nanofibrous materials for wound care
TRANSCRIPT
Introduction
Wound is defined as “a disruption in the normal con-tinuity of a body structure” [1]. Wounds vary from acute trauma to chronic ulcers, from superficial cuts to deep incisions, from small abrasions to serious burns. Although different types of wound dressings are used for different wounds to accommodate the healing process, they should always meet a number of expectations, including being able to: (1) Prevent and/or control infection in wounds and/or the surrounding areas; (2) provide a suitable degree of moisture within the environment of the ulcer, so as to create a desirable biological medium of optimal conditions for the com-plex processes of wound healing; (3) improve comfort and protect the wound and its surroundings from mechanical trauma; (4) absorb secretions, if needed; and (5) accelerate wound healing, if possible.
Cotton gauze has traditionally been used as wound dressing materials as it has good absorption and soft hand, but can be over permeable and make the wounds desiccate. It may also adhere to the wound bed and cause pain when it is being removed [2].
Modern dressings are usually made from new materi-als, including the multilayered. However, as a result of the absorption of wound exudates, dressing materials often produce enough moisture to favor the growth of bacteria, and therefore hinder healing [3].
Nowadays we have a variety of functionalized dress-ings that are capable of releasing antimicrobial agents, and/or providing tissue regeneration agents (growth factors), and therefore accelerate wound healing. Among them, electrospun nanofiber dressings are the most significant because of their high porosity, ultra softness, large surface-to-volume ratios, and great flex-ibility, which have accounted for the development of a wide variety of natural and synthetic polymers. Also, they exhibit higher drug encapsulation efficiency and better structural stability than other drug carriers.
Electrospinning is a technique for producing ultra fine (in micros and nanos) fibers as a result of charg-ing and ejecting a polymer melt or solution through a spinneret under a high voltage electric field (up to 30 kV) and solidifying or coagulating it to form a filament. Although electrospinning was introduced as early as 1934, the process has recently recaptured
Cutaneous and Ocular Toxicology, 2010; 29(3): 143–152
R E V I E W A R T I C L E
Nanofibrous materials for wound care
Wen Zhong1,2, Malcolm M.Q. Xing3, and Howard I. Maibach4
1Department of Textile Sciences, 2Department of Medical Microbiology, Faculty of Medicine, 3Department of Mechanical Engineering, Faculty of Engineering and Department of Biochemistry and Medical Genetics, University of Manitoba, Winnipeg, Canada, and 4Department of Dermatology, University of California, San Francisco, CA, USA
AbstractNanofibrous membranes are highly soft materials with high surface-to-volume ratios, and therefore can serve as excellent carriers for therapeutic agents that are antibacterial or accelerate wound healing. This article overviews research and development of nanofibrous dressing materials for wound care, in addi-tion to a discussion of natural and synthetic polymers used in fabricating nanofibrous dressing materials, and a description of in vitro and in vivo evaluation methods for the performance and cytotoxicity of these materials. Natural polymers are usually believed to be less cytotoxic than synthetic polymer in wound care. However, most natural polymers exhibit relatively low mechanical strength than synthetic polymers. As a result, they are usually crosslinked or blended with synthetic polymers so as to somewhat affect their biocompatibility.
Keywords: Nanofibers; electrospinning; wound care; cytotoxicity
Address for Correspondence: Wen Zhong, Department of Textile Sciences, Department of Medical Microbiology, University of Manitoba, Winnipeg, MB R3T 2N2 Canada. Tel.: +1 204 474 9913; Fax: +1 204 474 7593. E-mail: [email protected]
(Received 04 December 2009; revised 23 January 2010; accepted 15 February 2010)
ISSN 1556-9527 print/ISSN 1556-9535 online © 2010 Informa UK LtdDOI: 10.3109/15569527.2010.489307 http://www.informahealthcare.com/cot
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1556-95271556-9535© 2010 Informa UK Ltd10.3109/15569527.2010.489307
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attention because nano-size fibers can be produced from both natural and synthetic polymers [4,5]. The process of electro-spinning fibers is as follows: the spinning solution is delivered to a stainless spin-neret via a syringe pump; a high voltage power sup-ply applies voltage between the spinneret and a fiber collector; the pendant drop of polymer solution at the nozzle of the spinneret becomes highly electrified, and the repulsive electrostatic forces overcome the surface tension, causing a charged jet of fluid to be ejected. Charges on the polymer jet cause the jet to “splay” or split into finer filaments, which are collected at the collector after the solvent is evaporated.
Here we overview different materials (natural and synthetic) that have been used in the design and devel-opment of nanofiber wound dressings, and the various methods of in vitro and in vivo evaluation of wound dressing materials.
Materials for wound dressings
Natural polymers
Collagen is a biocompatible and biodegradable natural polymer that exists abundantly in the con-nective tissue of animals. Extensive work has utilized collagen for wound care. Rho et al. [6] fabricated an electrospun nanofiber scaffold from type I collagen. The collagen scaffold crosslinked by glutaraldehyde vapor enhanced its mechanical stability, and the scaf-fold treated with type I collagen or laminin improve cell adhesion and spreading of normal human kerati-nocytes, and therefore may be a good candidate for wound dressing materials. Powell et al. [7] compared the skin substitutes made from freeze-dried and electrospun collagen scaffolds. In vitro evaluations showed no significant variance in cell proliferation, surface hydration, or cellular organization between the two types of scaffolds. Electrospun nanofibrous skin substitutes reduced wound contraction as com-pared to freeze-dried collagen scaffolds. Adhirajan et al. [8] developed a gelatin microspheres-impreg-nated collagen scaffold for wound dressing to attenu-ate matrix metallo protienases (MMPs) and bacteria growth: gelatin microspheres were conjugated with a MMP inhibitor 2,3-dihydroxybenzoic acid (DHBA) to suppress MMPs, which persistently elevated and retarded the process of wound closure; the micro-spheres were also loaded with an antibiotic doxycy-cline to inhibit bacteria growth. Shanmugasundaram et al. [9] investigated the efficiency of controlled delivery of silver sulfadiazine from collagen scaffold (SSDM-CS) in infected deep partial thickness burn wounds, and confirmed that controlled delivery of
silver is effective in the healing process because it helps modulate MMP activity.
Gelatin, a denatured collagen, is another fre-quently used biomaterial for electrospun nanofiber mats for wound care because of its biocompatibility, biodegradability and low cost. Powell and Boyce [10] developed electrospun gelatin nanofiber mats for surgical replacements. In vitro analysis showed that the porosity and interfiber distance in the fiber mat play an important role in tissue regeneration: gelatin nanofiber mats with interfiber distances between 5 and 10 µm were shown to have higher cell viability, optimal cell organization, and excellent barrier forma-tion, as compared to mats with interfiber distances larger than 10 µm. Rujitanaroj et al. [11] fabricated electrospun type A gelatin nanofibers containing silver nanoparticles as an antibacterial agent: AgNO3 was added to the electrospun gelatin solution, and nAg was formed when the nanofiber mats had been aged for at least 12 hours. The mats were crosslinked with glutaraldehyde (GTA) to further improve their stabil-ity. Antibacterial activity of these silver nanoparticle-loaded gelatin nanofiber mats showed greatest against Pseudomonas aeroginosa, followed by Staphylococcus aureus, Escherichia coli, and methicillin-resistant S. aureus, respectively. Xu and Zhou [12] also produced gelatin nanofibers containing silver nanoparticles by electrospinning gelatin/AgNO3/formic acid system, followed with UV irradiation to reduce Ag+ into nAg. Sikareepaisan et al. [13] introduced electrospun gela-tin nanofibers containing asiaticoside, a herbal extract of Centella asiatica (L.) Urban, a medicinal plant used in traditional medicine for its wound healing ability. Based on the unit weight of the actual amount of asia-ticoside present in the specimens, the total amount of asiaticoside released from the fiber mat speci-mens was lower than that from the film counterparts while, based on the unit weight of the specimens, an opposite trend was observed. Other researchers used gelatin in combination with synthetic polymers in the design and development of blended nanofibers for wound care. For example, Poly(L-lactide) (PLLA) [14], Polyurethane [15] and poly (ε-caprolactone) (PCL) [16] were used in blended nanofibers with different blend-ing ratios. In vitro data showed that the adhesion and proliferation of fibroblasts enhance with the increase of gelatin ratios.
Silk processed from cocoons of silkworm has been used as a biomaterial (like sutures) for thousands of years. In the last few decades, biocompatibility problems became noticeable because silk fiber is an implantable material. It was suggested that sericin may be the likely cause [17]. With all sericin removed, silk fibroin was processed from cocoons and applied in various biomedical uses including wound dressing:
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Schneider et al. [18] reported on an electrospun silk fibroin nanofibrous membrane loaded with epidermal growth factor (EGF), suggesting that EGF was incorpo-rated into the nanofibers and released in a sustained manner. Such biofunctionalized silk mats were dem-onstrated to accelerate wound reepithelialization, and are therefore suitable for wound dressing purposes, especially for patients who have got chronic wounds. Silver nanoparticles were also incorporated into silk fibroin nanofibers to fabricate antibacterial wound dressings [19]: the electrospun silk fibroin nanofiber membrane was immersed into AgNO3 solution, fol-lowed with photo reduction.
Hyaluronic acid (HA) is a natural material exist-ing in extracellular matrix. It is extensively applied in biomedical end uses because of its biocompat-ibility and biodegradability [20]. However, HA fibers quickly dissolve or decompose in water, which limits its applications in wound dressings. Xu et al. [21] tried to enhance the stability of HA fibrous membranes via crosslinking or adding gelatin to the formula, but this did not bring about significant improvement in the water resistant of the HA membranes, but the increase of gelatin content did.
Cellulose is an abundant natural polymer long been used for wound dressings in clinical applica-tions [22,23]. Cellulose or cellulose acetate (CA) has also been used in the development of nanofibrous wound dressings because its ultrafine fibers provide maximum comfort and an excellent substrate (with a high surface-to-volume ratio) for antibacterial agents. Son et al. [24,25] developed CA nanofibers containing Ag nanoparticles on the surface: CA polymer solution with a small amount of silver nitrate (AgNO3) was electrospun into nanofibers, and then subjected to UV irradiation to generate Ag nanoparticles on the surface of nanofibers. The Ag nanoparticles had an average diameter of 21 nm and strong antibacterial activity. Suwantong et al. [26] incorporated into CA electrospun nanofibers asiaticoside (AC), the herbal extract from the medicinal plant Centella asiatica. The drug release from the drug-loaded nanofibers was shown to be at a higher rate than that from the as-cast films.
Chitin is another abundant natural polymer derived from crab and shrimp shells, and chitosan, a N-deacetylated derivative of chitin, has extensively been used in biomedical applications including wound dressings because of its biocompatibility, biodegrad-ability and antibacterial functions [27]. Chitosan is more often used in combination with other polymers in electrospinning. Chen et al. [28,29] electrospun col-lagen/chitosan nanofiber mats for wound dressings. The nanofibrous mats were crosslinked by glutaral-dehyde vapor. However, the crosslinking treatment reduced tensile strength and water sorption of the
mats. The dressing materials exhibited good in vitro biocompatibility. In vivo animal studies revealed that this composite dressing provide better wound heal-ing properties than gauze and commercial collagen sponge dressings. Modified chitosan or chitosan derivatives have also been used in the development of dressings. Ignatova et al. electrospun hybrid nanofib-ers containing chitosan (original or quaternized chi-tosan) and polyThe [30] or poly(vinyl alcohol) [31]. These chitosan-based nanofibers showed higher inhibition against bacteria S. aureus and E. coli than the corresponding solvent-cast films. The hybrid mats suppressed adhesion of pathogenic S. aureus. Zhou et al. [32] synthesized a water soluble N-carboxyethyl chitosan to avoid the usage of organic solvents in elec-trospining, so as to eliminate any toxic residue of the solvents that may affect the biomedical application of the fiber mats. Hybrid nanofibers containing carbox-yethyl chitosan and poly(vinyl alcohol) were prepared from their aqueous solutions. In vitro evaluations showed that the fibrous mats promote cell attachment and proliferation, and are therefore useful for wound care.
Other natural polymers that have been utilized in the design and development of nanofibrous wound dressings include Poly-N-Acetylglucosamine [33], alginate-based materials [34], poly(3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) [35], etc. One of the most important advantages of these natural polymers is that they are usually highly biocompatible and promote cell adhesion and proliferation. However, most natural polymers exhibit relatively low stiffness and mechani-cal strength. Accordingly, they are usually crosslinked or blended with synthetic polymers so as to somewhat affect their biocompatibility.
Synthetic polymers
Poly(vinyl alcohol) (PVA) is a non-biodegradable synthetic polymer traditionally used in wound dress-ings. Electrospun PVA nanofibers were developed as a new dressing material. Hong [36] electrospun PVA/AgNO3 aqueous solution into non-woven webs and then treated the webs by means of heat or UV radia-tion to obtain Ag nanoparticle-loaded PVA mats. The heat treatment also improved crystallinity of the elec-trospun PVA fiber web and prevented the web from getting dissolved in moisture, and is therefore essential for its application in wound dressings. Taepaiboon et al. [37] suggested that the drug-loaded PVA elec-tropsun mats exhibited much better release of the model drugs (four types of non-steroidal anti-inflam-matory drug with varying water solubility property) than the drug-loaded as-cast films. PVA is also used
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in combination with natural polymers (alginate [34], chitosan [31,32,38]) and other synthetic polymers [39] in the development of nanofibrous dressings.
Poly(caprolactone) (PCL) is a biodegradable and biocompatible polymer frequently used in biomedical applications. Merrell et al. [40] incorporated Curcumin (with anti-cancer, anti-oxidant and anti-inflammatory activities) into PCL nanofibers for diabetic wound dressings. In vitro and in vivo (mice model) evalua-tions suggested that curcumin-loaded PCL nanofib-ers are bioactive and, because of their antioxidant and anti-inflammatory properties, have potential for a wound dressing material. Choi et al. [41] electrop-sun amine-terminated block copolymers composed of PCL and poly(ethylene glycol) (PEG). Recombinant human epidermal growth factor (EGF) was chemically conjugated to the surface of the nanofibers. In vitro and in vivo experiments showed that the EGF-loaded nanofibers promote keratinocyte proliferation and wound healing. PCL has also been used in compos-ite nanofibers for wound care product development [16,42].
Poly(lactide) (PLA) and poly(L-lactic acid) (PLLA) are another group of biodegradable and biocompat-ible polymers extensively used in biomedicine. He et al. [43] used a coaxial electrospinning technique to fabricate nanofibers with a core/shell structure, where PLLA constitutes the shell while tetracycline hydro-chloride (TCH), an antibiotic, is the core. A sustained TCH release from these fiber mats was observed, indi-cating their potential application in wound dressing. Thakur et al. [44] used a dual spinneret electrospinning apparatus to fabricate a dual drug release electrospun PLLA mat containing lidocaine as the anesthetic and mupirocin as the antibiotic. The anesthetic was shown to be eluted through a burst release mechanism for immediate pain relief. The antibiotic was released simultaneously through a diffusion-mediated mecha-nism for extended antibiotic activity. The drug release profile also suggested that presence of the two drugs in the same polymer matrix functions to alter the release kinetics of at least one drug, and should there-fore be regarded as preferable in fabricating wound dressings. Hong et al. [45] designed a composite fiber non-woven mat for semi-occlusive wound dressings, including PLLA as the fibrous matrix, and poly(vinyl pyrrolidone)-iodine (PVP-I), TiO(2) nanoparticles, and zinc chloride as, respectively, the antimicrobial, odor-controlling, and antiphlogistic agents. In this design, PLLA, PVP, TiO(2) nanoparticles and zinc chloride were added to the electrospinning solution to form the composite fiber, which was then treated with iodine vapor to combine iodine with PVP to produce the PVP-I complexes. As a result, the PVP-I appeared on the fiber surface to endow the non-woven mat with
water absorbability, antimicrobial activity, and adhe-sive ability. This design was believed to have potential applications in the initial stage of dressing cankerous or contaminated wounds. PLA is also frequently used in combination with other polymer to fabricate hybrid nanofibrous dressings [30,46].
Other synthetic polymers that have been utilized in the development of nanofibrous wound dress-ings include the non-biodegradable polyurethane [15,47,48] and the biodegradable poly(ethylene oxide) (PEO) [49,50]. Copolymers are also used to obtain synergy effects from two different synthetic polymers. For example, poly(lactide-co-glycolide) (PLGA) was investigated as an antibiotic-loaded nanofiber mat for wound care [51]. Electrospun poly(vinyl pyrrolidone) (PVP) and poly(ethylene oxide)(PEO)/PVP copoly-meric nanofibers with complex-bound iodine were developed for antibacterial wound dressings [49,50]. Generally speaking, synthetic polymers have better mechanical strength than natural polymers, and allow researchers more flexibility in the design and develop-ment of new products. However, it is always a chal-lenge to minimize cytotoxicity in such products.
Evaluation of electrospun nanofibers for wound dressings
In vitro models
Anti-inflammation, biocompatibility and wound healing promotion are among the desired properties of wound dressing materials. In vitro tests are usually employed to evaluate the antibacterial activity and cytotoxicity of these materials:
A variety of antibacterial tests have been reported to demonstrate the antibacterial activities of modified or drug-loaded nanofibrous dressings. The Kirby-Bauer disc method (or disk diffusion method) provides a straight forward means to assess the susceptibility of bacteria to antibiotic-loaded nanofibrous membranes [11,44]. In the Kirby-Bauer test, the sample disks that had been cut from nanofibrous mats were placed on agar plates streaked with bacterial (for example, S. aureus and E. coli) suspension and incubated in an incubator at 37°C. After a certain period of time, the inhibitory effect of each sample disk is evaluated by referring to the diameter of an area of clearing where growth of bacteria is not observed (known as the inhibition zone). The viable cell-counting method gives bacterial inhibition efficiency of antibacterial fibrous mats [8,31]. With this method, a preculture of bacteria is grown overnight and then diluted with fresh media to a stand concentration for antibacte-rial testing. Nanofibrous samples are cut, sterilized
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Nanofibrous materials for wound care 147
and exposed to the bacteria suspension. At a specific time, the surviving bacteria are counted by the spread plate method. A similar quantitative method is used to determine the antimicrobial activity of the specimen by shaking samples in a concentrated bacterial sus-pension (in dynamic contact conditions) for a certain contact time [52].
The MTT (3-[4,5-di-methylthiazol-2-yl]-2,5- diphenyl tetrazolium bromide) assay is the most frequently used method for evaluating cell viability. Human or animal dermal fibroblasts (for example: 3T3, a mouse embryonic fibroblast cell line; HFF-1, a human foreskin fibroblast cell line) or keratinocytes are usually used to assess the cytotoxicity of a nanofibrous dressing mate-rial. Nanofibers [14,32,42] or nanofiber extracts [8,28] are placed into multi-well plate for cell culture. After a certain period of time, cells are cultured with MTT. The purple formazan crystals so formed are then dissolved in dimethyl sulfoxide (DMSO), and the absorbance can be measured with a microplate spectrophotometer to give the cell viability percentages. The MTS (3-[4,5-dimethylthiazol-2-yl]-5-[3-carboxymethoxyphenyl]-2-[4-sulfophenyl]-2H-tetrazolium) assay is similar to the MTT method except that the product is water soluble, so that no organic solvent is required [16].
Cell adhesion onto nanofibrous mats can be evalu-ated by seeding fibroblasts from a suspension onto the samples. After culturing for a certain period time, the fiber mats are harvested, and are rinsed to remove the non-adherent cells so as to fix the adherent cells. After that, the cells are dyhydrated and observed under a microscope [32]. Cell morphology/proliferation for fibroblasts cultured on the nanofibrous mats can also be observed under a microscope [14,15]. Histological analyses are also used to examine the interaction between nanofibers and cells [7].
For the evaluation of wound healing properties of dressings, Schneider et al. [18] used a human skin-equivalent wound healing model that mimics the structure of human skin and is able to heal by means of the same molecular and cellular mechanisms found in vivo. This skin-equivalent was produced by seed-ing both human dermal fibroblast and normal human keratinocytes into a collagen gel.
In vivo models
Animal models, usually rat or mouse, are used in the evaluation of wound dressings. Usually, a full thick-ness of wound with a small surface area is cut from the back of the animal. Then the wound dressing materi-als are transplanted to the wounded area. At certain intervals, the wound healing can be assessed by refer-ring to the percentage of wound healing (B/A, where
A is the initial wound area and B is the wound area after treatment with dressings) [28]. Photographs and computer software may be used to analyze the wound area [7,35]. Diabetic mice or mice with burning wound were created to study the effect of nanofibrous wound dressings on specific wound types [41]. Other assays or evaluations that were performed for in vivo models include immunohistochemistry assay, quantification of cell proliferation and migration, etc. [53].
Cytotoxicity of nanofibrous materials for wound care
Cytotoxicity is an important parameter in the evalua-tion of nanofibrous materials for wound care, because wound dressings are expected to accelerate wound healing rather than causing extra damages or reactions in the wound sites. Cytotoxicity of the materials may be caused by the fiber materials themselves, or by the additives that have been added during the fabrication process of the dressing materials.
Natural polymers have been known for their good biocompatibility and low cytotoxicity. For example, collagen is a major structural protein of the extra-cellular matrix in animals and has been regarded as a material with excellent biocompatibility [6,7]. The biocompatibility or cytotoxicity of collagen may be affected by its purity, and is related to its sources and the additives used in the fabrication, however. To date, there has been no standardized method for purification of collagen, and the presence of non-collagenous impurities may lead to immunogenic responses to collagen [54]. Bovine dermal is among the major sources for the extraction of collagen, and there have been reports about the presence of bovine collagen allergy in a small percent of people (less than 5%) [55]. Clinical use of bovine dermal collagen for wound care has been reported to induce no adverse reactions [56–59], but may do so in its more invasive applications, e.g., in the injectable soft tissue augmen-tation for cosmetic treatment [55,60,61]. Crosslinking is a procedure often used to enhance the mechanical strength of collagen materials. Glutaraldehyde (GTA) vapor is a chemical crosslinking agent that has been used to introduce a high level of crosslinking into the electrospun collagen fibers. It may, nevertheless, strongly influence cytotoxicity of the material due to the adverse reactions caused by residual and revers-ible fixation [62]. It has recently been suggested that the zero-length crosslinking agents EDC (1-Ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride)/ NHS (N-hydroxysuccinimide) and the biocatalyst (Transglutaminases) may be the best treatments for collagen crosslinking [63].
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For gelatin, a water soluble denatured collagen, it is essential to have it crosslinked so as to maintain its sta-bility in its wound care end uses. The choice of crosslin-ing agents again becomes a question. Glutaraldehyde is known for its cytotoxicity at high concentrations [64]. Glyceraldehyde and genipin have been shown to be good crosslinking agents for their low toxicity and reliability [65].
Silk is another natural material that has been used in biomedical products (like sutures) for centuries. During the last two decades, there were concerns about the silk-related biocompatibility [66]. However, more recent studies suggested that sericin proteins at the silk surface are the major cause of the biocompat-ibility problem, while fibroin fibers at the core have good biocompatibility [17,67]. As a result, electrospin-ning has been found to be a useful method to regener-ate biocompatible silk fibroin fibers from raw silk for biomedical end uses including wound care [18,68].
Derived from crab and shrimp shells, chitin and chitosan have been regarded as biocompatible and antibacterial, and therefore are good materials for wound care [27]. Some researchers further investigated the effect of molecular weight and the use of solvent on the cytotoxicity of chitosan. As a result, chitosans of a lower molecular weight (MW) were found to exhibit a smaller toxic effect than chitosans of a higher MW on human keratinocyte cells in vitro [69], and chitosan films prepared in lactic acid were shown to induce a lower level of skin irritancy than chitosan films pre-pared in acetic acid through contact with abraded and intact skin of rabbits [70]. Another in vivo study suggested that biocompatibility of chitosan scaffolds as dermal replacements can be variable depending on the deacetylated degree as well as on animal species [71]. From revelations that are not altogether consist-ent, it follows that further studies on the toxic effect of chitosans on human skins are necessary.
Cellulose or its derivatives have been used in wound dressing for a long time. A comparative study on cellulose and its derivatives showed that their in vivo tissue biocompatibilities are dependent on the degree of crystallinity and chemical structure of the sample [72], but the tissue reaction is relatively low for all samples, suggesting that cellulose may come to exhibit good biocompatibility through physical or chemical treatment [72,73].
Also for their biocompatibility or low cytotoxicity, a wide variety of synthetic polymers have found uses in the development of nanofibrous wound dressings, as described in previous sections. Cytotoxicity assay was conducted in most of the reported researches. And dif-ferent synthetic or natural polymers have been used in combination to enhance the function or safety of the wound dressing materials produced therefrom. However,
it is desirable that: (1) further studies, comprehensive in nature, should be had so as to compare the degrees of biocompatibility or cytotoxicity relating these polymers, and (2) new cytotoxicity evaluation approaches, in addi-tion to those conducted on animal models, should be developed so as to make it possible for clinical trials to be practiced on human subjects.
Concluding comments
A new generation of wound care dressings has been developed to provide a better wound healing environ-ment. Electrospun nanofibers are excellent materials for wound care because of their high porosity, soft-ness and surface-to-volume ratios. A wide variety of polymer materials, including both natural and syn-thetic polymers, have been used in the development of nanofibrous dressings. Generally speaking, natural polymers have high biocompatibility and low cytotox-icity. However, most natural polymers exhibit relatively low stiffness and mechanical strength. Accordingly, they are usually crosslinked or blended with synthetic polymers so as to somewhat affect their biocompatibil-ity. On the other hand, synthetic polymers have better mechanical strength than natural polymers, and allow researchers more flexibility in the design and develop-ment of new products. However, it is always a challenge to minimize cytotoxicity in such products. Different therapeutic agents, such as antibacterial agents and growth factors, were incorporated into the nanofibrous dressings to enhance their functions in wound care. In vitro and in vivo methods were adopted to evaluate the performance of these functionalized dressings, includ-ing their cytotoxicity. Their eventual clinical adoption will be awaited until controlled efficacy and safety/cytotoxicity evaluations.
Declaration of interest
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
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Nanofibrous materials for wound care 151
Appendix
Literacy summary tableTopics Results/comments ReferenceIntroductionwound concepts [1]Wound dressing Reviews on wound dressings [2,3]Electrospinning Reviews on electrospinning [4,5]Materials for wound dressingNatural polymersCollagen Biocompatible and biodegradable natural polymer that exists abundantly
in connective tissue in animals. Antibacterial agents can be incorporated.[6–9]
Gelatin Denatured collagen. Biocompatible and biodegradable. Ag and herbal extracts were incorporated to provide antibacterial functions.
[10–13]
Gelatin used in combination with synthetic polymers. [14–16]Silk Regenerated from cocoons of silkworm. Biocompatible and biodegradable.
Silver nanoparticles and growth factors were incorporated into silk nanofibers for antibacterial and accelerating wound healing.
[17–19]
Hyaluronic acid (HA) A natural material existed in extracellular matrix. Biocompatible and biodegradable. Quickly dissolve or decompose in water; needs crosslinking to increase its stability.
[20,21]
Cellulose or cellulose acetate
Silver nanoparticles or herbal extracts were incorporated to provide antibacterial functions.
[22–26]
Chitosan Derived from crab and shrimp shells. Biocompatible, biodegradable and antibacterial. Modified chitosan or chitosan derivatives were developed for wound dressings. Often used in combination with other polymers.
[27–32]
Other natural polymers Poly-N-Acetylglucosamine, alginate-based materials, poly(3-hydroxybutyrate- co-3-hydroxyvalerate (PHBV), etc.
[33–35]
Synthetic polymersPoly(vinyl alcohol) (PVA) Non-biodegradable synthetic polymer traditionally used in wound dressings.
Ag was loaded as antibacterial agent. Often used in combination with natural or other synthetic polymers.
[31,32,36–39]
Poly(caprolactone) (PCL) Biocompatible and biodegradable. Antibacterial agents and growth factors were incorporated into PCL nanofibers to accelerate wound healing. Also used in combination with other polymers.
[16,40–42]
Poly(lactide) (PLA) Biocompatible and biodegradable. Anesthetic, antibiotic, and other therapeutic agents were incorporated.
[30,43–46]
Other synthetic polymers Non-biodegradable polyurethane, biodegradable poly(ethylene oxide) (PEO) and poly(lactide-co-glycolide) (PLGA), etc.
[15,47–51]
Evaluations of Electrospun nanofibers for wound dressingsIn vitroAntibacterial tests Kirby-Bauer disc method (qualitative), viable cell-counting method (quantitative). [8,11,31,44, 52]Cell viability test (MTT or MTS)
Human or animal dermal fibroblasts were used to assess cytotoxicity of a nanofibrous dressing material.
[8,14,16,28,32,42]
Cell adhesion /morphology/proliferation
Fibroblasts that cultured on dressings materials were examined. [7,14,15,32]
Wound healing properties A human skin-equivalent wound healing model that mimic the structure of human skin.
[18]
In vivoAnimal models Rats or mice were used. Wound healing can be assessed by percentage of
wound healing, immunohistochemistry assay, quantification of cell proliferation and migration, etc.
[7,28,35,41,53]
Cytotoxicity of nanofiber materials for wound care collagen A biomaterial with excellent biocompatibility. The biocompatibility or cytotoxicity of
collagen may be affected by its purity, and is related to its sources and the additives used in the fabrication.
[6,7,54–63]
Gelatin Cytotoxicity may be affected by crosslinking agents. [64,65]
Appendix continued on next page
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Topics Results/comments ReferenceSilk The sericin proteins at the silk surface are the major cause of the biocompatibility
problem, while fibroin fibers at the core have good biocompatibility.[17,18,66–68]
Chitin/chitosan Cytotoxicity may be affected by molecular weight, solvent or deacetylated degree. [27,69–71]Cellulose Cytotoxicity is dependent on the degree of crystallinity and chemical structure of the
sample.[72,73]
Appendix Continued.
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