The ideal dressing material is bio-inert andkeeps the wound site moist. It is equally importantthat no regenerative tissue is peeled off on theremoval of the dressing. 2-Methacryloyloxyethylphosphorylcholine (MPC) has a phospholipidpolar group that mimics a biomembrane. We pre-pared poly [MPC-co-n-dodecyl methacrylate(DMA)] (PMD), using conventional radical poly-merization with 2,2’-azobisisobutyronitrile as an ini-tiator, and coated it on polyurethane (PU;Tecoflex� 60 Thermedics Inc.) membrane. Full-thickness surgical wounds were made on the dor-sal skin of rats and wound healing was comparedunder the following three conditions: air-exposedcontrol (no dressing), PU dressing, and PMDdressing. At 3, 4 and 7 days after the operation, thewound sizes of the PMD dressings were smallerthan the non-dressed wound, and at 6 and 7 daysafter the operation, the wound sizes of PU dressingwere smaller than that of the air-exposed group.But there were no significant difference betweenthe PMD dressing group and PU dressing group.Histologically, scab formation was not observed on

the PU or PMD-dressed wounds. However, in theair-exposed control, a scab was formed and re-epithelialization of the wound site was prevented.Additionally, no damage was observed in the his-tological section of PMD dressed wound after thewound was cured. These results indicate thatPMD dressing (PMD-coated PU membrane) hasthe potential to provide an inert environment forwound healing as well as PU.

Key words: dressing, wound healing, bio-inert,moist, MPC

Introduction

Several materials are currently used for wounddressing. Although myths still remain concerningwound healing, the factors, which affect healing, havebeen identified. Humidity, temperature and oxygentension of the wound site affect healing1-5. Althoughremedies for wounds depend on their type, degree andsite, theoretically, if a wound dressing protects thewound site and maintains ideal conditions for healing,wound healing will be promoted. Infection of thewound should obviously be avoided, and it is nowaccepted that dryness at the wound site inhibits heal-ing. Indeed, in a wet environment, wounds healsmoothly, and the newly formed epidermis is of goodquality1-5. Thus, dressing material is required not only to

Original Article

Evaluation of 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer-coateddressing on surgical wounds

Osamu Katakura1,3, Nobuyuki Morimoto2,3, Yasuhiko Iwasaki2,3, Kazunari Akiyoshi2,3

and Shohei Kasugai1,3

1) Oral Implantology and Regenerative Dental Medicine, Graduate school, Tokyo Medical and DentalUniversity2) Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University3) Center of Excellence Program, Frontier Research on Molecular Destruction and Reconstruction of Toothand Bone, Tokyo Medical and Dental University

J Med Dent Sci 2005; 52: 115–121

Corresponding Author: Osamu KatakuraOral Implantology and Regenerative Dental Medicine, GraduateSchool, Tokyo Medical and Dental University, 1-5-45, Yushima,Bunkyo-ku, Tokyo 113-8549, JapanTel: +81-3-5803-4664 Fax: +81-3-5803-5934E-mail: ok.mfc@tmd.ac.jpReceived December 6, 2004; Accepted March 18, 2005

be bio-inert but also to seal the wound and prevent des-iccation. It is also apparent that various cells andendogenous growth factors at the wound site play animportant role in the healing process4,5. The dressingmaterial should be bio-inert and protect the wound siteand should ideally possess the ability to maintain opti-mal conditions at the wound site, consequently accel-erating the healing process.

On the other hand, 2-methacryloyloxyethyl phos-phorylcholine (MPC) is a synthetic molecule, which hasphosphorylcholine in its structure that mimics the cel-lular surface6-24. Poly [MPC-co-n-dodecyl methacry-late (DMA)] (PMD) is an MPC polymer with the follow-ing unique characteristics. Protein absorption onto thePMD surface is negligible, and although fibroblasts arenot able to attach to the PMD sheet, fibroblasts main-tain their proliferative capacity while in contact with it7-11.For example, fibroblasts do not attach to MPC polymerand they do not spread on its surface; however, afterleaving the MPC polymer surface, they start to prolifer-ate on a more appropriate substrate. Importantly,PMD has high blood compatibility and does not activateplatelets12-23. Thrombus formation is inhibited if anartificial blood vessel made of a synthetic polymer, suchas polyurethane, is coated with MPC polymer14-23.Thus, no blood coagulation cascade is elicited whenblood comes into contact with the PMD surface. In addi-tion, PMD has moisture-retaining ability6,7,8. Based onthese unique characteristics of PMD, we speculatedthat a PMD dressing might provide an ideal set of con-ditions for wound healing. The purpose of this studywas to evaluate whether PMD could be applied as adressing material.

Materials and Methods

Polyurethane membranePolyurethane (Tecoflex� 60 Thermedics Inc.) is

commercially available, and 2.5 % polyurethane in chlo-roform was prepared. The solution was poured into acasting dish and left in a sealed chloroform environmentovernight. To evaporate the chloroform completely,the disk-shaped polyurethane membranes were driedunder vacuum overnight and sterilized under ultravioletlight.

Attaching the PMD membrane to the polyurethanesurface

PMD was synthesized from MPC and dodecyl-methacrylate (Fig. 1). PMD membranes were pre-

pared using a solvent evaporation technique from a 1wt% chloroform solution on a Teflon� plate. The platewas left in a sealed chloroform environment overnightand then dried under vacuum for 1 day. These mem-branes were attached to the polyurethane membraneusing ether as a solvent and sterilized under ultravioletlight. The surfaces of the PMD membranes weresmooth and clean (Fig. 2). Thirty minutes before weapplied the PMD dressings to the surgical wounds, thedressings were soaked in sterilized PBS solution toequilibrate their surfaces. This procedure made thephosphate residues protrude from the outer surface.

RetainerWe used a retainer to hold the dressing membrane

on the wound site. The retainer was made from a plas-tic plate 0.2 mm thick. A hole 10 x 10 mm in size wasmade at the center of the retainer. The retainer holdingthe dressing membrane covered the wound site andwas attached to the dorsal skin with a suture (Fig. 3).

Surgical procedureThirty male Wistar rats, 15 weeks old, weighing

approximately 230 g, were used. They were divided intothree groups. A full-thickness square surgical wound,

O. KATAKURA et al. J Med Dent Sci116

Figure 1. Chemical structure of PMD

Figure 2. SEM image of PMD membrane surface. (x 5000)

10 x 10 mm in size, was made on the dorsal skin ofeach rat under Nembutal anesthesia. In the air-exposed control group, only the retainer was applied tothe wound site. In the polyurethane group, the woundwas covered with the polyurethane membrane that wehad prepared, then fixed with another polyurethanemembrane (TegadermTM, 3M); the retainer was thensutured to the dorsal skin. In the PMD group, thewounds were first covered with the PMD membranes.The PMD membrane was then held in place withTegadermTM (3M) and the retainer (Fig. 3).

Wound size measurementThe cephalocaudal and horizontal axis of the

wound were measured every day. The wound area andwound area ratio were calculated using the followingformulas.

Wound area (mm2) = size of cephalocaudal axis(mm) x size of horizontal axis (mm)

Wound area ratio (%) = [wound area on measure-ment day / wound area on day 0] x 100

For statistical analysis, repeated measure ANOVAand Tukey test were used. Statcel 2 (OMS-Publishing)was used for all statistical analysis in this study.

Histological examinationFour days, 7 days and 1 month after the operation,

the tissue of the wound site was harvested and fixed in10% formalin. The specimens were embedded inparaffin, cut into 4-Òm frontal sections, and stained withhematoxilin and eosin. Masson trichrome stainingwas carried out to observe the collagen fiber alignment.

Results

In the present study, we used polyurethane (PU)membrane because PU itself is currently used as a

dressing. This group was positive control of thisexperiment. And as the experimental group, PMD,one type of MPC polymer, was attached to thepolyurethane (PU) membrane because PMD polymermembrane alone has low mechanical strength.

Changes in wound area ratio are shown in Figure 4.The wound area ratio of the PMD group was smallerthan that of the air-exposed control group after at 3, 4and 7 days after the operation. The differencebetween air-exposed control group and PU group wasobserved at 6 and 7 after the operation. Furthermore,the speed of reduction of the wound area ratio of thePMD group has a tendency to be high before the fifthoperative day. Therefore there was no significant dif-ference between the PMD and PU groups all throughthe experimental period. One week after the operation,the wound area ratio of the air-exposed control groupwas approximately 60%, whereas the value of bothPMD and PU groups was approximately 30%.

In the air-exposed group, a scab was observed overthe wounds 4 days and 7 days after the operation. Inthis group, the new epithelium migrated under thescab. In PU and PMD group, no scab was formed, andthe epithelial tissue had migrated smoothly beneath themembranes at 4 days and 7 days after the operation(Figs. 5,6). At Days 4 and 7, uniform infiltrations ofinflammatory cells were observed in all three groups.However, one month after the operation, the epidermisin the PMD groups was thicker than in the air-exposedcontrol and PU dressing group. In all groups noappendages were regenerated after the wounds werecompletely healed (Fig. 7). In the dermis, more bloodvessels were regenerated in PMD group, comparedwith the other groups. And no anomaly was observed in

117EVALUATION MPC DRESSING ON SURGICAL WOUNDS

Figure 3. Cross-sectional images of dressings and retainer. Theupper and the lower drawing demonstrated the layers in the dressingsused for the PU group and the PMD group, respectively.

Figure 4. Wound area ratio after the operation. A Value of p ＜ 0.05is accepted as statistically significant. *: p ＜ 0.05; **: p ＜ 0.01.

O. KATAKURA et al. J Med Dent Sci118

Figure 5-1. Low-magnification histological images on Day 4. (A) Open group wound, (B) PU group wound, (C) PMD group wound. The arrowsindicate the edges of the epithelial tissue that has migrated from the peripheral tissue of the wound. Hematoxylin and eosin staining (x 20).

Figure 5-2. High-magnification histological images on Day 4. (A) Open group wound, (B) PU group wound, (C) PMD group wound. Hematoxylinand eosin staining (x 100).

119EVALUATION MPC DRESSING ON SURGICAL WOUNDS

Figure 6. High-magnification histological images on Day 7. (A) Open group wound, (B) PU group wound, (C) PMD group wound. Hematoxylinand eosin staining (x 200).

Figure 7. High-magnification histologial images of 1 month after the operation. (A) Open group wound, (B) PU group wound, (C) PMD groupwound. Hematoxylin and eosin staining (x 100).

the collagen bundles of the dermis of the PMD group(Fig. 8).

Discussion

The present results demonstrated that PMD and PUdressing enhanced wound closure than air-exposedgroup, although there was no difference in wound clo-sure rate between the PMD and PU dressing allthrough the experimental period. In the air-exposedcontrol, a scab was formed and wound closure wasextremely retarded. Histological observation revealedthat in both the PMD and PU group, the re-epithelial-ized areas at the wound site were smooth.

Since the number of animals is limited, quantitativedata in histological images was not obtained unfortu-nately. However, the following histological differenceswere observed. The epidermis of PMD groups wasthicker than in the air-exposed control and the PUgroup at 1 month after the operation. The alignment offibrous bundles in the dermis of the PMD grouplooked mature than the PU group. In addition to that,large number of regenerated blood vessels wasobserved in the PMD group compared to the PU andair-exposed groups. Although these finding indicatehigh potentiality of PMD as a dressing material, further

additional experiments including quantitative histologi-cal analyses are required to confirm this.

PMD is unique bio-compatible material and we hadexpected PMD dressing would be superior as wounddressing. However, contrary to our expectation, signif-icant difference between PU group and PMD group wasnot detected. Present results demonstrated at least thatPMD dressing is not inferior to the ordinary PU dressingi.e. that PMD dressing is comparable to PU dressing.

Mechanical property of PMD is easily manipulated bychanging the ratio of MPC and DMA. Utilizing PMD, it isalso possible to prepare a dressing, which retains mois-ture and/or which releases a drug or growth factor toenhance wound healing. It is likely that newly-designed dressing of PMD would be superior to theordinary dressing.

Conclusively, this is the first report of the PMDapplication to wound and although we could not provePMD dressing is superior to PU dressing, the poten-tiality of PMD as a dressing material was suggested inthe present study.

References1. Winter GD. Formation of the scab and the rate of epithlization

of superficial wounds in the skin of the young domestic pig.Nature 1962;193:293-294.

2. Hinman CD, Maibach H. Effect of air exposure and occlusion

O. KATAKURA et al. J Med Dent Sci120

Figure 8. Histological images after 1 month after the opration. (A) Open group wound, (B) PU group wound, (C) PMD group wound. Massontrichrome staining (x 200).

on experimantal human skin wounds. Nature 1963;200:377-379.

3. Kannon GA, Garrett AB. Moist Wound healing with occlusivedressings —a clinical review. Dermatologic Surgery 1995;21:583-590.

4. Pollack SV. Wound healing; a review of environmental factorsaffecting wound healing. The journal of dermatologic surgery1979;5:477-481.

5. Martin P. Wound healing —Aiming for perfect skin regenera-tion. Science 1997;276:75-81.

6. Ishihara K, Ueda T, Nakabayashi N. Preparation of phospho-lipid polymers and their properties as hydrogel sheet. Polym J1990;22:355-360.

7. Ishihara K, Iwasaki Y. Reduced protein adsorption on novelphospholipid polymers. Journal of Biomaterial Applications1998;13:111-127.

8. Ishihara K, Nomura H, Mihara T, et al. Why do phospholipidpolymers reduce protein adsorption?: Journal of BiomedicalMaterials Research 1998;39:323-330.

9. Ishihara K, Ishikawa E, Iwasaki Y, et al. Inhibition of fibroblastcell adhesion on substrate by coating with 2-methacryloy-loxyethyl phosphorylcholine polymers. J. Biomater. Sci.Polymer Edn 1998;8:801-816.

10. Ishihara K, Ishikawa E, Iwasaki Y, et al. Inhibition of fibroblastcell adhesion on substrata by coating with 2-methacryloy-loxyethyl phosphorylcholine polymers. J. Biomater. Sci.Polymer Edn 1999;10,10:1047-1061.

11. Ishihara K, Iwasaki Y, Nakabayashi N. Novel biomedicalpolymers for regulating serious biological reactions:Materials Science and Engineering 1998;6,4:253-259.

12. Ishihara K, Fukumoto K, Iwasaki Y, et al. Modification of poly-sulfone with phospholipid polymer for improvement of theblood compatibility. 1. Surface characterization. Biomaterials1999;20:1545-1551.

13. Iwasaki Y, Sawada S, Nakabayashi N, et al. The effect of thechemical structure of the phospholipid polymer on fibronectinadsorption and fibroblast adhesion on the gradient phospho-lipid surface. Biomaterials 1999;20:2185-2191.

14. Iwasaki Y, Ishihara K, Nakabayashi N. Newly designed poly-mers for artificial organs. Recent Res. Devel. in polymer sci-ence 1997;37-50.

15. Iwasaki Y, Ijuin M, Mikami A, et al. Behavior of blood cells incontact with water-soluble phospholipid polymer. Journal ofBiomedical Materials Research 1999;46:360-367.

16. Iwasaki Y, Nakabayashi N, Nakatani M, et al. Competitiveadsorption between phospholipid and plasma protein on aphospholipid polymer surface. J. Biomater. Sci. Polymer Edn1999;10(5):513-529.

17. Ishihara K, Tanaka S, Fukukawa N, et al. Improved blood com-patibility of segmented polyurethanes by polymeric additiveshaving phospholipid polar groups. I. Molecular design ofpolymeric additives and their functions. Journal ofBiomedical Materials Research 1996;32(3):391-399.

18. Iwasaki Y, Ishihara K, Nakabayashi N, et al. Platelet adhesionon the gradient surfaces grafted with phospholipid polymer. J.Biomater. Sci. Polymer Edn. 1998;9(8):801-816.

19. Ishihara K, Oshida H, Endo Y, et al. Hemocompatibility ofhuman whole blood on polymers with a phospholipid polargroup and its mechanism. Journal of Biomedical MaterialsResearch 1992;26:1543-1552.

20. Yoneyama T, Sugihara K, Ishihara K, et al. The vascular pros-thesis without pseudointima prepared by antithrombogenicphospholipid polymer. Biomaterials 2002;23:1455-1459.

21. Iwasaki Y, Uchiyama S, Kurita K, et al. A nonthorombogenicgas-permeable membrane composed of a phospholipid poly-mer skin film adhered to a polyethylene porous membrane.Biomaterials 2002;23:3421-3427.

22. Iwasaki Y, Aiba Y, Morimoto N, et al. Semi-interpenetratingpolymer networks composed of biocompatible phospholipidpolymer and segmented polyurethane: Journal ofBiomedical Materials Research 2000;52:701-708.

23. Ishihara K, Fukumoto K, Iwasaki Y, et al. Modification of poly-sulfone with phospholipid polymer for improvement of theblood compatibility. 2. Protein adsorption and platelet adhe-sion. Biomaterials 1999;20:1553-1559.

24. Iwasaki Y, Sawada S, Ishihara K, et al. Reduction of surface-induced inflammatory reaction on PLGA/MPC polymerblend. Biomaterials 2002;23:3897-3903.

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