Enhanced surface hydrophilicity of polylactic acid sutures treated by lipase and chitosan

Abstract

Polylactic acid (PLA) surgical sutures are a new type of absorbable sutures that can be degraded and absorbed in the body. However, there is high hydrophobicity for the surface of PLA sutures, which leads to poor biocompatibility and cellular affinity. In order to increase the hydrophilicity, the PLA sutures were etched by lipase firstly, and then grafted with chitosan. The results indicate that the optimal conditions of treating PLA sutures by lipase were as follows: 45℃ reaction temperature, 4.5 g/L concentration of lipase and 8 h reaction time. The sutures were etched by lipase and then formed some grooves and a number of hydroxyl (-OH) bonds, which led to increased surface area and hydrophilicity, but a drop in mass and strength. The optimal conditions of grafting chitosan onto PLA sutures were as follows: 4 h reaction time and 3 g/L concentration of chitosan. The chitosan grafted and loaded on the surface of PLA sutures, and in some areas of the sutures the chitosan reunited, which led to a rough surface and large friction coefficient. Finally, the hydrophilicity of the PLA sutures, treated by lipase and then grafted with chitosan, was greatly improved.

Keywords

polylactic acid absorbable suture surface hydrophilicity chitosan lipase

Polylactic acid (PLA) surgical sutures are a type of absorbable sutures that can be degraded automatically and absorbed by the body during wound healing. In addition, the PLA sutures do not need to be removed after the wound has healed, so patients can avoid the acute pain caused by removing sutures.^1–3 PLA surgical sutures are widely used in surgical operations on human internal organs, and some operations for special body parts, such as perineum incision, circumcision, infantile skin suture, abdominal wall colostomy and operations on the face, neck and other exposed parts, and some operations performed on the occasion of contamination and infection.^4–6

However, the molecule of PLA is a type of large ester molecule possessing many ester bonds, which leads to high hydrophobicity for PLA sutures, so that the PLA sutures possess poor biocompatibility and cellular affinity.^7–9 Therefore, in order to improve the biocompatibility and cellular affinity of PLA sutures, it is necessary to increase its hydrophilicity.

Some experts have adopted two methods to improve the hydrophilicity of PLA sutures as follows. (a) The surface of PLA sutures were etched and then formed some cracks and holes, which increased the superficial area of the sutures, and some hydrophilic groups, such as the hydroxyl group (-OH) and the carboxyl group (−COOH), may also be generated on the surface of the sutures, so that its hydrophilicity improved.^10–12 For instance, Yang et al.¹³ applied sodium hydroxide mixed with ethanol at 0.25 mol/L to treat the surface of PLA, which led to hydrolytic reactions on the surface of PLA, and the hydrophilicity was enhanced eventually. Benyathiar et al.¹⁴ used the gamma rays and electron beams to radiate and etch the surface of PLA, which improved its hydrophilicity. According to this method, in order to gain higher hydrophilicity, it is necessary to enhance the degree of etching on the PLA sutures, but that will seriously worsen its mechanical properties.

(b) Some hydrophilic substances were grafted on the surface of PLA sutures to enhance its hydrophilicity.^15–18 For instance, Zhang et al.¹⁹ took hydrophilic polyvinylpyrrolidone (PVP) to graft on the surface of PLA sutures, and then the water absorption rate of the sutures increased by six to eight times. Zhou et al.²⁰ grafted maleic anhydride monomers to the main chain of PLA, which obviously improved the hydrophilicity of PLA. Li and Sun²¹ grafted acrylate β-hydroxyethyl to PLA, which significantly improved its hydrophilicity and degradation. According to this method, because the surfaces of PLA sutures are hydrophobic and very smooth, the amount of hydrophilic material grafted to surface of PLA sutures is very small, which limits the significant improvement its hydrophilicity.

We should reference the advantages of the above methods of hydrophilic modification, and try to overcome their weaknesses. If PLA sutures were first etched slightly, which led to roughness of the suture surface, the mechanical properties of the PLA sutures would not drop too much (by over 5%), which would overcome the deficiency of method (a) above. Moreover, through the processing of etching, the surface area of PLA sutures would increase, and some active group would be formed on the surface of the sutures, so that, if some hydrophilic polymer materials were grafted to the etched surface of the sutures, the amount and fastness of hydrophilic polymer materials grafted would increase, which would overcome the deficiency of method (b) above. Finally, the hydrophilicity of PLA sutures would be greater.

When selecting the main materials, we should pay special attention to the properties of environmental protection, high efficiency and natural biomass. In the first step, the enzyme (e.g. lipase), which has the advantages of being non-toxic, offering environmental protection, having high efficiency and so on, was a good choice for etching PLA sutures.²² In the second step, in order to graft some hydrophilic materials to the PLA sutures, chitosan, which possesses high hydrophilicity, is non-toxic, has good biocompatibility, has antibacterial properties and so on, was a good choice.^23–25

Based on the above ideas, the technical scheme in this article, as shown in Figure 1, was that the PLA sutures should be etched by lipase firstly, which could improve the roughness, surface area and chemical activity of PLA sutures. Then, the chitosan was grafted onto the PLA sutures, which could improve its hydrophilicity.

Figure 1.

The technical scheme in this article. PLA: polylactic acid.

In this article, the processing of lipase treatment was researched and optimized, and the influences of technological parameters, such as temperature, lipase concentration and time, on the structure and properties of PLA sutures were analyzed. In addition, the changes of the physical and chemical structures of etched PLA sutures were represented by Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). After that, the processing of grafting chitosan onto the surface of PLA sutures was studied and optimized, and the effects of parameters, including chitosan concentration, time and temperature, on the structure and properties of the PLA sutures were analyzed. In addition, the physical and chemical structures of grafted PLA sutures were characterized by SEM and FTIR.

Experimental details

Materials

The original PLA sutures with 33.72 tex linear density, 32.51 cN/tex breaking strength and 28.48% breaking elongation is provided by the Zhejiang Xingao Company in China, which will be treated to increase the hydrophilicity in this article. The lipase, with L3126 trade mark and 30–90 U/mg enzymatic activity in powder form, is provided by the American Sigma Company in the USA, which will be used to treat and etch the original PLA sutures in the first step. The chitosan, of food grade in powder form, is provided by Zhengzhou Wanbo Chemical Products Co. Ltd in China, and will be grafted onto the surface of etched PLA sutures.

Some chemical reagents applied in this experiment frequently, such as potassium dihydrogen phosphate (KH₂PO₄), glacial acetic acid (CH₃COOH), sodium hydroxide (NaOH), absolute ethyl alcohol (C₂H₆O) and deionized water (H₂O) are common reagents, so their specifications, manufacturers and so on will not be detailed here.

Experimental process

The process of treating PLA sutures by lipase

In order to wash the impurities or oil residues from surface of the original PLA sutures, 10 m PLA sutures were dipped into 30 mL anhydrous ethanol, and then they were oscillated for 1 h in an Ultrasonic Oscillator (KQ-250E, Shanghai Yuezhong Instrument Co., Ltd) under conditions of 20℃ and 40 KHz. After that, the PLA sutures were rinsed with absolute ethyl alcohol, and then dried in an Air-circulating Oven (DHG-9070, Ningbo Textile Instrument Factory) at 45℃ for 1 h.^26,27 Finally, the cleaned and dried PLA sutures were obtained.

A total of 6.805 g KH₂PO₄ and 1.656 g NaOH were mixed with some deionized water (H₂O) to reach 1 L total volume, and then the pH of the mixed liquid was titrated to 7.8 using 0.1 mol/L standard NaOH solution, which was finally called the buffer solution. After that, a certain quality (e.g. 0.05, 0.25, 0.45 or 0.65 g) of lipase was mixed with the above buffer solution to reach 100 mL total volume, so the different concentrations (e.g. 0.5, 2.5, 4.5 and 6.5 g/L) of lipase solutions were gained.^28,29

A total of 10 m of the above cleaned and dried PLA sutures were immersed into 100 mL of the above lipase solutions, and then they were oscillated for a certain period of time (e.g. 2, 4, 6, 8, 10, 12 or 14 h) in a Constant Temperature Bath Oscillator (SHA-C, Ningbo Textile Instrument Factory) at 45℃.^30,31 After that, the PLA sutures were rinsed with H₂O and dried naturally; these sutures were then referred to as the PLA sutures treated by lipase.

The processing of grafting chitosan onto the surface of PLA sutures

A total of 3 mL of glacial acetic acid (CH₃COOH) was added with deionized water (H₂O) until it reached 1000 mL total volume in order to form 3 g/L solution of CH₃COOH, which could dissolve the chitosan.^32,33 Then a certain mass of chitosan was mixed with the above solution of CH₃COOH to form different concentrations (e.g. 1, 2, 3, 4 or 5 g/L) of chitosan solutions.

The above PLA sutures treated by lipase were immersed into the chitosan solutions, and reacted under nitrogen atmosphere at a certain temperature (e.g. 20℃, 30℃, 40℃, 50℃ or 60℃) for a certain time (e.g. 2, 3, 4, 5 or 6 h), and then the sutures were rinsed using absolute ethyl alcohol (C₂H₆O) to remove the unreacted matters. After that, the sutures were dried in a Vacuum Drying Oven (DZF - 6020, Ningbo Textile Instrument Factory) under vacuum conditions at 45℃ for 1 h.^34,35 Finally, PLA sutures grafted with chitosan were obtained.

Characterization

Surface morphology

SEM investigations were performed on a JSM-6510LA (Kabuskiki Kaisha, Japan) at 7 kV accelerating voltage to evaluate the morphologies of the PLA sutures and fiber. All the samples were mounted on aluminum holders and sputter coated with a thin layer of gold prior to observation.

Chemical structure

The chemical structure of PLA sutures was assessed by FTIR. The infrared spectra were obtained via FTIR (TL-8000) with a resolution of 4 cm⁻¹ that scanned 50 times from 600 to 4000 cm⁻¹ at room temperature. All samples were taken using the conventional KBr disk method.

Tensile mechanical properties of PLA sutures

The strength of PLA sutures was measured by an Electronic Yarn Strength Tester (YG061-150, Laizhou City Electronic Instrument Co., Ltd, China) at 500 mm/min test speed and 500 mm test length.

Hydrophilicity of PLA sutures

The sample of PLA sutures was dried in a Drying Oven (DHG-9070, Shanghai Yiheng Science Instruments Co., Ltd) for 1 h, and then the drying quality of the sample was recorded as “m₀.” After that, the sample of PLA sutures was exposed under standard atmospheric conditions (20℃, 60% relative humidity) for 12 h, and then the quality of moisture absorption was recorded as “m₁.” So, the moisture absorption rate (recorded as “W”) of PLA sutures was calculated according to Equation (1)^36–38

\begin{matrix} W = \frac{m_{1} - m_{0}}{m_{0}} \times 100 % \end{matrix}

(1)

The moisture absorption rate (W) indicates the hydrophilicity of PLA sutures, and the greater W, the better the hydrophilicity.

Mass loss rate and weight gain rate of PLA sutures

The quality of the original PLA sutures was recorded as “m.” The sutures were treated and etched by lipase, and then the quality would decrease and be recorded as “m′.” So, the mass loss rate (recorded as “W'”) of PLA sutures was calculated according to Equation (2)^39,40

\begin{matrix} W' = \frac{m - m'}{m} \times 100 % \end{matrix}

(2)

After that, the PLA sutures were grafted with chitosan, and then the quality of the sutures would increase and be recorded as “m*.” So, the weight gain rate (recorded as “W*”) of the PLA sutures was calculated according to Equation (3)^41,42

W^{*} = \frac{m^{*} - m}{m} \times 100 %

(3)

Surface friction of PLA sutures

The surface friction coefficient of PLA sutures was observed by a yarn friction coefficient tester (LFY-110) at 2 cm/min; the friction piece was a rubber roller.

Results and discussion

The structure of PLA sutures treated by lipase

The surface structure of PLA sutures treated or not by lipase was observed by SEM, and is shown in Figure 2.

Figure 2.

Scanning Electron Microscopy images of polylactic acid sutures: (a) original suture; (b) suture treated by 4.5 g/L lipase. (Color online only.)

The surface of the original PLA sutures, as shown in Figure 2(a), was very smooth and clean, but the surface of PLA sutures treated by lipase, as shown in Figure 2(b), appeared to have many etched gullies and protruding blocks (indicated by blue arrows). This indicates that the lipase could indeed decompose and etch the fiber surface of PLA sutures, which made the surface of the fiber form some grooves and blocks, and then the surface of the sutures became rough and the surface area of the sutures increased. As a consequence, the hydrophilicity of the sutures and the binding force between sutures and chitosan were both improved. On the contrary, the etched gullies and protruding blocks on the surface of PLA sutures were the mechanically weak structure of the sutures, where the sutures were easily broken, so the strength of the sutures was reduced.

The process optimization of treating PLA sutures by lipase

In the processing of treating PLA sutures by lipase, there were three processing parameters, namely the concentration of lipase, reaction time and temperature, which have great influence on the result of treating PLA sutures by lipase. When the concentration of lipase was 2.5 g/L, and the reaction time was 6 h, the effect of reaction temperature on the breaking strength of PLA sutures is as shown in Figure 3.

Figure 3.

The effect of reaction temperature on the tensile breaking strength of polylactic acid sutures.

Figure 3 shows that with the increase of reaction temperature, the strength of PLA sutures would decrease first and then increase, and the PLA sutures would have the lowest strength at 45℃ reaction temperature. This is because just at the appropriate temperature (e.g. 45℃), the lipase would have the highest activity, which led to the fastest speed of decomposing and etching for the sutures, so the strength of PLA sutures would be the lowest at 45℃ reaction temperature. On the contrary, too low or too high reaction temperature would inhibit the activity of lipase, which led to a slower decomposition rate and lower strength for the PLA sutures. Therefore, the optimal reaction temperature for the lipase to decompose and etch PLA sutures was chosen as 45℃.

When the reaction temperature was 45℃ and the concentrations of lipase were 0.5, 2.5, 4.5 and 6.5 g/L, respectively, with the increasing of reaction time, the strength of PLA sutures is as shown in Figure 4.

Figure 4.

The strength of polylactic acid sutures treated by lipase at different concentrations and reaction times. (Color online only.)

Figure 4 shows that the strength of PLA sutures gradually decreased with time, and the decreasing rate of strength was slow early on, but fast later on. This is because the lipase could decompose and etch the PLA sutures, and then some grooves appeared on the surface of suture, as shown in Figure 2(b). In this way the surface area of the sutures would increase, which could increase the contact area of the sutures with lipase, so the degradation rate of PLA sutures caused by lipase became faster and faster, and the decreasing rate of strength was rising fast.

In addition, it is known from Figure 4 that with the increasing of enzyme concentration, the strength of the sutures became lower and lower, which was due to the more and more violent reaction of decomposition on the surface of the sutures.

Based on the above analysis, it is known that the strength of the sutures will decline after being treated by lipase. Thus, the question is how to optimize the concentration of lipase and the reaction time. We should set up a limitation on the strength of sutures. In addition, a drop in strength that is too small means insufficient decomposition and etching on the surface of the sutures, which means that the lipase could not play its important role; on the contrary, a drop in strength that is too large means over-violent decomposition and etching on the surface of the sutures, which means that the original strength of the sutures would worsen. So, we set up the drop of strength as 5%.

In Figure 4, the red horizontal dotted line represents the line of dropping by 5% relative to the original strength. The intersections of the red horizontal dotted line and the strength lines were “the best reaction time” in different concentrations of lipase, as follows: 0.5 g/L concentration of lipase for about 14 h; 2.5 g/L concentration of lipase for about 10 h; and both 4.5 and 6.5 g/L concentrations of lipase for about 8 h. In order to improve the working efficiency, we should choose the shortest reaction time, that is, 8 h in 4.5 or 6.5 g/L concentration of lipase. In addition, in order to save lipase, we should choose the lower concentration of lipase, that is, 4.5 g/L. Therefore, the optimum reaction time and concentration of lipase were as follows: 4.5 g/L concentration of lipase for 8 h.

The chemical structure of PLA sutures treated by lipase

After being treated by lipase, the chemical structure of the sutures would be changed. The FTIR of PLA sutures, treated or not by lipase, is shown in Figure 5.

Figure 5.

Fourier Transform Infrared Spectroscopy: (a) original polylactic acid (PLA) suture; (b) PLA suture treated by lipase.

Figure 5 shows that the curves of “a” and “b” appeared in the absorption band of C = O in saturated fatty acid ester at 1750 cm⁻¹, which indicated that the sutures, before or after treatment by lipase, always contained PLA possessing an ester bond. In addition, the characteristic absorption peaks of the hydroxyl group (-OH) appeared at about 1070 cm⁻¹, and this characteristic absorption peak of sutures treated by lipase (in the “b” curve) was larger than that of the original sutures (in the “a” curve), which indicates that the number of hydroxyl groups (-OH) in the sutures, after treatment by lipase, would increase. Based on the above analysis and the related literature,^43–46 it is speculated that the increased hydroxyl group (-OH) may be produced by the chemical reaction of hydrolysis of PLA, as shown in Figure 6, in which the molecule of PLA was hydrolyzed under the action of lipase, and the number of -OH bonds increased.

Figure 6.

Hydrolysis of polylactic acid under the action of lipase.

The hydrophilicity and mass loss rate of PLA sutures treated by lipase

Through the above analysis, it is known that in PLA sutures treated by lipase there appeared some grooves and blocks, and then the surface area and the hydroxyl groups of the sutures both increased. Thus, the hydrophilicity of PLA sutures would change as shown in Figure 7.

Figure 7.

The moisture absorption rate of polylactic acid (PLA) sutures.

Figure 7 shows that the moisture absorption rate of the original PLA sutures was extremely low, which indicated the hydrophobicity of the original PLA sutures. However, the moisture absorption rate of PLA sutures treated by lipase rose significantly, which showed that the hydrophilicity of PLA sutures treated by lipase was improved. This is because the increased surface area of PLA sutures treated by lipase had a better attraction for water molecules, and the increased hydroxyl groups of PLA sutures treated by lipase had good hydrophilicity; therefore, the hydrophilicity of PLA sutures treated by lipase was significantly improved.

In addition, after treatment by lipase, the PLA sutures were partly hydrolyzed, so the mass of PLA sutures would decrease inevitably, as shown in Figure 8.

Figure 8.

The mass loss rate of polylactic acid sutures treated by lipase.

Figure 8 shows that with the time of treating sutures by lipase, the mass loss rate of PLA sutures increased, which means the mass of sutures declined continuously. In addition, the greater the concentration of lipase, the more mass-loss of PLA sutures. This is because the PLA molecules were decomposed continuously on the surface of the PLA sutures.

The process optimization of grafting chitosan onto PLA sutures

In processing of grafting chitosan onto PLA sutures, the important conditions, including the reaction temperature and time, were set with full-factor experiments at 3 g/L concentration of chitosan. The increased mass of PLA sutures was a direct result of grafting chitosan onto the sutures, so the weight gain rate of sutures was used as a judgment criterion to evaluate and optimize the reaction temperature and time. The weight gain rates of sutures at different reaction temperatures and times are shown in Figure 9.

Figure 9.

The weight gain rate of sutures at different reaction temperatures and times.

Figure 9 shows that with the reaction temperature increasing from 20℃, the weight gain rates of sutures increased, which was because the chemical reactions of grafting chitosan onto PLA sutures also increased with increasing temperature. However, when the reaction temperature rose to 40–60℃, the weight gain rates of sutures were almost the same, which was because the rate of grafting reaction reached its maximum at 40–60℃, and the temperature had little influence on the chemical reaction speed. So, the reaction temperature should be set from 40℃ to 60℃ in order to achieve a greater suture weight gain rate. Furthermore, in order to save thermal energy, the reaction temperature was optimized at 40℃. In addition, Figure 9 indicates that with the reaction time increasing, the weight gain rates of sutures increased rapidly in the early stage and slowly in the later stage. This is because during the reaction course of grafting chitosan onto PLA sutures, in the early stage, there were many unsaturated chemical bonds and strong chemical reactions, and therefore the weight gain rates of sutures increased quickly. However, in the later stage, the chemical bonds were almost saturated, and the reaction was completed, so the weight gain rates of sutures increased slowly. Under the condition of 40℃ optimum reaction temperature, the reaction rate reached nearly maximum after 4 h reaction time and the increasing weight gain rates of sutures tended to vary little; therefore, the reaction time was optimized at 4 h.

Moreover, the concentration of chitosan is one of intrinsic factors that affect the weight gain rates of sutures. Under the optimum conditions of 40℃ and 4 h, the influence of chitosan concentration on the weight gain rates of sutures was as shown in Figure 10.

Figure 10.

The weight gain rate of sutures at different concentrations of chitosan.

Figure 10 shows that with an increasing concentration of chitosan, the weight gain rate of sutures increased quickly at first, and then increased slowly. This is because with the concentration of chitosan increasing, the amount of chitosan that contacts and then grafts with PLA sutures was increasing, so the weight gain rates of the sutures increased obviously. However, when the chitosan concentration increased over 3 g/L, the PLA sutures, which had grafted with too much chitosan, were close to saturation, so the weight gain rate of the sutures increased slightly. In order to graft as much chitosan as possible onto the PLA sutures, that is, to make the weight gain rates of sutures as large as possible, the concentration of chitosan should be larger than 3 g/L (3–5 g/L). Furthermore, based on the consideration of saving chitosan, the best concentration of chitosan was 3 g/L.

The physical structure of PLA sutures grafted with chitosan

The physical appearance of PLA sutures grafted with chitosan would change. SEM images of surface of PLA sutures are shown in Figure 11.

Figure 11.

Scanning Electron Microscopy images of polylactic acid (PLA) sutures: (a) original PLA sutures; (b) PLA sutures grafted with chitosan (×500); (c) PLA sutures grafted with chitosan (×2000); (d) PLA sutures grafted with chitosan (×5000).

Figure 11(a) indicates that the surface of the original PLA sutures was smooth and clean. Figures 11(b)–(d) show that the surface of the sutures was coated with chitosan, which means that the PLA sutures gained the characteristics of chitosan, such as high hydrophilicity, biocompatibility, antibacterial properties and so on. In addition, Figures 11(b)–(d) also show that the chitosan reunited in certain places of the sutures, which led to their rough surface.

The chemical structure of PLA sutures grafted with chitosan

There would be some chemical reactions when the PLA sutures were grafted with chitosan. The FTIR of PLA sutures grafted with chitosan is shown in Figure 12.

Figure 12.

Fourier Transform Infrared Spectroscopy: (a) polylactic acid (PLA) sutures treated by lipase; (b) chitosan, (c) PLA sutures grafted with chitosan.

Figure 12(a), which shows the FTIR of PLA sutures treated by lipase, contains the characteristic peaks of PLA, including the absorption peak of the “C-O” bond in “C-O-H” at 1089 cm⁻¹, the telescopic vibration absorption peak of the “C-O” bond in “C-O-C” at 1186 cm⁻¹, the absorption peak of “C = O” in the ester bond at 1752 cm⁻¹ and the telescopic vibration absorption peak of the “-OH” bond at 3457 cm⁻¹. Figure 12(b), which shows the FTIR of chitosan, contains the characteristic peaks of chitosan, such as the telescopic vibration absorption peak of “-NH₂” at 1655 cm⁻¹, the telescopic vibration absorption peak of the “C-” bond at 902 and 1025 cm⁻¹ and the telescopic vibration absorption peak of the “-OH” bond at 3457 cm⁻¹. Figure 12(c), which shows the FTIR of PLA sutures grafted with chitosan, has not only the characteristic absorption peaks of PLA at 1089 and 1186 cm⁻¹, but also the characteristic absorption peak of chitosan at 1625 cm⁻¹ of “-NH₂,” which indicates that the chitosan was grafted onto the PLA sutures. The absorption peak of the “-NH₂” bond moved from 1655 cm⁻¹ (Figure 12(b)) to 1625 cm⁻¹ (Figure 12(c)), which was because the amino (-NH₂) in chitosan reacted with the hydroxyl (-OH) in PLA and the electron cloud migrated. The absorption peak of the “-OH” bond moved from 3457 cm⁻¹ (Figures 12(a) and (b)) to 3409 cm⁻¹ (Figure 12(c)), which was because the hydroxyl (-OH) in chitosan reacted with the hydroxyl (-OH) in PLA to form a hydrogen bond and the electron cloud density was averaged. The telescopic vibration absorption peak of the “C-O” bond at 902 and 1025 cm⁻¹ in Figure 12(b) did not appear in Figure 12(c), which was because the peaks of the “C-O” bond at 902 and 1025 cm⁻¹ may be masked by the near peaks of other bonds. Based on the above analysis and the related literature,^47–50 it is speculated that the PLA molecule on surface of PLA sutures and the chitosan molecule would be joined in two patterns, as shown in Figure 13.

Figure 13.

The polylactic acid (PLA) sutures and the chitosan were joined in two patterns.

The hydrophilicity and friction coefficient of PLA sutures grafted with chitosan

When the chitosan was grafted onto the surface of PLA sutures, the hydrophilicity of the sutures would change with the increasing concentration of chitosan, as shown in Figure 14.

Figure 14.

The moisture absorption rate of polylactic acid sutures at different concentration of chitosan.

Figure 14 demonstrates that with increasing concentration of chitosan, the moisture absorption rates of PLA sutures rose, owing to the increased chitosan loaded on the surface of the PLA sutures. At 3 g/L concentration of chitosan, the moisture absorption rate of PLA sutures reached the maximum (about 0.6%). Over 3 g/L chitosan, the moisture absorption rate of PLA sutures increased very slowly. Thus, the moisture absorption rate of PLA sutures treated by 3 g/L chitosan was almost 24 times the moisture absorption rate of PLA sutures that were untreated (at 0 g/L concentration of chitosan in Figure 14), which indicates that the hydrophilicity of sutures treated by chitosan was greatly improved.

The surface roughness of PLA sutures could be represented by the friction coefficient; the friction coefficients of sutures in different concentrations of chitosan are shown in Figure 15.

Figure 15.

The friction coefficients of polylactic acid sutures at different concentrations of chitosan.

It is known from Figure 15 that the friction coefficient of the original PLA sutures (at 0 g/L concentration of chitosan) was very small (about 0.1), which means a smooth surface for the PLA sutures. However, with the increasing concentration of chitosan, the friction coefficient of PLA sutures treated by chitosan gradually increased, which means that the resistance of stitching wounds using PLA sutures would increase.

Conclusions

The optimal conditions of treating PLA sutures by lipase were as follows: 45℃ reaction temperature, 4.5 g/L concentration of lipase and 8 h reaction time. The lipase could decompose and etch the surface of the PLA sutures, and then some grooves appeared on the surface of the sutures, which led to increasing surface area, falling mass and dropping strength. The number of hydroxyls (-OH) in the sutures, after treatment by lipase, increased, which led to improved hydrophilicity. The optimal conditions of grafting chitosan onto PLA sutures were as follows: 4 h reaction time and 3 g/L concentration of chitosan. The chitosan grafted and loaded on the surface of PLA sutures, and in some areas of the sutures the chitosan reunited, which led to a rough surface and large friction coefficient for the PLA sutures. Finally, the hydrophilicity of PLA sutures treated by lipase and then grafted with chitosan was greatly improved.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the MOE (Ministry of Education in China) Project of Humanities and Social Sciences (grant number 18YJC760051), the 2017 Shanxi Philosophy and Social Science Project (grant number 201702), the Program for the Philosophy and Social Sciences Research of Higher Learning Institutions of Shanxi (PSSR) (grant number 201803060), the Shanxi Social Science Association Important Project (grant number SSKLZDKT2016055), the Shanxi University Students Innovation And Entrepreneurship Training Program Project (grant number 2018099), the Students Innovation And Entrepreneurship Training Program Project of Taiyuan University of Technology (grant number 18049), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi, China (grant number 2015125) and the Youth Foundation of Taiyuan University of Technology, china (grant number 2015QN042). Shuqiang Liu would also like to acknowledge financial support through the International Visit Program of Taiyuan University of Technology.

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