a promising strategy for wound regenerative therapy. In this article, the roles of adult stem cells - as the therapeutics and the vehicles in these advanced biomimetic drug delivery systems for wound regeneration medicine - are scrutinized to indicate their mechanisms, characteristics, broad applicability and future lines of investigation.

Polypeptide grafted hyaluronan: synthesis and characterization

Poly(l-leucine) grafted hyaluronan (HA-g-PLeu) has been synthesized via a Michael addition reaction between primary amine terminated poly(l-leucine) and acrylate-functionalized HA (TBAHA-acrylate). The precursor hyaluronan was first functionalized with acrylate groups by reaction with acryloyl chloride in the presence of triethylamine in N,N-dimethylformamide. (1)H NMR analysis of the resulting product indicated that an increase in the concentration of acryloylchoride with respect to hydroxyl groups on HA has only a moderate effect on functionalization efficiency, f. A precise control of stoichiometry was not achieved, which could be attributed to partial solubility of intermolecular aggregates and the hygroscopic nature of HA. Michael addition at high [PLeu-NH(2)]/[acrylate](TBAHA) ratios gave a molar grafting ratio of only 0.20 with respect to the repeat unit of HA, indicating grafting limitation due to insolubility of the grafted HA-g-PLeu. Soluble HA-g-PLeu graft copolymers were obtained for low grafting ratios (<0.039) with <8.6% by mass of PLeu and were characterized thoroughly using light scattering, (1)H NMR, FT-IR, and AFM techniques. Light scattering experiments showed a strong hydrophobic interaction between PLeu chains, resulting in aggregates with segregated nongrafted HA segments. This yields local networks of aggregates, as demonstrated by atomic force microscopy. Circular dichroism spectroscopy showed a β-sheet conformation for aggregates of poly(l-leucine).

Thermally responsive injectable hydrogel incorporating methacrylate-polylactide for hydrolytic lability

Injectable thermoresponsive hydrogels are of interest for a variety of biomedical applications, including regional tissue mechanical support as well as drug and cell delivery. Within this class of materials there is a need to provide options for gels with stronger mechanical properties as well as variable degradation profiles. To address this need, the hydrolytically labile monomer, methacrylate-polylactide (MAPLA), with an average 2.8 lactic acid units, was synthesized and copolymerized with N-isopropylacrylamide (NIPAAm) and 2-hydroxyethyl methacrylate (HEMA) to obtain bioabsorbable thermally responsive hydrogels. Poly(NIPAAm-co-HEMA-co-MAPLA) with three monomer feed ratios (84/10/6, 82/10/8, and 80/10/10) was synthesized and characterized with NMR, FTIR, and GPC. The copolymers were soluble in saline at reduced temperature (<10 degrees C), forming clear solutions that increased in viscosity with the MAPLA feed ratio. The copolymers underwent sol-gel transition at lower critical solution temperatures of 12.4, 14.0, and 16.2 degrees C, respectively, and solidified immediately upon being placed in a 37 degrees C water bath. The warmed hydrogels gradually excluded water to reach final water contents of approximately 45%. The hydrogels as formed were mechanically strong, with tensile strengths as high as 100 kPa and shear moduli of 60 kPa. All three hydrogels were completely degraded (solubilized) in PBS over a 6-7 month period at 37 degrees C, with a higher MAPLA feed ratio resulting in a faster degradation period. Culture of primary vascular smooth muscle cells with degradation solutions demonstrated a lack of cytotoxicity. The synthesized hydrogels provide new options for biomaterial injection therapy where increased mechanical strength and relatively slow resorption rates would be attractive.