Surface modification of polylactic acid and poly (D,L-lactide-co-glycolide) biodegradable materials via chitosan-coating treatment: A new approach for developing novel antibacterial acupoint catgut embedding materials

Abstract

Surgical site infections are liable to cause significant postoperative morbidity and increase health costs. However, polylactic acid and poly (D,L-lactide-co-glycolide) acupoint catgut-embedding therapy materials used to stimulate certain points for curing diseases are typically devoid of antibacterial activity. Here we report novel polylactic acid and poly (D,L-lactide-co-glycolide) embedding materials with effective antibacterial properties via chitosan-coating treatment that retained their inherent excellent characteristics. To achieve more comprehensive properties, polylactic acid polymer chips and poly (D,L-lactide-co-glycolide) multifilaments were adopted to fabricate embedding materials, and their preparation processes were studied. Meanwhile, a new one-dip-one-rolling coating system was designed and established in this work. Afterwards, characterizations such as fundamental properties, mechanical properties and biocompatibility were explored. The results showed that both the polylactic acid and poly (D,L-lactide-co-glycolide) groups with covered chitosan layers showed a different increase in weight and diameter. FT-IR analysis indicated that there were interactions between the pure embedding materials and chitosan molecules, the present of hydroxyl group (-OH) and some polar bonds such as (N-H) amide II and (C=O) amide I would be benefit for the hydrophilicity of modified materials. Chitosan-coated polylactic acid monofilaments in group 2 (breaking strength = 31.26 ± 1.62 cN/dtex, breaking elongation = 34.82 ± 5.21%) and chitosan-coated poly (D,L-lactide-co-glycolide) braiding threads in group 2 (breaking strength = 92.58 ± 1.69 cN/dtex, breaking elongation = 73.39 ± 2.38%) showed better mechanical properties. Antibacterial efficacy and cell cytocompatibility of polylactic acid and poly (D,L-lactide-co-glycolide) groups were greatly enhanced after coating. The degradation behaviors were slowed down, providing a longer-lasting effect during the treatment process. In conclusion, the modified polylactic acid and poly (D,L-lactide-co-glycolide) embedding materials with good, comprehensive performance presented great potential in acupoint catgut-embedding therapy clinical applications.

Keywords

surgical site infections polylactic acid poly(D L-lactide-co-glycolide)antibacterial properties chitosan

Acupoint catgut-embedding therapy (ACET) research is a promising acupuncture treatment, characterized by the insertion of biodegradable materials into some parts of human body.^1,2 Once inserted, embedded materials in the acupoints will produce a series of stimulation effects to cure the disease concerned.³ In this field, ACET has attracted much attention because of its inherent advantages, such as its simple, effective, and lasting effects.^4–6 However, there are still many potential problems for existing ACET materials. For example, surgical site infections (SSIs) are challenging complications after surgical procedures, which not only lengthen the time of hospitalization, but also increase the cost for patients, whereas embedding materials commonly used in clinics are typically short of antibacterial activity.^7–9 Moreover, defects of poor mechanical properties,¹⁰ allergic reactions,¹¹ and lack of functional characteristics¹² have been cited as the major reasons for the decline in the production of ACET materials.¹³ Based on this, it is essential for researchers to study embedding materials for ACET applications.

Biodegradable materials can be divided into natural and synthetic types according to their origin.^14,15 Catgut, a typical representative of natural embedding materials, has the advantages of good biodegradability, low cost, and easily accessibility.¹⁶ However, it is high risk to apply catgut material in the clinical treatment of allergy as it has variable quality. Synthetic embedding materials such as polylactic acid (PLA)^17,18 and poly (D,L-lactide-co-glycolide) (PLGA),^19,20 have multiple strengths such as excellent biodegradability²¹ and biocompatibility etc.²² They have also been authorized by the US Food and Drug Administration to be used in clinical settings.²³ Unfortunately, as reported by recent studies, degradation products of synthetic polymers such as PLA (Figure 1(a)) and PLGA (Figure 1(b)) generate acid products, which often cause inflammation when implanted.²⁴ Hence, modifications are required to combine the advantages of natural and synthetic biodegradable materials, while avoiding their disadvantages.

Figure 1.

Schematic drawing of molecular structures: (a) polylactic acid (PLA); (b) poly (D, L-lactide-co-glycolide) (PLGA); and (c) chitosan.

Chitosan (Figure 1(c)) is a linear, high molecular weight heteropolysaccharide,²⁵ consisting of N-acetyl-glucosamine and N-glucosamine units.²⁶ With its abundant reserve, chitosan is the second most important natural polymer globally (the first is cellulose) and has been widely extracted from marine arthropods (prawns, crabs, shellfish, etc).²⁷ As shown in our previous reports, chitosan-coating modification can be used to improve the surface roughness, mechanical properties, and biocompatibility of PLA- and PGA-embedding monofilaments.^28,29 Moreover, there are many applications of chitosan in the surface modification of textile materials for different purposes, including salt-free neutral dyeing of cotton,^30,31 antibacterial cotton and wool,³² dyeing of wool,³³ and antibacterial wound dressings.³⁴ In recent years, applying chitosan coating on the surface of biodegradable material has become of interest. Zeng et al.³⁵ studied the potential application of chitosan coating on three-dimensional porous PLA scaffold, proving this method to be useful in enhancing biocompatibility and cell adhesion properties. Han et al.³⁶ investigated the effects of chitosan-coating technology on PLA films using a phase separation method. The results showed the hydrophilicity of composites was improved whereas the thermal stability was slightly reduced. The degradation rates could also be tuned in terms of structure through changing the pore sizes of PLA-based films. Moreover, the film showed strong antibacterial activity against Escherichia coli. Wang et al.³⁷ fabricated biodegradable PLGA-chitosan core-shell nanocomposites with a narrow size distribution, and achieved good results in terms of drug-carrying capacity and sustained release performance. Feng et al.³⁸ illustrated a new method for manufacturing PLA-aligned scaffolds modified by chitosan, and the viability, adhesion, length, and migration behaviors of osteoblasts in vitro was greatly improved. However, current studies are mainly concerned with coating chitosan on products such as scaffold, film, and nanocomposites, etc. and only examined the short-term modification effects. Few papers studied the effects of chitosan coating on linear materials including PLA monofilament and PLGA braiding threads. Moreover, there was lack of detailed research on in vitro comprehensive behaviors observed in the long term.

Motivated by the surface modification of biodegradable materials using the chitosan-coating method to develop novel antibacterial embedding materials and retain their advantages, this paper first fabricated and compared four sets of PLA monofilaments and PLGA braiding threads. It then evaluated their comprehensive properties, including fundamental properties (surface morphology, weight and diameter changings, FT-IR analysis: For exploring the emergence of new functional groups of modified materials), mechanical properties (tensile, flexibility), and biocompatibility (cytotoxicity, antibacterial efficacy, and degradation). Finally, the feasibility of the chitosan-coating method on the PLA monofilament and PLGA braiding threads was evaluated based on these experimental results.

Materials and methods

Materials

PLA polymer chips were provided by Shenzhen Esun Industrial Co, Ltd (Shenzhen, China). The density was 1.25 g/cm³, the melting point was 150℃, the melt index was 10–12 g/10 min, flexural modulus is 3110 MPa. The PLGA multifilament (molar ratio LA: GA = 10: 90, inherent viscosity = 1.7 dl/g, diameter = 0.043 mm, USP 8-0) and chitosan (molecular mass 448 KD, degree of deacetylation 80-85%) were provided by Tianqing Biomaterial Company (Shanghai, China). Acetic acid solution (99.5% (v/v)) was supplied by Lingfeng Chemical Reagent Co., Ltd (Shanghai, China). All chemicals were analytically pure and used without further purification.

Preparation of PLA monofilaments and PLGA braiding threads

Figure 2 shows the preparation process of PLA monofilaments and PLGA braiding threads. In our previous work,³⁹ PLA monofilaments were achieved by a two-step melt-spinning method under the optimum preparation parameters (spinning speed = 150 m per min, drawing ratio = 4). Firstly, PLA polymer chips weighing 5 g were squeezed into the melting zone from the feeding port; the temperature of melting zone was set as 185℃, which was higher than the melting temperature of PLA polymers beads. The molten polymer then went through a conduit to form a polymer fluid and was extruded out of the forming zone. Second, the as-spun filament experienced a drawing process by the front and rear rollers, and the degree of macromolecular orientation was greatly improved. Finally, the PLA monofilaments were wound onto a rotation shaft at a winding speed of 60 m per min after the drawing process (Figure 2(a)). In addition, the PLA monofilaments with two different diameters produced were coded as PLA1 and PLA2. As shown in Figure 2(b) PLGA braiding threads were produced using a two-dimensional spindle threads braiding machine with a gear ratio of 130/36. Each PLGA braiding thread was formed by 12 pieces of PLGA multifilament, and the carriers move continuously according to the braiding trajectory. In the present work, two types of PLGA braiding threads with different diameters were produced, and they were coded as PLGA1 and PLGA2.

Figure 2.

Illustration of preparation process for: (a) PLA monofilaments; and (b) PLGA braiding threads; PLA: polylactic acid; PLGA: poly (D, L-lactide-co-glycolide).

Antibacterial modification of PLA monofilaments and PLGA braiding threads

The solution dip-rolling method was regarded as the most simple and efficient way to deposit some functional coating agents on textiles and is useful for improving mechanical properties and biocompatibility.^40,41 In this context, a one-dip-one-rolling chitosan-coating system was designed and adopted for the surface modification of PLA monofilaments and PLGA braiding threads. To protect the molecular structure of the PLA and PLGA groups, the concentration of acetic acid solution (99.5% (v/v)) was diluted to 3% (v/v) by deionized water. The coating bath was prepared by dissolving appropriate amounts of chitosan powder in acetic acid solution 3% (v/v) and stirring the dispersion for 1 h at 60℃ . As shown in Figure 3, samples were first immersed in the coating agents for some time using the dipping roller, and then pressed to remove the spare chitosan molecules covering their surface, which contributed to forming the smoother chitosan-coating layer. Finally, the treated samples went through the drying system and were dried by a drum wind dryer. The drying temperature should be less than 55℃ because of the melting temperature of PLA was 55–65℃. In the present work, four types of chitosan-coated PLA monofilaments and PLGA braiding threads (CS-PLA1, CS-PLA2, CS-PLGA1, and CS-PLGA2) were obtained. Their properties were then tested and compared to evaluate the efficiency of this chitosan-coating method.

Figure 3.

Schematic drawing of chitosan modification process for PLA monofilaments and PLGA braiding threads using one-dip-one-rolling coating method; PLA: polylactic acid; PLGA: poly (D, L-lactide-co-glycolide).

Testing method and indicators

Structure characterizations

Surface morphology

A scanning electron microscope (SEM; Hitachi TM 3000, Japan) was used to investigate the surface property by observing and analyzing images of samples.

Weight and diameter changes

Weight changes of samples before and after coating modification were measured and calculated. In this work, an electronic balance (FA2004A-200G, Jingtian Instrument Factory, Shanghai, China) was adopted to estimate each sample per 10 cm length. The results were expressed with average and standard deviations for 10 replicates. Weight change rate could be calculated using equation (1) as follows.

Weight changeing rate (%) = \frac{W_{1} - W_{0}}{W_{0}} \times 100 %

(1)

In which W₁ (g) refers to the weight of samples after coating modification and W₀ (g) refers to the weight of samples before coating modification.

Diameter changes of samples before and after coating modification were measured and calculated. In this work, an optical microscope (DA1-180M, Yongxin optics Co., Ltd, Ningbo, China) and ImageJ software was used to estimate each sample. The results were expressed with average and standard deviations for 10 replicates. The diameter change rate could be calculated using equation (2) as follows.

Diameter changeing rate (%) = \frac{D_{1} - D_{0}}{D_{0}} \times 100 %

(2)

In which D₁ (mm) refers to the weight of samples after coating modification and D₀ (mm) refers to the weight of samples before the coating modification.

FT-IR analysis

FT-IR analysis was used to explore the interaction mechanism between the pure monofilaments or braiding threads and chitosan molecules. A PerkinElmer spectrum 100 FT-IR spectrometer was adopted and the wavenumber range of 4000 cm⁻¹ to 450 cm⁻¹ was recorded and analyzed in this work.

Mechanical characterizations

According to the Chinese National Standard GB/T 6529,⁴² samples should be conditioned for 24 h at standard atmospheric conditions (temperature = 20℃ ± 2℃, relative humidity = 65% ± 2%) before the test; the experimental process was also required to be conducted in standard atmospheric conditions.

Tensile property

Breaking strength and breaking elongation were used to evaluate the tensile properties of samples, and an electronic single-yarn tensile tester (Model YG061f, Shandong Laizhou Electron Instrument Co., Ltd., Laizhou, China) was adopted in this research. The operation was carried out according to the Chinese National Standard GB/T 14344-2008.⁴³ The detailed testing parameters were as follows: the gauge length was 500 mm, tensile speed was 300 mm per min, and pre-tension was 1 cN/tex. The final result was expressed with an average value and a standard deviation of 10 replicates.

Flexibility

The bending stiffness was used to represent flexibility of samples, and the test standard was according to cantilever beam method.⁴⁴ To achieve accurate results, test parameters were selected after several trials. Meanwhile, the final result was expressed with an average value and a standard deviation of 10 replicates. Bending stiffness could be calculated using equation (3) as follows.

Bending stiffness (cN \cdot {cm}^{2}) = \frac{F \times L^{3}}{f}

(3)

In which B ( $cN \cdot cm$ ²) refers to the bending stiffness; L (cm) refers to the length of samples hanging in the air; F (cN) refers to the loading force on the hanging part of samples; and f (cm) refers to the deflection of samples’ loading end.

In vitro characterizations

Cell cytotoxicity

The cell cytotoxicity of PLA monofilaments and PLGA braiding threads were evaluated using the Cell Counting Kit-8 (CCK-8) cell proliferation method. Samples were immersed in alcohol solution (99.5% (v/v)) for 1 h, then washed with phosphate-buffered saline (PBS) solution three times before the test. The detail operations were as follows. Firstly, Dulbecco’s modified Eagle’s medium was used to culture the samples and gain the leach liquor after a culture of 48 h under the standard condition at 37℃. Then the second generation rat fibroblasts (2 × 10⁴ ml) were seeded into a 96-well plate, and the leach liquor was added to each well after good adherence of the cells. Finally, a CCK-8 assay was carried out to measure the cell viability of samples. Afterwards, the optical density (OD) value was achieved using a microplate reader at a wavelength of 450 nm. The relative growth rate (RGR) could be calculated by equation (4) as follows.

RGR (%) = \frac{{OD}_{1}}{{OD}_{2}} \times 100 %

(4)

In which OD₁ refers to the OD values of the sample and OD₂ refers to the OD values of the control.

Antibacterial efficacy test

Based on the standard of ISO 20645:2004, the antibacterial efficacy of chitosan-coated PLA monofilaments and PLGA braiding threads was evaluated in this work. Firstly, Staphylococcus aureus and Escherichia coli at a concentration of 1 × 10⁶ CFU per ml were evenly distributed on the sterilized agar plate. The samples were then transversely placed against the agar surface to ensure close contact. Finally, the plates were incubated at 37℃ for 24 h and examined to determine their zones of inhibition. The inhibition zone diameter could be calculated using equation (5) as follows.

d (mm) = \frac{H - h}{2}

(5)

In which d (mm) is the inhibition zone, H (mm) is the total diameter of the specimen and inhibition zone, and h (mm) is the total diameter of the specimen.

Degradation behavior

Samples were sterilized and freeze dried for 12 h before testing. PBS (pH 7.4) solutions consisted of NaCl 8 g/L, KClO 2 g/L, Na₂ HPO₄•12H₂O 2.9 g/L and KH₂PO₄ 0.2 g/L was used to simulate the internal human body environment. Samples were immersed in the PBS solution and taken out for 1 week, then washed with deionized water and dried in a freeze drier. Finally, the weight loss of each sample was measured and calculated by equation (6) as follows.

Weight loss (%) = \frac{W_{1} - W_{0}}{W_{0}} \times 100 %

(6)

In which W₁ (g) refers to the weight in the process of degradation and W₀ (g) refers to the initial weight.

Statistical analysis

Statistical analysis was performed using Origin (Origin Lab, USA). The results were averaged and expressed as means and standard deviation. Analysis of variance was used for determining the statistical difference between samples. The data in the figures are marked by (*) for p < 0.05.

Results and discussion

Structure properties of prepared PLA monofilaments and PLGA braiding threads

Surface morphology

SEM images of coated and uncoated PLA monofilaments and PLGA braiding threads are shown in Figure 4. The uncoated group (PLA1, PLA2, PLGA1, and PLGA2) wase observed to have relatively smooth surfaces except some tiny impurities, which may be caused by the fabricating process. As for the coated group (CS-PLA1, CS-PLA2, CS-PLGA1, and CS-PLGA2), there were mainly two parts covering their surface; one was the pure embedding monofilament or braiding thread, the other was the chitosan layer. Moreover, the chitosan-coated PLA group (CS-PLA1, CS-PLA2) was found to have some fragments covering their surfaces, whereas the surfaces of the coated PLGA group (CS-PLGA1, CS-PLGA2) presented more chitosan molecules. This was because the PLA group had a smoother surface structure compared to the PLGA group, so less chitosan solution could adhere to the surface in the coating process. As for the PLGA group, the large specific surface area and the gaps between the threads were both beneficial for the attachment of chitosan molecules. In summary, all the samples exhibited more surface roughness after the coating treatment, but PLGA group adhered to more chitosan due to its complex surface structure compared to the PLA group.

Figure 4.

Scanning electron microscope images of surfaces of chitosan coated and uncoated PLA monofilaments and PLGA braiding threads, PLA group: PLA1, PLA2; CS-PLA group: CS-PLA1, CS-PLA2; PLGA group: PLGA1, PLGA2; CS-PLGA group: CS-PLGA1, CS-PLGA2; CS: chitosan coated; PLA: polylactic acid; PLGA: poly (D, L-lactide-co-glycolide).

Weight and diameter change rates

Figure 5 shows the weight and diameter change rates of PLA monofilaments and PLGA braiding threads after coating modification. Both the weight and diameter change rates of PLGA group (PLGA1, PLGA2) were observed to have larger change values than the PLA group (PLA1, PLA2). These findings were agreement with the surface structure changes of the PLA and PLGA groups. From the observation of the SEM images, the PLGA group had more surface roughness than the PLA group, which was conducive to the threads achieving more chitosan coating in the modified process, causing the larger weight and diameter changes. Moreover, PLGA2 (PLA2) presented larger weight and diameter change rates than PLGA1 (PLA1) due to its larger amount of chitosan coating. It illustrated that the initial diameters had a different effect on the coating of the samples, and a larger specific surface area would cause the larger amount of coating for both the PLA and PLGA groups.

Figure 5.

The weight and diameter changing rates of PLA monofilaments and PLGA braiding threads after coating modification, PLA group: PLA1, PLA2; CS-PLA group: CS-PLA1, CS-PLA2; PLGA group: PLGA1, PLGA2; CS-PLGA group: CS-PLGA1, CS-PLGA2; CS: chitosan coated; PLA: polylactic acid; PLGA: poly (D, L-lactide-co-glycolide).

FT-IR analysis

Figure 6 shows the FT-IR analysis of chitosan-coated and uncoated PLA monofilaments and PLGA braiding threads. The PLA group was observed to have four absorption bands: 1184.2 cm⁻¹, 1758.7 cm⁻¹, 2965.9 cm⁻¹ and 3504.3 cm⁻¹. The sharp band at 1184.2 cm⁻¹ was attributed to the bending vibration of (C-O), and the absorption bands at 1758.7 cm⁻¹, 2965.9 cm⁻¹, and 3504.3 cm⁻¹ confirmed the presence of (C=O), (C-H), and (O-H). Therefore, it was concluded that the PLA monofilament had the typical structure of aliphatic polyester. In contrast, the CS-PLA group was similar to the changing curves of PLA group, but some new peaks such as 1118.7 cm⁻¹, 1581.3 cm⁻¹ and 1631.4 cm⁻¹ appeared, which was attributed to the bands of (C-O-C), (N-H) amide II bending groups, and (C=O) amide I in the chitosan structure. The PLGA group had typical sharp peaks at 2965.9 cm⁻¹, which was attributed to the bands of (C-O-C), and it illustrated that PLGA had the chemical band of ester. In addition, the CS-PLGA group showed characteristic peaks at 2881.6 cm⁻¹ and 1424.8, which indicated the chemical bands of (CH₂) existed, and the absorption bands at 1751.4 cm⁻¹ proved the characterization of (C=O). These findings revealed that the chitosan had successfully covered the surface of PLA and PGA groups, and there was interaction between the pure PLA (PLGA) and chitosan molecules.

Figure 6.

FT-IR analysis of chitosan coated and uncoated PLA monofilaments and PLGA braiding threads; PLA group: PLA1, PLA2; CS-PLA group: CS-PLA1, CS-PLA2; PLGA group: PLGA1, PLGA2; CS-PLGA group: CS-PLGA1, CS-PLGA2; CS: chitosan coated; PLA: polylactic acid; PLGA: poly (D, L-lactide-co-glycolide); Part of the characteristic absorption bands: 3504.3 cm-1: (-OH); 1631.4 cm-1: (C=O) amide I; 1581.3 cm-1: (N-H) amide II; 1118.7 cm-1: (C-O-C).

Mechanical properties

Tensile property

Figure 7 shows the tensile properties of chitosan-coated and uncoated PLA monofilaments and PLGA braiding threads. Both the PLA and PLGA groups were observed to have an increase in their breaking strength and breaking elongation values after chitosan-coating treatment, whereas the degree of change was different. In detail, the uncoated PLA group (PLA1 and PLA2) presented breaking strength values of 12.43 ± 1.64 cN/dtex and 17.85 ± 0.78 cN/dtex, respectively, whereas the coated PLA group (CS-PLA1 and CS-PLA2) was 24.62 ± 2.35 cN/dtex and 31.26 ± 1.62 cN/dtex, respectively. Meanwhile, sample CS-PLGA1 (75.64 ± 3.23 cN/dtex) showed significant difference from that of PLGA1 (breaking strength = 63.21 ± 3.17 cN/dtex). These findings revealed that the breaking strength of the PLA and PLGA groups would be improved by the chitosan-coating treatment, which was caused by the chitosan layers and fragments covering their surfaces, and the coating also enhanced the degree of arrangement for PLGA braided threads, further causing the increase of their breaking strength values.

Figure 7.

Tensile properties of chitosan coated and uncoated PLA monofilaments and PLGA braiding threads, PLA group: PLA1, PLA2; PLGA group: PLGA1, PLGA2; PLA: polylactic acid; PLGA: poly (D, L-lactide-co-glycolide).

A similar change pattern was observed for the breaking elongation of PLA and PLGA groups. The coated PLA group (CS-PLA1 = 27.56% ± 4.23%, CS-PLA2 = 34.82% ± 5.21%) exhibited much larger breaking elongation values compared to the uncoated PLA group (PLA1 = 13.25% ± 1.78%, PLA2 = 18.92% ± 3.25%). As for the PLGA group, CS-PLGA2 (73.39% ± 2.38%) exhibited a significant difference to PLGA2 (56.41% ± 1.83%), whereas CS-PLGA1 (67.24% ± 4.64%) had larger breaking elongation values than PLGA1 (47.23% ± 2.76%). It illustrated that the chitosan coating was beneficial for the breaking elongation of the PLA and PLGA groups, and the PLGA group had larger breaking elongation values than the PLA group. The reason may be that the extension of inner large molecular chains for PLA and PLGA groups were improved with the chitosan-coating treatment, causing an increase in their breaking elongation values.

In sum, chitosan-coating treatment was beneficial for the tensile properties of both the PLA and PLGA groups, and the PLGA group showed better tensile properties than the PLA group.

Flexibility

To evaluate the flexibility of the PLA and PLGA groups, the values of bending stiffness were measured and discussed in this research. As shown in Figure 8, the PLGA group had larger bending stiffness than the PLA group. In particular, CS-PLGA2 (18.32 ± 3.04 cN•mm²) showed significantly more difference than CS-PLA2 (7.38 ± 1.86 cN•mm², p < 0.05), and CS-PLGA1 (15.19 ± 2.06 cN•mm²) presented much larger bending stiffness values than CS-PLA1 (5.67 ± 1.32 cN•mm², p < 0.05). In addition, CS-PLA2 was observed to have a much larger bending stiffness than PLA2 (3.41 ± 1.24 cN•mm², p < 0.05), and CS-PLGA2 showed significant difference to PLGA2 (12.64 ± 0.39 cN•mm², p < 0.05).

Figure 8.

Bending stiffness of chitosan coated and uncoated PLA monofilaments and PLGA braiding threads, PLA group: PLA1, PLA2; PLGA group: PLGA1, PLGA2; PLA: polylactic acid; PLGA: poly (D, L-lactide-co-glycolide).

These findings indicated that chitosan-coating treatment strengthened the bending stiffness for both the PLA and PLGA groups, and the PLGA group was stiffer and had less flexibility than the PLA group after coating treatment. The reason was that the coated chitosan layer could enhance the rigidity of the PLA and PLGA groups, and the arrangement of the molecules had become more orderly after coating, which is also beneficial for the improvement of bending stiffness of samples. In addition, from the surface structure testing results, the PLGA group had a larger surface roughness and specific surface areas compared to the PLA group, thus they could adhere to more chitosan coating in the modified process, further causing the larger bending stiffness values. Hence, this chitosan modification method was proved to have a high prospect for enhancing the rigidity of both the PLA and PLGA groups, which was conducive to bearing resistance during the embedding process.

In vitro characterizations

Cell cytotoxicity

The cell cytotoxicity of chitosan-coated and uncoated PLA monofilaments and PLGA braiding threads was determined by their leach liquors, which was incubated with the C2C12 rat fibroblasts for 48 h. As shown in Figure 9, both the PLA and PLGA groups were observed to have no toxicity, with more than 75% of cells being viable. Moreover, the chitosan-coated samples showed larger cell viability than uncoated samples. CS-PLA1 (94.2 ± 1.28%) had the larger cell viability values than PLA1 (83.5 ± 1.34%), and CS-PLGA2 (96.3 ± 1.32%) showed a larger difference than PLGA2 (87.4 ± 2.49%). These results agree with the theory that chitosan promotes cell proliferation and biocompatibility, and samples from the CS-PLGA group with better cell viability had greater potential in the application of ACET materials.

Antibacterial efficacy test

Zones of inhibition were adopted to evaluate the antibacterial properties of PLA monofilaments and PLGA braiding threads (Figure 10). As shown in Figure 11, the uncoated samples (PLA group and PLGA group) presented no antibacterial activity, whereas the coated samples (CS-PLA group and CS-PLGA group) were observed to have effective antimicrobial activity against Escherichia coli and Staphylococcus aureus. It was revealed that chitosan-coating treatment successfully improved the antibacterial activity of PLA monofilaments and PLGA braiding threads. In addition, the CS-PLGA group showed a much larger zone of inhibition than the CS-PLA group, which indicated the PLGA group had a better antibacterial capacity compared to the PLA group. This was because the PLGA group were shown to have larger amounts of chitosan than the PLA group after coating treatment.

Figure 9.

C2C12 rat fibroblast viability after incubated with chitosan coated and uncoated PLA monofilaments and PLGA braiding threads for 48 h, PLA group: PLA1, PLA2; CS-PLA group: CS-PLA1, CS-PLA2; PLGA group: PLGA1, PLGA2; CS-PLGA group: CS-PLGA1, CS-PLGA2; CS: chitosan coated; PLA: polylactic acid; PLGA: poly (D, L-lactide-co-glycolide).

Figure 11.

Antibacterial activity of chitosan coated and uncoated PLA monofilaments and PLGA braiding threads, PLA group: PLA1, PLA2; CS-PLA group: CS-PLA1, CS-PLA2; PLGA group: PLGA1, PLGA2; CS-PLGA group: CS-PLGA1, CS-PLGA2; CS: chitosan coated; PLA: polylactic acid; PLGA: poly (D, L-lactide-co-glycolide).

Figure 10.

Zones of inhibition: (a) schematic drawing; and (b) photograph.

Degradation behavior

To evaluate the degradation behaviors of the PLA group and PGA group after implanting, samples were immersed in PBS solutions and weighted every 7 days. Afterwards, weight loss was calculated and analyzed.

As shown in Figure 12, the CS-PLA and CS-PLGA groups were observed to have much smaller weight loss values than the PLA and PLGA groups. For example, the CS-PLA group began to degrade in the 28th day, whereas it was the 14th day in the PLA group. These findings illustrated that chitosan-coating treatment slowed down the degradation rate of samples because the chitosan-coating layer covering the surface of samples prevented direct contact between embedding materials and water molecules, which greatly slowed down the first stage of hydrolysis. Moreover, samples were found to have the different degradation behaviors. The PLA group had almost no changes during the experiment period, whereas the PLGA group first performed a relatively slow degradation in the 21 days, then accelerated and lost much of its weight in the next 3 weeks. Finally, the degradation rate decreased and the samples lost most weight on the 70th day. These findings could be explained by the theory of You et al.⁴⁵ who reported that the degradation of PLGA was slow in the induction period and became greater in time. This was because water molecules attack the chemical bonds before entering the internal structure of the PLGA, and the breakdown of chemical bonds exposed a high specific surface area, which promoted the further invasion of hydrolysis.

Figure 12.

Weight loss of the polylactic acid group and poly (D,L-lactide-co- glycolide) group. PLA group: PLA1, PLA2, CS-PLA1 and CS-PLA2; PLGA group: PLGA1, PLGA2, CS-PLGA1 and CS-PLGA2; CS: chitosan coated; PLA: polylactic acid; PLGA: poly (D, L-lactide-co-glycolide).

In sum, this chitosan-coating treatment could slow down the weight-loss behavior of both the PLA and PLGA groups. This is beneficial for the lasting effect of embedding materials in the human body and provides a new insight into controlled degradation research in the ACET field.

Conclusion

In this research, we prepared novel PLA and PLGA embedding materials using chitosan-coating treatment. To gain better comprehensive performance, their preparation processes were studied and a new one-dip-one-rolling coating system was established. Characterizations such as structure, mechanical, and biocompatibility properties were measured to verify the feasibility of surface modification technology for developing novel antibacterial embedding materials. The following results could be concluded from this work.

By measuring structure characterizations, the surfaces of both the PLA and PLGA groups were shown to cover chitosan layers and some fragments; the CS-PLGA group achieved more chitosan coating than the CS-PLA group due to its rougher surface structure. The FT-IR analysis results verify the existence of chitosan molecules and indicated that there were few new chemical bonds after surface modification.

Via the analysis of mechanical properties, the tensile properties and bending stiffness of PLA and PLGA groups were greatly improved after chitosan coating, and the CS-PLGA group presented the better mechanical performance than the CS-PLA group, caused by the larger amount of chitosan coating to which PLGA braiding threads adhered.

The biocompatibility evaluations demonstrated that all the samples were non-toxic and the modified PLA and PLGA groups demonstrated effective antibacterial efficacy against Staphylococcus aureus and Escherichia coli. Their weight-loss behavior was delayed after coating treatment, which was beneficial for the lasting effect of embedding materials in the treatment period. Overall, the modified groups have alternative potential for product application in novel antibacterial ACET materials.

These findings suggested that chitosan coating was an easily operated and efficient method for PLA and PLGA groups to achieve requirements of antibacterial properties, while retaining the other advantages. More than that, the CS-PLGA group with a larger amount of more chitosan coating seemed more suitable for ACET applications.

In summary, this study may inspire advancements in the design and fabrication of PLA and PLGA embedding materials with antibacterial activity to satisfy clinical applications.

Footnotes

Acknowledgement

The first author thanks the scholarship support from China Scholarship Council.

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 is supported by the 111 Project - Biomedical Textile Materials Science and Technology (B07024) and The Fundamental Research Funds for the Central Universities (BCZD2018008).

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