CONG Hui (叢 慧) ，ROE Simon，MENTE Peter，RUFF Gregory，KING W Martin

1 College of Textiles，North Carolina State University，Raleigh NC 27695，USA

2 College of Veterinary Medicine，North Carolina State University，Raleigh NC 27607，USA

3 Department of Biomedical Engineering，North Carolina State University，Raleigh NC 27695，USA

4 Vilcom Circle，Chapel Hill，NC 27514，USA

5 College of Textiles，Donghua University，Shanghai 201620，China

Introduction

Monofilament and braided surgical sutures are commonly used in surgeries to close wound.However potential problems associated with knot，such as knot security，suture extrusion，infection，and ischemia are still difficult to solve［1-3］.In order to avoid tying knots，competing technologies in wound closure market include staples，closure strips，and tissue adhesives［4］.Barbed surgical sutures are innovative type of knotless sutures that have a plurality of barbs (see Fig.1)projecting out the suture main body.The barbs can penetrate and anchor with surrounding tissue all along the suture's length，thus eliminating the need for tying a knot.Barbed surgical sutures have been approved by the US Food＆Drug Administration in 2004 for use in wound closure and tissue apposition，especially in plastic and cosmetic surgery［5-9］.While this barbed suture technology is widely accepted clinically for skin wound closure，its suitability in other applications，such as tendon repair，has yet to be proven［10］.The current study focused on two objectives:(1)to fabricate barbed sutures from nylon and polypropylene(PP)monofilament sutures with different barb geometries;(2)to evaluate the anchoring performance of the prepared sutures in both porcine skin and tendon tissues using an in vitro pull-out test［5］.The pull-out test has been done with simulant skin tissues，polyurethane foams，not yet in live tissue，especially in tendon tissues.

Fig.1 Barbs projecting out from the suture surface

1 Experimental

1.1 Materials

Nylon and PP size “O” (diameter = 0.38 mm)monofilament sutures were obtained from Ethicon Inc.Fresh porcine dermis and superficial digital flexor tendons were harvested from the School of Veterinary Medicine at North Carolina State University and used within 12 h.

1.2 Methods

Both nylon and PP monofilament sutures were fabricated into barbed sutures using a barb cutting instrument specially designed and assembled in our laboratory (see Fig.2).The monofilament suture was held under tension against a fixed stainless steel wheel etched with V-shape grooves (Part No.1).Barbs were inserted on the suture surface by a cutting blade(Part No.2)，fixed underneath a rotating wheel which was controlled by a rotating handle (Part No.3)and driven through a synchronous timing belt.Three screw micrometers controlled the position of the knife and hence the barb geometry (Part No.4).

Fig.2 The mechanical cutting instrument

The recommended barb geometries for porcine skin and tendon tissues with a 40% and 50% cut depth of the suture diameter respectively were taken from a previous study by Ingle et al［5］.The target cut angle of 170° and frequency of 2.5 barbs/cm were kept constant.In order to reduce the variables，10 barbs were inserted in each cut section.The barb geometry was measured using a Nikon H550S optical microscope at 40 magnification.The average barb cut angle and cut depth measurements were obtained from the microscopic images using MB Ruler image analysis software.The tensile properties of both the unbarbed and barbed sutures were tested on a TestResources 311Q tensile testing machine using a modified ASTM D2256 test method.The anchoring performance of the nylon and PP barbed sutures was determined in skin and tendon tissues using the in vitro pullout test illustrated in Fig.3，and Dsand Dtare the distances from the needle insertion point to the needle exit point of the skin and tendon tissue samples respectively.

Fig.3 Pull-out test models:(a)skin/suture and(b)tendon/suture

2 Results and Discussion

2.1 Barb geometry

After fabricating the barbed sutures，the single barb geometry was observed under a compound optical microscope and measured by image software.The results of the measured cut depth and cut angle compared with the target values are shown in Fig.4.The geometric configurations of the barbs for all four types of sutures show close agreement with the target values.The coefficients of variation of the cut depth were around 10% and those for the cut angle were all less than 1%.

Fig.4 Target values and experimentally measured results for barbed suture geometry，in terms of (a)cut depth and (b)cut angle

2.2 Suture material properties

Microscopic images of a single barb cut into the monofilament sutures are shown in Fig.5.A clear and sharp cut line is visible for each suture when viewed by a low power optical microscope.Near the base of the cut line，dark areas marked with arrows indicate regions of stress concentration，which has resulted from the cutting and bending of the barbs.Barbs on the nylon suture have a tendency of collapse back onto the surface，whereas barbs on the PP sutures appear to stand out more readily.This is likely caused by differences in the microstructure of the two suture materials in terms of stiffness.The Young's moduli of nylon and PP were found to be 0.43 GPa and 1.05 GPa，respectively.

Fig.5 Light microscopic images of (a)nylon and (b)PP barbed sutures

The experimentally measured ultimate tensile forces of the barbed and control sutures are listed in Table 1.After cutting barbs，the ultimate tensile force decreased because of the reduced load bearing area.Comparing the percentage loss in ultimate tensile force with the corresponding cut depth，the nylon suture appeared to lose more tensile force than the PP suture.

Table 1 Ultimate tensile force of barbed and control sutures and the comparison between percentage loss in ultimate tensile force and the percentage cut depth

Figure 6 shows the different modes of suture failure under tensile loading.A“clean”and sharp broken end was formed when the nylon suture broke under tension;when a PP barbed suture failed，it formed fibrillated filaments at the broken end.

Fig.6 Barbed suture fragments after the tensile testing

2.3 Tissue anchoring performance

In the tissue/suture pull-out test，the maximum pull-out force was taken as the tissue anchoring performance of the barbed suture.Table 2 showed the anchoring ability of nylon and PP barbed sutures in both porcine skin and tendon tissues.It showed that，PP barbed sutures had a superior anchoring ability with the surrounding tissue than nylon sutures.Those sutures with a greater cut depth showed lower ultimate tensile force when inserted in tendon tissue.However they showed superior pull-out force compared with that of skin tissue.Theeffectiveness of barbed suture anchoring is determined by the extent of barb engagement rather than the suture ultimate tensile force.

Table 2 Ultimate tensile force of barbed and control sutures

3 Conclusions

Two different barb configurations were successfully fabricated for both nylon and PP monofilament sutures by using a specially designed mechanical cutting instrument.The Young's modulus of the suture material is likely to influence the ability of cut barbs to project out and so engage with the surrounding tissue.Based on the comparison of the ultimate tensile force between barbed and unbarbed control sutures，no general relationship was found between the percentage loss in tensile force and the cut depth.The barbed suture fragments left after tensile testing showed two different suture failure modes.Given the same tissue，PP barbed sutures had a superior anchoring ability than nylon sutures.Despite having a lower ultimate tensile stress the 50% cut depth barbed suture gave a superior tissue anchoring performance in tendon tissue than the“stronger”40% cut depth barbed suture in skin tissue.In future studies，comparisons among more types of sutures will be needed and a more effective way to improve barb engagement is a critical concern.

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