Biomechanical Skin Property Evaluation for Wounds Treated With Synthetic and Biosynthetic Wound Dressings and a Newly Developed Collagen Matrix During Healing of Superficial Skin Defects in a Rat Model

Introduction. There is a high prevalence of superficial wounds such as partial-thickness burns. Treatment of these wounds frequently includes temporary application of wound dressings. The aim of this study was to compare a newly developed collagen matrix with commonly used temporary skin dressings for treatment of partial-thickness skin defects. Materials and Methods. Through a skin dermatome, 42 standardized superficial skin defects were generated on the back of 28 adult male Lewis rats. The wounds were treated with a synthetic wound dressing (Suprathel, Polymedics Innovations Inc, Woodstock, GA) (n = 14), a biosynthetic skin dressing (Biobrane, Smith & Nephew, Hull, UK) (n = 14), or a newly developed bovine collagen matrix, Collagen Cell Carrier (Viscofan BioEngineering, Weinheim, Germany) (n = 14). Biomechanical properties of the skin were determined and compared every 10 days over a 3-month period of using the Cutometer MPA 580 (Courage + Khazaka Electronic GmbH, Cologne, Germany). Results. As opposed to healthy skin, statistically significant differences were detected between days 10 and 30, and between days 60 and 80, for calculated elasticity (Ue), firmness of skin (R0), and overall elasticity (R8). After 3 months, no statistically significant differences in skin elasticity were detected between the different wound dressings. Conclusions. The presented results give an opportunity to compare the wound dressings used for treatment with respect to skin elasticity and reveal the potential of the bovine collagen matrix in the treatment of superficial skin defects; therefore the results facilitate further evaluation of collagen matrix in surgical applications and regenerative medicine.

The incidence of acute superficial skin defects, such as partial-thickness burn wounds, is high. Extended superficial and superficial-partial skin defects were usually treated with the application of temporary wound dressings.^1-3 These dressings imitate important features of natural skin⁴ and are meant to protect the wound, accelerate wound healing, reduce pain, and improve the functional and aesthetic outcomes;^5,6 however, the choice of the most suitable wound dressing remains a daily challenge in outpatient clinics. 

In addition to synthetic temporary wound dressings, biological substances such as collagen are frequently used in the production of wound dressings.⁷ Manufactured collagen scaffolds play an important role in tissue repair applications in regenerative medicine.^8-11 Specific requirements such as mechanical stability, biocompatibility, and biodegradation have made collagen type I one of the most widely used natural polymers in the fields of biomedicine and tissue engineering.^12,13 

The biosynthetic skin dressing (Biobrane, Smith & Nephew, Hull, UK) and the synthetic wound dressing (Suprathel, Polymedics Innovations Inc, Woodstock, GA) are 2 commonly used skin dressings. The biosynthetic dressing consists of an inner porcine collagen type I and an outer silicone layer,¹⁴ while the synthetic dressing consists of a copolymer-foil.⁴ The newly developed collagen matrix (Collagen Cell Carrier, Viscofan BioEngineering, Weinheim, Germany) is based on bovine collagen type I, and previous studies^15,16 demonstrated its in vitro and in vivo biocompatibility. 

In a previous study,¹⁷ the authors evaluated the alteration of biomechanical properties of skin during healing of untreated partial-thickness wounds. The aim of this study was to compare different wound dressings with regard to skin elasticity.

In this context, the authors evaluated the effect of commonly used, temporary wound dressings and the newly developed collagen matrix with regard to the alteration of biomechanical skin properties in the course of healing of superficial wounds.

Animals
Twenty-eight adult male Lewis rats (Char-les River, Erkrath, Germany) were used in this study. The initial mean body weight was 280 ± 12 g. The experiment was approved by the local animal committee and research committee.

Wound dressings
The biosynthetic skin dressing is composed of an inner collagen type I layer and an outer silicone layer. This semipermeable wound dressing allows wound fluid to pass through the membrane and remain on the wound until complete reepithelialization.¹⁴ The synthetic wound dressing consists of a copolymer from polylactide, trimethylene carbonate, and ε-caprolactone. It is absorptive and offers high plasticity with an immediate adaption to the wound bed at body temperature.⁴ The collagen matrix is a novel, thin, mechanically stable and chemically noncrosslinked fibrillar bovine collagen type I scaffold.^15,16 

Wounding procedure
Surgery was performed under narcosis, using intraperitoneal anesthesia.¹⁸ The back of each rat was carefuly shaved, and 2 superficial wounds 2 cm x 2.5 cm were created on the back of each rat in a standardized manner using a skin dermatome,¹⁹ with a set depth of 0.3 mm. Afterwards, all rats were randomly divided into 2 groups. In the first group (n = 14), one wound was covered with the biosynthetic dressing and the other with the synthetic dressing. In the second group (n = 14), the wound was covered with the collagen matrix (Figure 1). All wound dressings were fixed with a stapler, and they were left on the wound until the dressings detached themselves from the surface. These values were compared to untreated wounds from the authors’ previous study¹⁷ and normal, nonwounded skin. 

Measuring apparatus
The biomechanical properties of the skin were determined using the Cutometer MPA 580 (Courage + Khazaka Electronic GmbH, Cologne, Germany)^20,21 skin measurement device, which consists of a main unit and a handheld probe with a 6 mm suction head (Figure 2). The probe applies a controlled vacuum and measures skin deformation. For the measurements in this study, the time-strain mode (Modus 1) was used with an application of 300 mbar load for 3 seconds (on-time), followed by a relaxation time of another 3 seconds (off-time). The depth of skin penetration into the opening of the central suction head was measured using an optical system. For skin elasticity analyses, the following parameters were considered:

• R0 = Uf: maximal skin extension; firmness of skin (represents the passive behavior of the skin to force). Lower values represent higher firmness.
• R8 = Ua: pliability; overall elasticity (complete relaxation after the pressure is cut off); with an approximation toward R0, elasticity increases.
• Ue = (R7 x R0) / R5: elasticity; later also “calculated elasticity” (this value is not displayed numerically by the device).
• Measurements were performed through the same investigator and under the same environmental conditions after 10, 20, 30, 40, 50, 60, 70, 80, and 84 days. As a reference value, normal, nonwounded skin was measured apart from the wound. 

Statistical analysis
For statistical analysis, the Wilcoxon rank-sum test (Mann–Whitney U test) was used to compare values. Statistical significance was set at 5% (P ≤ 0.05). The analysis was performed using SPSS software version 20.0 (IBM, Armonk, NY). 

At a macroscopic level, complete wound healing of all wounds could be detected within the experimental period.

Biomechanical properties of skin
During wound healing, the alteration of biomechanical skin properties was evaluated using the skin measurement device. Evaluations took place every 10 days from day 10 to day 80 and additionally on day 84. At the first measurement (day 10), all wound dressings had detached themselves from the wound surface. 

Compared to healthy skin, calculated elasticity (Ue), firmness of skin (R0), and overall elasticity (R8) showed an initial decrease in elastic parameters 10 days after surgery for all wounds. Between days 20 and 30, skin elasticity parameters increased, particularly in wounds covered with the biosynthetic dressing or the synthetic dressing. Elasticity parameters in wounds covered with the collagen matrix demonstrated a delayed increase and delayed maximum when compared to the synthetic and biosynthetic dressing. The maximum elasticity was reached on day 30 for the synthetic and biosynthetic dressings and on day 40 for the collagen dressing. After reaching the maximum, skin elasticity values tended to decrease slowly in the later course of wound healing. Highest values were reached for the biosynthetic dressing regarding R8 and Ue, and for the synthetic dressing regarding R0 (Figure 3, Figure 4). 

Compared to healthy skin, statistically significant differences were detected between days 10-30 and days 60-80 (Figure 5). After 3 months, no statistically significant differences in skin elasticity were detected between the different wound dressings. Figure 6 demonstrates the changes in Ue during healing of wounds treated with the synthetic dressing, biosynthetic dressing, and collagen matrix, as well as untreated wounds. 

This study compared 3 different wound dressings (a synthetic dressing, a biosynthetic dressing, and a novel collagen matrix) in the treatment of partial-thickness skin defects. The authors specifically examined skin elasticity in the course of wound healing by means of a skin measurement device, and they were able to demonstrate the complex changes in the biomechanical properties of the skin during the course of healing these wounds and to show there were no statistically significant changes 3 months after wounding.

A change in the biomechanical properties of skin is a function of tissue structures. It is well known the Uf and Ue parameters are related to the stretching of elastic and collagen fibers.²² These values are dependent on alterations in the ground substance of the dermis.²³ The measuring of changes in the biomechanical skin properties provides objective data of skin pliability, thereby permitting an accurate insight into functional outcomes.^21,24 So far, changes in biomechanical skin properties have been well described for scars and keloids.^24,25 Moreover, it is known that cutaneous scarring depends on wound treatment modalities²⁶; therefore, the evaluation of these values may improve the understanding of wounds and also allow early comparisons of different wound dressings.

Owing to the creation of partial-thickness wounds, an initial increase was observed in all elastic parameters on day 10. Between days 20 and 30, wounds treated with the synthetic and biosynthetic dressings demonstrated a remarkable increase in elasticity. This may be due to the increasing depth of skin within the scope of wound healing. However, between days 30 and 40, these values notably decreased. These changes may have resulted from the acceleration of scar formation, hence the increase in skin rigidity. 

The skin measurement device displays 5 parameters: Uf, Ua, Ur (retraction), Ue, and Uv (visco-elasticity). Draaijers and coauthors²² found these values were highly correlated to each other and, therefore, they suggested the use of 1 parameter is sufficient for the evaluation of elasticity of scars.²² However, Ue, R0, and R8 were generally used in skin elasticity analysis because they were assumed to be the most representative skin measurement device values for skin elasticity.^22,27 

In a previous study,¹⁷ the authors of the current paper compared these wound dressings with the quality of newly formed skin, the epidermal cell count, and the thickness of skin in a rat model. The wounds treated with the biosynthetic and synthetic dressings demonstrated comparable values, whereas the wounds treated with the novel collagen matrix demonstrated higher epidermal cell count and thicker neoepidermis. The epidermal thickness of the collagen matrix bore comparison with healthy skin.

The biosynthetic and synthetic wound dressings used in this study are both clinically proven to be efficient in the treatment of superficial skin defects at donor sites after the harvesting of split-thickness skin grafts in ulcers, and particularly in burn injuries.^14,28 Another study²⁹ has already demonstrated that after 3 months there are no statistically significant differences in skin elasticity between wounds treated with the biosynthetic dressing and those treated with the synthetic dressing. Wisser and colleagues³⁰ found the use of temporary skin dressings during wound healing can reduce pain and limit the functional and cosmetic consequences of scarring.

The newly developed collagen matrix is a 20 µm thin planar scaffold based on fibrillar and bovine collagen type I. It was extensively studied in previous in vitro experiments and demonstrated excellent biocompatibility as well as mechanical characteristics and cell biological properties in cell culture.¹⁶ This biomaterial is chemically noncrosslinked and exhibits a high stability with similar mechanical properties to crosslinked scaffolds containing tendon fiber.¹⁶ In previous in vivo experiments, the collagen type I-based biomaterial also demonstrated high biocompatibility, low irritation of tissue, complete resorption, and replacement by autologous tissues.¹⁵ The novel collagen matrix sheets can also be glued and sewed to adjacent tissue. These sheets exhibit certain deformability and therefore adapt easily to the surface of the host tissue; and due to low material thickness, the transparency of this biomaterial allows for the inspection of the underlying tissue during grafting.

The findings of this study help to compare the wound dressings utilized with respect to skin elasticity. Statistically significant changes could be detected only during healing; however, after 3 months no statistically significant differences were detected between the different wound dressings. Hence, the new collagen matrix could be an interesting biomaterial for promoting epidermal wound healing and could be useful in the treatment of other applications such as split-thickness skin graft donor sites. This study provides a basis for further evaluation of the new scaffold for biomedical and clinical applications in the fields of tissue engineering and regenerative medicine.

Affiliations: BG-Trauma Center, Eberhard Karls University, Department of Plastic and Hand Surgery, Reconstructive and Aesthetic Surgery, Tuebingen, Germany; and HELIOS Klinikum Wuppertal GmbH, University Hospital Witten/Herdecke, Germany, Department of Plastic and Hand Surgery, Reconstructive and Aesthetic Surgery, Wuppertal, Germany

Correspondence:
Manuel Held, MD
Department of Plastic, Reconstructive, Hand and Burn Surgery 
BG-Trauma Center 
Schnarrenbergstr. 95
72076 Tuebingen, Germany
ManuelHeld@hotmail.com

Disclosure: Support was provided solely from institutional and/or departmental sources. the Institute of Anatomy (Tuebingen) provided animal housing and BG-Trauma Center provided the material. The authors have no financial or other conflicts of interest.