Wound dressings are designed to protect wounds from further damage and promote healing. Most wounds can be classified as epidermal, superficial partial-thickness, deep partial-thickness, or full-thickness. Treatment approaches differ according to the depth of the damaged skin layers.1
The standard therapy to treat superficial partial-thickness wounds often involves the application of epidermal substitutes. These products may consist of synthetic material and have to be removed in the course of wound healing.2 For example, the temporary, synthetic wound dressing Suprathel (PolyMedics Innovations, Denkendorf, Germany), is effective in treating partial-thickness burns and split-thickness skin graft donor sites.3 This common wound dressing is an absorptive, synthetic wound cover that consists of a copolymer foil of D,L-lactidetrimethylenecarbonate and caprolactone. It offers high plasticity with an immediate adaptability to the wound bed at body temperature; moisture permeability prevents the accumulation of wound exudate.
In contrast, biologic substances such as collagen and gelatin are often used in the production of wound dressings. They can be produced in a three-dimensional matrix, which allows cells to migrate into the scaffold and promote wound healing. Importantly, wound dressings made of these materials do not have to be removed.
A natural biopolymer, collagen has the advantage of low toxicity and a low chronic inflammatory response (in contrast to synthetic products).4 In the past, it has been shown that collagen-derived wound dressings can be directly involved in cellular interactions as hemostatic and chemotactic stimuli and serve as a support structure for cells and blood vessels that enhance the building of a neodermis.5,6 Gelatin is a derivative of collagen, which is obtained by controlled hydrolysis.7 Both biopolymers have similar, including biocompatibility, biodegradability, and a lack of antigenicity.
This study examined a novel gelatin-collagen nonwoven scaffold (Freudenberg Group New Technologies SE & Co KG, Weinheim, Germany) and its effect on the healing of superficial dermal wounds in comparison with a synthetic dressing (Suprathel) and control wounds, which were treated with a semipermeable, sterile Opsite foil.
MATERIALS AND METHODS
The experiments were performed on six female Göttingen minipigs (A/S, Dalmose, Denmark). All animals were treated according to the German Law on the Protection of Animals, and the study was performed with permission from the Baden-Württemberg Animal Welfare Committee. The minipigs weighed 25.8 (±2.5) kg and were an average age of 47 weeks (±8 days) old. They had access to water ad libitum and were fed 400 g of a standard minipig diet (SDS SMP; Special Diets Services, Witham, Essex, United Kingdom) per day.
After shaving and sterilizing the pigs’ flanks, three squares measuring 2.4 cm in length were marked on one side of each minipig and labeled according to the planned treatment. Three superficial wounds with a depth of 0.5 mm were created under anesthesia on the flank of each pig using a skin dermatome. The standard distance between wounds measured 2.0 cm to avoid cross-contamination. The side and position of the treated and untreated wounds were randomized. Immediately after wounding, the biologic and synthetic wound dressings were placed on two of the three wounds, respectively. The third wound functioned as an untreated control. A semipermeable, sterile Opsite foil was placed covering all wounds to prevent wound infection and dislocation of the different wound dressings (if applicable).
The animals were conditioned to wear custom-made swine jackets (Ellegaard Minipig Jacket Large Full Body; Lomir Biomedical Inc, Notre Dame de L'Ille Perrot, Quebec, Canada) for postoperative bandage protection. These jackets were placed over the wound bandages and secured with Fixomull stretch (BSN Medical GmbH, Hamburg, Germany) in the front and the back of the minipig’s dorsum and around the forelegs to prevent slipping.
The Nonwoven Scaffold
The novel biologic gelatin-collagen nonwoven scaffold was developed in cooperation with the Freudenberg Group New Technologies SE & Co KG. Based on a compound of collagen and gelatin (>90%), it is produced through an industrialized spinning procedure leading to bimodally distributed fibers with a diameter of approximately 2 and 10 μm and a pore size of 35 to 70 μm. The gelatin and collagen make the noncrosslinked nonwoven scaffold bioresorbable. After coming into contact with the wound fluid, the gelatin dissolves and mixes with the wound fluid.
After an experimental period of 7 days, the elasticity of new epidermis was evaluated using the Cutometer MPA 580 (Courage & Khazaka Electronic GmbH, Cologne, Germany). The Cutometer is an electronic instrument that assesses skin elasticity based on a suction and elongation measuring principle. This device generates negative pressure, which draws the skin into a hollow aperture in the center of a probe and estimates skin penetration depth with an optical measuring system. For the current study, a vacuum of 450 mbar over 2 seconds with a 6-mm aperture diameter on the handheld probe and a 2-second relaxation time was chosen.
Biopsies for histologic evaluation were taken after 7 days. Five-micrometer-thick paraffin-fixed slides were prepared and stained with hematoxylin and eosin. Epidermal thickness and the total number of epidermal cells within a section of 100-m width in the wound’s center were evaluated.
The collected data of each group were examined using nonparametric methods. After analysis of regression and variance, P < .05 was considered statistically significant.
Within the experimental period of 7 days, complete wound healing of all superficial wounds could be detected.
Mean Epidermal Thickness
Histologic analysis showed an increased mean epidermal thickness of wounds treated with the biologic dressing (31.38 m, P = .0010) and the synthetic dressing (22.78 m, P = .8395) compared with the untreated wounds (22.30 m). The difference between the untreated wounds and those treated with the biologic dressing was statistically significant (P < .05; Figure 1).
Epidermal Cell Count
The mean epidermal cell count revealed a higher number of cells in wounds treated with the biologic dressing (76.17 cells, P = .0382) and the synthetic dressing (55.83 cells, P = .0950) compared with untreated wounds (46.33 cells). The difference in epidermal cell count between the untreated wounds and those treated with the biologic dressing was also statistically significant (P < .05; Figure 2).
Skin elasticity was determined using Cutometer absolute parameters Uf (elastic deformation), Ur (elastic recovery), and Ua (total recovery), as well as relative parameters such as Ur/Ue (elastic function), Ur/Uf (gross elasticity), Ua/Uf (biologic elasticity), and Uv/Ue (viscoelastic ratio; Table).
There were no statistically significant differences among the two different treatments and the control group in terms of skin elasticity. Nevertheless, the following trends were visible: wounds treated with the nonwoven scaffold showed better results than wounds treated with the synthetic dressing in terms of elastic deformation, elastic function, and total recovery; whereas superficial skin defects treated with the synthetic dressing showed better biologic elasticity and a higher viscoelastic ratio and gross elasticity. The untreated wounds showed better results than both treated wounds in terms of elastic deformation and total recovery (Figures 3 and 4).
For wound healing studies, pigs are a preferred animal model because of their large body surface and similarities to human skin. The epidermal thickness, for instance, is very similar between humans (50–120 μm) and pigs (30–140 μm), as well as the relative thickness of the skin.8 Further, the skin architecture and physiology are alike.9 The Göttingen minipig was especially interesting because they are often used in research8,10–14 and have multiple advantages compared with domestic pigs: they are small, easy to handle, and genetically and phenotypically controlled.8 Especially interesting is their nonpigmented skin, which simplifies wound observations. Disadvantages include high costs for acquisition and upkeep.
The histologic evaluation of all wounds treated with the biologic nonwoven dressing yielded better results regarding the number of epidermal cells and the epidermal thickness compared with the wounds treated with the synthetic dressing. These results emphasize that the dissolved gelatin on the wound enhances the activation of macrophages and has hemostatic effects, as well as accelerating and improving wound healing and tissue regeneration.15–17 A thicker epidermis (ranging between 30 and 140 μm, the range of intact epidermis) leads to a more stable epidermis. Gelatin can be produced more cheaply than collagen,7,18,19 which makes this combined product interesting.7 Further, the nonwoven scaffold, because it is a biologic product, can remain on the wound and does not have to be removed like synthetic dressings, making the wound dressing easier to apply in a clinical setting.
Unfortunately, the Cutometer measurements do not show such clear results. This might be attributable to the fact that skin elasticity and its effects on the dermis in terms of scar formation are best detected during long-term follow-up because scar formation can take up to 2 years.20,21 However, the hypothesis that injured epidermal structures influence the dermis and play an important role in scarring is supported by Mustoe and Gurjala.22 Moreover, the efficiency of silicone gel occlusion on scar reduction and prophylaxis23 indicates that the tested wound dressings may influence not only the epidermal thickness and cell count but also long-term dermal healing.
The histologic findings regarding epidermal thickness and the number of epidermal cells reveal significant support for the use of the biologic nonwoven scaffold. Further, the biologic product can remain on the wound and does not have to be removed like synthetic products, simplifying the handling in the clinical setting and reducing patient pain related to dressing changes.
Because the Cutometer measurements did not show definitive results after 7 days, the effect of the nonwoven dressing on skin elasticity and scar formation should be evaluated in a long-term follow-up study to gain further knowledge regarding skin repair. Based on these findings, the nonwoven scaffold could be a promising material to promote epidermal wound healing and also be useful for the treatment of split-thickness skin graft donor sites.
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Keywords:Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.
collagen; gelatin; minipig; scaffold; superficial wounds; Suprathel; wound healing