Effect of the conversion degree and multiple healing on the healing efficiency of a thermally reversible self‐healing polymer

In this paper, the effect of the cross‐links' conversion degree on the healing efficiency of a thermally remendable polymer based on the Diels‐Alder (DA) reaction was studied quantitatively. By using the differential scanning calorimetry (DSC) results along with the Kissinger method, the conversion degree of the thermally reversible cross‐links was predicted as a function of time and temperature. For investigating the healing efficiencies at different conversion degrees, three‐point bending specimens were fabricated under certain curing conditions, which guaranteed the formation of both reversible and irreversible bonds. Afterward, specimens failed under three‐point bending test and healed up to certain conversion degrees. The results revealed on average 15%, 38%, and 83% recovery of flexural strength and 89%, 91%, and 93% recovery of flexural modulus at conversion degrees of 0.6, 0.8, and 1, respectively. Moreover, by repeating the damaging and healing procedure, it was shown that the synthesized polymer has the capability to be healed several times.     

Bioactive polymeric formulations for wound healing

Ricinoleic acid (RA) has potential to promote wound healing because of its analgesic and anti‐inflammatory properties. This study investigates the synthesis and characterization of RA liposomes infused in a hydrogel for topical application. Lecithin liposomes containing RA were prepared and incorporated into a chitosan solution and were subsequently cross‐linked with di‐aldehyde β‐cyclodextrin (Di‐β‐CD). Chitosan/Di‐β‐CD concentrations and reaction temperatures were varied to alter gelation time, water content, and mechanical properties of the hydrogel in an effort to obtain a wide range of RA release profiles. Hydrogel cross‐linking was confirmed by spectroscopy, and liposome and carrier hydrogel morphology via microscopy. Chitosan, Di‐β‐CD, and liposome concentrations within the formulation affected the extent of matrix swelling, mechanical strength, and pore and overall morphology. Higher cross‐linking density of the hydrogel led to lower water uptake and slower release rate of RA. Optimized formulations resulted in a burst release of RA followed by a steady release pattern accounting for 80% of the encapsulated RA over a period of 48 hours. However, RA concentrations above 0.1 mg/mL were found to be cytotoxic to fibroblast cultures in vitro because of the oily nature of RA. These formulations promoted wound healing when used to treat full thickness skin wounds (2 cm²) in Wister male rats. The wound contraction rates were significantly higher compared to a commercially available topical cream after a time period of 21 days. Histopathological analysis of the RA‐liposomal chitosan hydrogel group showed that the epidermis, dermis, and subcutaneous skin layers displayed an accelerated yet normal healing compared to control group.     

Carbon tetrachloride-induced crack healing in polycarbonate

Crack healing induced by carbon tetrachloride in polycarbonate has been studied at temperatures in the range of 40–60°C. The carbon tetrachloride treatment reduces the glass transition temperature of polycarbonate. Crack healing is observed because the effective glass transition temperature in polycarbonate is reduced to below the test temperature by the carbon tetrachloride treatment. Two distinctive stages of crack healing are divided based on the recovery of mechanical strength and fractograph. The first stage corresponds to the progressive healing due to the convolution of wetting and self-diffusion, which has a constant crack closure rate. Immediately following the first stage, the second stage, corresponding to the self-diffusion of polymer chain, enhances the quality of healing behavior. The transport of carbon tetrachloride in polycarbonate consists of case I (concentration gradient controlled) and case II (relaxation controlled) behaviors. The direction of case II is opposite to that of case I. The solubility decreases with increasing temperature, but diffusivity and velocity for mass transfer, crack closure rate, and diffusion coefficient for the diffusion front have the opposite trend. The first stage of crack healing is controlled by case II transport. The transport of carbon tetrachloride changes the fracture behavior of polycarbonate from ductile to brittle. A comparison of crack healing in polycarbonate and poly (methyl methacrylate) is made. © 1994 John Wiley & Sons, Inc.     

Craze growth and healing in polystyrene

The kinetics of craze growth and craze healing were studied by dark-field optical microscopy in monodisperse molecular weight polystyrene (PS) that varied in molecular weight from 88,000 to 1,334,000. The following observations were made. (1) G₁ the virgin growth rate, decreased rapidly with increasing molecular weight until M_n ～ 200,000 and then remained constant. (2) G₁ decreased with increasing craze density. (3) The growth rates of approaching craze tips decreased when the craze tips overlapped, and the effect was less for crazes whose parallel growth paths were greater than 40 μm apart. (4) Complete craze healing was observed by comparison of the nucleation times, τ₂, and growth rates, G₂, of healed individual crazes with the craze kinetics of the virgin sample. (5) The extent of healing was characterized using four cases in which τ and G were measured as a function of healing time, temperature, constant stress, and molecular weight. (6) Craze healing times were found to increase with molecular weight and were analyzed in terms of the modified molecular weight of the craze zone. (7) Significant bond rupture was determined to occur during crazing by comparison of healing times with stress relaxation and diffusion data. (8) Craze healing studies provide insight into both crack healing and fracture of glassy polymers.     

Chemical healing in partially cross-linked polyamides

Evidence is presented for chemical healing—healing resulting from chemical reactions between neighboring macromolecules located at the interface surface—which has recently been reported in linear polycondensates. In order to distinguish the contribution of the solid-state reaction from the healing effect, samples (nylon-11, nylon-12, and nylon-6,6) with strongly restricted chain mobility (by crystallization and chemical cross-linking in the amorphous zones) have been used. It has been found that under the same healing conditions the cross-linked samples display better welding than the un-cross-linked ones. The conclusion is drawn that the observed healing effect is due mainly to the solid-state chemical reactions at the contact surfaces and not to the diffusion of macromolecules. The results obtained can be considered as an indication of the existence of “chemically released diffusion” in the solid state, i.e., motion of chain segments owing to the interchain and/or intrachain chemical reactions, mainly of the exchange type.     

Self‐healing in Nanocomposite Hydrogels

Polymer hydrogels with characteristics distinct from those of solid materials are one of the most promising candidates for smart materials. Here, we report that a nanocomposite hydrogel (NC gel) consisting of a unique polymer/clay network structure, can exhibit complete self‐healing through autonomic reconstruction of crosslinks across a damaged interface. Mechanical damage in NC gels can be repaired without the use of a healing agent, and even sections of NC gels separated by cutting, from whichever the same or different kinds of NC gel, perfectly (re‐)combine by just contacting the cut surfaces together at mildly elevated temperatures. In NC gels, the autonomic fusion of cut surfaces as well as the self‐healing could be achieved not only immediately after being cut but also after a long waiting time.

     

D-glucosamine-based supramolecular hydrogels to improve wound healing

A simple supramolecular hydrogel based on D-glucosamine, a naturally occurring aminosaccharide, promises new biomaterials for applications such as wound healing.     

Multifunctional fibrous wound dressings for refractory wound healing

Refractory wounds have always been an important issue to healthcare systems, whose healing process is always delayed by multiple factors, including bacterial infections, chronic inflammation, and excessive exudates, etc. Employing multifunctional wound dressings is recognized as an effective strategy to deal with refractory wounds, which has yielded promising outcomes in recent years. Among these advanced wound dressings, fibrous dressings have gained growing attention due to their unique merits. Such wound dressings have demonstrated great potential in delivering theranostic agents, such as antibacterial agents, anti-inflammatory drugs, growth factors, and diagnostic probes, etc., for the purposes of accelerating wound healing. This paper reviews the development of multifunctional fibrous dressings and their applications in treating refractory wounds. The construction approaches of novel fibrous dressing with capabilities of antibacterial, anti-inflammation, exudate management and diagnosis were also introduced. Furthermore, the existing problems and challenges are also discussed briefly.     

Diabetic wound healing activated by supramolecular cascade reaction

The nano-supramolecular cascade reactor based on sulfobutylether-β-cyclodextrin, not only displayed glucose-activated radical polymerization to form hydrogel in situ, but also produced hydroxyl radicals for drug-resistant bacteria elimination and wound protection, which successfully achieved diabetic wound healing and in situ therapy.

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Natural okra-based hydrogel for chronic diabetic wound healing

As a representative of chronic wounds, the long-term high levels of oxidative stress and blood sugar in chronic diabetic wounds lead to serious complications, making them the biggest challenge in the research on wound healing. Many edible natural biomaterials rich in terpenes, phenols, and flavonoids can act as efficient antioxidants. In this study, okra extract was selected as the main component of a wound dressing. The okra extracts obtained via different methods comprehensively maintained the bioactivity of multiple molecules. The robust antioxidant properties of okra significantly reduced intracellular reactive oxygen species production, thereby accelerating the wound healing process. The results showed that okra extracts and their hydrogel dressings increased cell migration, angiogenesis, and re-epithelization of the chronic wound area, considerably promoting wound remodeling in diabetic rats. Therefore, okra-based hydrogels are promising candidates for skin regeneration and wider tissue engineering applications.     

Rapid healing of crazes in ruptured rubber-toughened plastics

High-speed photographic studies in connection with photodensitometry were performed to study craze healing in ABS/MMA sheets occurring within 5 microseconds to 20 milliseconds after rupture. Observations on various parts of the deformed specimens, particularly at a plastic zone in the vicinity of a crack, show that there are two stages of healing; first, a relatively elastic stage, and second, a relatively viscoelastic stage. Results of the observation also indicate that the rate of healing is influenced by the magnitude of plastic strain to which the specimens were subjected. The healing mechanism near an advancing crack is discussed briefly.     

Metal-catalyzed transesterification for healing and assembling of thermosets

Catalytic control of bond exchange reactions enables healing of cross-linked polymer materials under a wide range of conditions. The healing capability at high temperatures is demonstrated for epoxy-acid and epoxy-anhydride thermoset networks in the presence of transesterification catalysts. At lower temperatures, the exchange reactions are very sluggish, and the materials have properties of classical epoxy thermosets. Studies of model molecules confirmed that the healing kinetics is controlled by the transesterification reaction rate. The possibility of varying the catalyst concentration brings control and flexibility of welding and assembling of epoxy thermosets that do not exist for thermoplastics.     

Chemical healing in poly(ethylene terephthalate)

A new molecular mechanism for the healing phenomenon in semicrystalline linear polycondensates (healing resulting from chemical reactions between macromolecules located in the interfacial surface) is demonstrated. Strips of commercial poly(ethylene terephthalate) are annealed at 258°C in order to avoid melt sticking. Two such strips are partially overlapped, pressed, and heated in a vacuum at 240°C for 10, 20, 30, and 100 h. By measuring the stress at break outside the contact area and the debonding shear stress the critical overlapping length is computed. It is concluded that transreaction contributes more than solid-state post-condensation to chemical healing.     

Molecular basis of healing and fracture at polymer interfaces

The interdiffusion of polymer chains across a polymer–polymer interface, and subsequent fracture to re-create the interface is reviewed. In particular, films formed via latex coalescence provide a very large surface area. Of course, latex film formation is a very important practical problem. Healing of the interface by interdiffusion is treated using the de Gennes reptation theory and the Wool minor chain reptation model. The self-diffusion coefficients of polystyrene and the polymethacrylates obtained by small-angle neutron scattering, SANS, direct non-radiative energy transfer, DET, and other techniques are compared. Reduced to 150,000 g/mol and 135°C, both polystyrene and poly(methyl methacrylate) have diffusion coefficients of the order of 10^?16?10^?17 cm²/sec. Variations in the diffusion coefficient values are attributed to the experimental approaches, theoretical treatments and molecular weight distribution differences. An activation energy of 55 kcal/mol was calculated from an Arrhenius plot of all polystyrene data reduced to a number-average molecular weight of 150,000 g/mol, using an inverse square molecular weight conversion method. Interestingly, this is in between the activation energies for the α and β relaxation processes in polystyrene, 84 and 35 kcal/mol, respectively. Fracture of polystyrene was considered in terms of chain scission and chain pull-out. A dental burr apparatus was used to fracture the films. For low molecular weights, chain pull-out dominates, but for high molecular weights, chain scission dominates. At 150,000 g/mol, the energy to fracture is divided approximately equally between the two mechanisms. Above a certain number average molecular weight (about 400,000 g/mol), the number of chain scissions remains constant at about 10²⁴ scissions/m³. Energy balance calculations for film formation and film fracture processes indicate that the two processes are partly reversible, but have important components of irreversibility. From the interdiffusion SANS data, the diffusion rate is calculated to be about 1 Å/min, which is nine orders of magnitude slower than the dental burr pull-out velocity of about 0.8 cm/sec.     

Ethanol-induced crack healing in poly(methyl methacrylate)

The crack healing induced by ethanol in poly(methyl methacrylate) (PMMA) has been studied at temperatures of 40–60°C. Crack healing occurs because the effective glass transition temperature of PMMA is reduced to below the test temperature by ethanol plasticization. It is found that crack closure rate is constant at a given temperature. The fracture strength of healed PMMA is lower than that of the original samples. By comparing the fracture stress with the morphology of the crack edge on the PMMA surface, we found that a high degree of swelling is responsible for the incomplete recovery of mechanical strength. The fractography of the completely healed sample shows a very different fracture morphology from that of virgin PMMA. The transport of ethanol in PMMA also is studied. At lower temperatures, transport is described by ideal Case II behavior. As the temperature increases, the kinetics shift from ideal Case II to anomalous behavior. The first stage of crack healing is controlled by Case I transport. © 1994 John Wiley & Sons, Inc.     

Development of honey hydrogel dressing for enhanced wound healing

Radiation at 25 and 50 kGy showed no effect on the acidic pH of the local honey, Gelam, and its antimicrobial property against Staphylococcus aureus but significantly reduced the viscosity. Honey stored up to 2 years at room temperature retained all the properties studied. Radiation sterilized Gelam honey significantly stimulated the rate of burn wound healing in Sprague-Dawley rats as demonstrated by the increased rate of wound contraction and gross appearance. Gelam honey attenuates wound inflammation; and re-epithelialization was well advanced compared to the treatment using silver sulphadiazine (SSD) cream. To enhance further the use of honey in wound treatment and for easy handling, Gelam honey was incorporated into our hydrogel dressing formulation, which was then cross-linked and sterilized using electron beam at 25 kGy. Hydrogel with 6% of honey was selected based on the physical appearance.     

Self‐healing of high elasticity block copolymers

A new route for adding self‐healing properties to soluble polymers is presented briefly. Self‐healing block copolymers (polystyrene‐block‐polybutadiene block‐polystyrene from Sigma‐Aldrich) were obtained by dissolving the polymer in a solvent that neither dissolves the microbubbles nor deactivate the Grubbs catalyst. The self‐healing block copolymer has been obtained by mixing the polymer, the solvent, the microbubbles filled with monomer (dicyclopentadiene), and the Grubbs' catalyst followed by the evaporation of the solvent. The structure of self‐healed high elasticity block copolymer has been investigated by optical and Scanning Electron Microscopy. Raman spectroscopy and mechanical data suggested that the block copolymer exhibits self‐healing features. Copyright © 2008 John Wiley & Sons, Ltd.     

Selection of healing agents for a vascular self-healing application

To increase the durability and reliability of thermosets, self-healing via a vascular network, is developed. A judicious choice of healing agents proves to be necessary to achieve the best recovery of properties. Four low viscosity two-component epoxy-amine healing systems were compared, to check which glass transition temperature range would be best to recover mechanical properties (T_g ranging from −8 to 68 °C). Interdiffusion experiments show that all systems react sufficiently slowly at room temperature to allow interdiffusion of epoxy and amine over more than 1 mm before the diffusion is stopped by vitrification. Swelling tests revealed that most of the selected healing agents diffuse into the surrounding matrix and swell it. This might be beneficial for crack closure and improved adhesion between healing system and matrix. Flexural tests demonstrated that, the higher the glass transition temperature of the fully cured healing system, the higher the healing capability.     

A self healing model based on polymer-mediated chromophore correlations

Here we present a model of self healing in which correlations between chromophores, as mediated by the polymer, are key to the recovery process. Our model determines the size distribution of the correlation volume using a grand canonical ensemble through a free energy advantage parameter. Choosing a healing rate that is proportional to the number of undamaged molecules in a correlated region, and a decay rate proportional to the intensity normalized to the correlation volume, the ensemble average is shown to correctly predict decay and recovery of the population of disperse orange 11-DO11 (1-amino-2-methylanthraquinone) molecules doped in PMMA polymer as a function of time and concentration as measured with amplified spontaneous emission and linear absorption spectroscopy using only three parameters that apply to the full set of data. Our model also predicts the temperature dependence of the process. One set of parameters should be characteristic of a particular polymer and dopant chromophore combination. Thus, the use of the model in determining these parameters for various materials systems should provide the data needed to test fundamental models of the underlying mechanism responsible for self healing.