Ammonium hydroxide modification of polylactic acid and polyglycolic acid monofilaments for acupoint catgut embedding therapy

Abstract

Polylactic acid (PLA) and polyglycolic acid (PGA) monofilaments have attracted much attention for their wide usage in acupoint catgut embedding therapy (ACET). Their application is restricted, however, by their poor hydrophilicity and cell attachment properties. In this study, PLA and PGA monofilaments were produced from polymer chips, and then modified by the application of an ammonium hydroxide solution to the surface. The modified PLA and PGA monofilaments were fully characterized with respect to their structural, mechanical, and in-vitro properties. The results showed that the surface roughness and hydrophilicity of the materials were greatly increased; surface modified samples of both materials exhibited the smallest contact angle values: 79.2° ± 2.5° (sample PLA2) and 75.9° ± 1.4° (sample PGA2). The weights and diameters and the tensile and flexibility properties of the materials changed little with surface modification, but their swelling ratios increased significantly. All the prepared samples were non-toxic (more than 75% of cells being viable). Surface modification also enhanced cell attachment: PLA2 (48.15% ± 2.16%) and PGA2 (59.43% ± 3.18%) showed the largest cell attachment values (cultured for 48 h) among the samples. In summary, the study proves the feasibility of ammonium hydroxide modification of PLA and PGA, which is beneficial for guiding future work on developing functional PLA or PGA materials for ACET.

Keywords

acupoint catgut embedding therapy (ACET)ammonium hydroxide hydrophilicity cell attachment surface modification

Introduction

Acupoint catgut embedding therapy (ACET) involves inserting biodegradable materials into the human body for the treatment of certain common diseases, including pseudomyopia,¹ obesity,² neuropathic pain,³ etc. ACET is regarded as one of the most promising methods to replace traditional acupuncture therapy, because it offers many advantages including long-lasting benefits, ease of operation, and significant curative effects.^4,5 As such, the ideal materials for ACET should satisfy a series of requirements, including: good easy-forming ability, swelling capability, availability, and biodegradable and biocompatible properties. Unfortunately, there is no ideal ACET material on the market, and the relevant standards are incomplete.⁶

In recent years, a variety of biodegradable materials have been widely used for ACET threads, which can be divided into natural and synthetic ACET materials according to their essential attributes.^7,8 Natural ACET materials, such as catgut⁹ and collagen,¹⁰ have many advantages including good degradability and biocompatibility, but they are easily susceptible to complications after surgery and are of low quality being essentially of protein.¹¹ The synthetic ACET materials, which include polylactic acid (PLA),¹² polyglycolic acid (PGA),¹³ and poly lactic-co-glycolic acid (PLGA),¹⁴ present better mechanical properties, biodegradability, and biocompatibility compared with natural biodegradable materials, but they often offer poor hydrophilicity and cell attachment capacity in the application process.^15,16 Hence, it is the objective of researchers to combine the advantages of natural and synthetic ACET materials, while avoiding their disadvantages.

Surface modification of polymers is a recent approach to enhance their hydrophilicity and cell adhesion. Schaub et al.¹⁷ applied the oxygen plasma modification method on poly-l-lactic acid (PLLA) nanofibers; the results showed that the hydrophilicity and cell attachment ability of plasma modified PLLA was greatly improved, and the amount of oxygen containing groups increased. Koo et al.¹⁸ applied the UV/ozone irradiation method in the surface modification of PLA, and found that the water contact angle decreased from 61° to 39° and surface energy slightly decreased with the increase of UV energy. Lin et al.¹⁹ coated PGA fibers with hyaluronic acid solutions, and found less inflammatory reaction upon in-vivo subcutaneous implantation, better cell adhesion to the scaffold, and decreased acidity of degradation products in vitro. Ospina-Orejarena et al.²⁰ produced collagen-functionalized PLA using a grafting method, and the nanofiber scaffolds were observed to have better bioactivity, mechanical properties, and cell attachment ratios compared with untreated ones. Costes et al.²¹ applied ammonium hydroxide solutions (NH₄OH, 28–30% NH₃ basis) on the phosphorus/nitrogen modification of lignin process, further enhancing its flame retardant effect in PLA. Gao et al.²² prepared Poly (D, L) Lactide (PDLLA)/bioactive glass composite films using surface modified bioactive glass particles through a solvent casting-evaporation method, and the modified particles showed better hydrophobicity and longer time of suspension in an organic matrix. Ammonia surface treatment is an effective and simple method used to improve the surface properties of biomedical materials in the tissue engineering field, especially for biodegradable materials such as PLA, PGA, PCL, etc. The ammonium hydroxide treatment will enhance the roughness of materials by surface etching due to its weak alkalinity, while not causing further damage to the inner structures of materials.^23–25 Although many surface modifications have been brought forward, these modified conditions are still relatively complex and they cannot satisfy the desired modification effects for ACET, including good swelling behavior, hydrophilicity, and biocompatibility. Seldom has research focused on the surface modification of linear PLA and PGA materials, the majority of studies cover films, meshes, and other products.

In this work, PLA and PGA monofilaments were first produced from polymer chips, and then subjected to surface modification by an ammonia solution. The comprehensive performances such as structural properties (surface morphology, X-ray photoelectron spectroscopy (XPS), Fourier Transform infrared spectroscopy (FTIR) analysis, pH values, weight and diameter, hydrophilicity), mechanical properties (tensile, flexibility, swelling), and biocompatibility (cell cytotoxicity, cell attachment ratio) were fully characterized. The feasibility of the ammonia-modified method on PLA and PGA monofilaments was evaluated based on these experimental results.

Materials and methods

PLA polymer chips were provided by Shenzhen Esun Industrial Co., Ltd., China. PGA polymer chips were purchased from PURAC Co., Ltd, Holland. The fundamental properties of the PLA and PGA chips are listed in Table 1. Ammonia solution (concentration of NH₃ = 25%, CO₂ < 0.001%, PO₄ < 0.0001%, SO₄ < 0.0002%) was provided by Shanghai Lingfeng Chemical Reagent Co., Ltd, China. Alcohol solution, with a purity of 75%, was purchased from Shanghai Lianyu Chemistry industry Co., Ltd, China. All the chemicals were analytically pure and used without further purification.

Table 1.

Fundamental properties of the PLA and PGA chips used to produce monofilament samples

Material	Density (g/cm³)	Intrinsic viscosity (dL/g)	Melting temperature (℃)	Flexural modulus (Mpa)
PLA	1.25	4.2	150	3110
PGA	1.63	1.3	213	2651

Note: The experimental materials used in this study were 100% medical materials 1MPa = 1000000Pa = 1000000N/m².

Production of PLA and PGA monofilaments

Based on previous reports, it is highly efficient and easy for PLA and PGA to be shaped into linear materials such as monofilaments, sutures, threads, etc.^26,27 In this study, PLA and PGA polymer chips were first immersed in alcohol solution (75% (v/v)) for 30 min to remove impurities, including oil and dust. They were then put into a screw extruder (HLY-6/18-C5; Dehong Rubber Machinery Co., Ltd, Shanghai, China) through a hopper at temperatures of 185℃ (PLA) and 240℃ (PGA), respectively. Raw monofilaments were prepared at a spinning speed of 250 m/min, and then transmitted to the drawing area (drawing ratio = 4) via guiding rollers. Finally, PLA and PGA monofilaments were produced by the function of front and rear rollers, and wound onto tubes at a winding speed of 200 m/min. In the present work, four types of monofilaments with different diameters were produced; the PLA monofilaments were coded as PLA 1 and PLA 2, respectively, while the PGA monofilaments were coded as PGA 1 and PGA 2, respectively.

Modification of PLA and PGA monofilaments

Dip-coating technology is regarded as the most promising method of surface modification of textile materials, further enhancing their surface roughness, mechanical properties, hydrophilicity, and cell adhesion via the interface reactions.^28,29 Hence, an ammonium hydroxide solution (NH₃ concentration = 25%) was adopted as the modifying agent in the surface treatment of the PLA and PGA monofilaments. In detail, the prepared samples were first immersed in an alcohol solution (75% (v/v)) for 30 min to remove surface impurities, and then they were immersed in the ammonium hydroxide solution for 20 min (after several trial experiments), followed by washing three times with deionized water. Finally, the modified samples were freeze-dried in a vacuum freeze-drying dryer (FD-1 A-50; Bilang Instruments Co., Ltd, Shanghai, China) for 24 h to remove the inner water. The resultant surface modified PLA monofilaments were coded as M-PLA 1 and M-PLA 2 and the surface modified PGA monofilaments were coded as M-PGA 1 and M-PGA 2. The properties of the PLA and PGA monofilaments, unmodified and modified, were then comprehensively compared to evaluate the efficiency of the ammonia modification method.

Characterizations

Structural properties.

Surface morphology. The surface morphologies of the PLA and PGA monofilaments, unmodified and modified, were examined by scanning electron microscopy (SEM) (Hitachi TM 3000, low vacuum secondary detection, 15 kV, 47 µA, Japan). To avoid the charging problems of fibers, the samples were set on round stainless steel holders with double-sided adhesive conductive tape and then coated with gold before observation.

XPS. XPS (Source gun type: Al K Alpha; vacuum 10-9 bar; wide resolution scan 100 eV; high resolution scan 30 eV; binding energy of C element 284.8 eV; Kratos AXIS Ultra DLD, Daojin, Japan) was adopted to analyze the surface chemical compositions of unmodified and modified filament samples.

FT-IR analysis. A PerkinElmer spectrum 100 FT-IR spectrometer was used to evaluate the interaction mechanism of prepared samples. The disks were obtained by compacting with a hydrostatic press at a force of 10 tons for 1 min. The FT-IR spectrum was recorded with a resolution of 4 cm⁻¹ at room temperature over 16 cumulative scans in a wave number range of 4000–750 cm⁻¹.

pH values. A pH meter type 84-1 A was adopted to evaluate the effect of ammonia surface modification on the PLA and PGA samples.

Weight and diameter. An electronic balance (FA2004A-200 G, Jingtian Instrument Factory, Shanghai, China) was used to weigh each sample per 10 cm in length, and the results were expressed with average and standard deviations for 10 replications. In addition, an optical microscope (DA1-180 M, Yongxin Optics Co., Ltd, Ningbo, China) and Image J software was used to measure the diameter of each sample. The results were expressed with average and standard deviations for 10 replications.

Surface hydrophobicity. An OCA15EC water contact angle measuring instrument (Defei Instrument Co. Ltd, Shanghai, China) was employed to evaluate the hydrophilicity of the samples. First, deionized water (2 µL) was dropped on to the surface of each sample, and a microscope was used to observe the angle between water and sample. Three different points of each sample were tested and their average values were used to express the surface hydrophilicity.

Mechanical properties.

Tensile properties. An electronic single-yarn tensile tester (Model YG061f, Shandong Laizhou Electron Instrument Co., Ltd., Laizhou, China) was adopted to evaluate the tensile properties (tensile strength and breaking elongation) of samples. Based on the Chinese National Standard GB/T 6529, samples were conditioned for 24 h at standard atmospheric conditions (temperature = 20 ± 0.2℃, relative humidity = 65 ± 2%) before testing. The details of the testing parameters were as follows: the gauge length was 500 mm, tensile speed was 300 mm/min, and pre-tension was 1 cN/tex. The final result was expressed with an average value and a standard deviation of 10 replications.^30,31

Flexibility. Based on the testing principle of the cantilever beam method, the bending stiffness of the samples was tested to evaluate the flexibility of the monofilaments.³² In order to gain more accurate results, the testing parameters were selected after several trials. The final result was expressed with an average value and a standard deviation of three replications.^33,34

Swelling behavior. Phosphate-buffered solution (PBS, pH = 7.4, NaCl 8 g/L, KCl 0.2 g/L, Na₂ HPO₄ċ12H₂O 2.9 g/L, and KH₂PO₄ 0.2 g/L) was employed to test the swelling ratio of the samples. Diameter before and after the swelling test was measured by an optical microscope and processed by Image J software. Three measurements were made and recorded as mean and standard deviation. The swelling rate was calculated using Equation (3) as follows.

Swelling ratio (%) = \frac{d_{1} - d_{0}}{d_{0}} \times 100 %

(3)

where d₀ (mm) refers to the diameter of the sample before expansion and d₁ (mm) refers to the diameter of the sample after expansion.

Cell compatibility

C2C12 muscle cells and rat fibroblasts were provided by the Center for type culture collection in Shanghai, China, and they were isolated and cultured to compare the biocompatibility of unmodified and modified PLA and PGA samples.^35,36

Cytotoxicity. Based on the GB/T 16886.5-2017 standard, the cell counting kit-8 (CCK-8) cell proliferation method was used to evaluate the cytotoxicity of the samples. In order to sterilize them and remove impurities, samples were immersed in alcohol solution (75% (v/v)) for 3 h, and then washed in the PBS solution. In detail, the samples were first placed into Dulbecco’s modified Eagle’s medium at a certain ratio and cultured under a standard culture condition at 37℃ for 48 h in the incubator to achieve leach liquor. Second-generation rat fibroblasts (1 × 10⁴ml) were then 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 and control (without any treatment). The optical density (OD) value was achieved by using a microplate reader at a wavelength of 450 nm. The relative growth rate (RGR) was calculated by Equation (4) as follows.

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

(4)

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

Cell attachment and cell morphology. Samples (1 cm) were immersed in alcohol solution (75% (v/v)) for 3 h, and then placed in the wells of 24-well plates, and the bottom surface of each well was fully covered by the disk. Each sample was repeated three times. C2C12 cells were harvested and seeded in the wells. Each well was inoculated with a total of 1 mL medium of 2.0 × 10⁶ cells/mL. After culturing for 24 h in a humidified incubator (37℃, 5% CO₂), the unattached cells were removed by washing with PBS three times, and the attached cells were fixed for 10 min in 3% glutaraldehyde at room temperature. The samples were rinsed in PBS, and then the cells were dyed with one drop of Giemsa stain (Sigma) for 30 min, and washed with distilled water. Cell attachment and morphology were observed with a phase contrast microscope (Inverted fluorescence microscope: Nikon, Shanghai, China). The percentages of C2C12 coverage on samples with modified and unmodified surfaces were calculated and analyzed by Image 6.0 software. This experiment was repeated three times and each specimen was measured at five different positions.

Results and discussion

Structural properties of prepared samples

Surface morphology

Figure 1 shows the SEM images of the PLA and PGA monofilament samples. Both the unmodified PLA (PLA 1 and PLA 2) and PGA (PGA 1 and PGA 2) groups were observed to have relatively smooth surfaces, while the modified PGA group exhibited some impurities and grooves on the sample surfaces; such damage may be caused in the spinning process, however. As for the modified monofilament samples, some serious grooves and peels were observed among both the PLA (M-PLA 1 and M-PLA 2) and PGA (M-PGA 1 and M-PGA 2) groups. This phenomenon was consistent with the findings of previous studies.^37,38 The surface roughness of both PLA and PGA monofilaments was greatly increased after modification, and the alkaline corrosion effectively altered the surface structures of the samples.

Figure 1.

SEM images of PLA and PGA monofilament samples.

XPS

The surface chemical compositions of the PLA and PGA samples were analyzed by XPS. Figure 2 shows the XPS spectra of the unmodified (control) and modified PLA and PGA. The surfaces of both the unmodified and the modified samples were dominated by C1s and O1s signals at 284 and 533 eV. XPS indicated that the atomic carbon/oxygen (C/O) ratio decreased, which may be caused by the introduction of various functional groups with different characteristics. Moreover, high resolution C1s spectrum images of the samples were collected for further investigation of changes in surface chemical bonds, in which there were peaks at 284.8 eV, 287.3 eV, and 288.5 eV, corresponding to C-C, C=O and O=C-O. Considering the decreased C/O ratio, chemical bonds such as C=O and O=C-O showed increasing trends at different degrees. In summary, all these data indicate that the surface modification of PLA and PGA with ammonia resulted in the increase of polar groups like hydrophilic groups (C=O and O=C-O) and a decrease of nonpolar groups like C-C.

Figure 2.

XPS survey spectra and high-resolution C1s XPS spectra of modified and non-modified PLA and PGA filament samples.

FT-IR analysis

Figure 3 shows the FT-IR analysis of the PLA and PGA samples. The PLA group was observed to have five absorption bands at: 3507.8 cm⁻¹, 2960.1 cm⁻¹, 1760.6 cm⁻¹, 1725.9 cm⁻¹, and 1089.5 cm⁻¹, respectively. The sharp bands at 3507.8 cm⁻¹, 2960.1 cm⁻¹, and 1089.5 cm⁻¹ contributed to the bending vibration of (O-H), (C-H), and (C-O), respectively. The absorption bands at 1760.6 cm⁻¹ and 1725.9 cm⁻¹ were attributed to the bands of (C=O). In detail, there existed some blue-shift appearances for the modified PLA, and the vibration amplitude of molecular groups was increased. In the case of the PGA group, some characteristic peaks such as 2962.4 cm⁻¹, 1440.5 cm⁻¹, and 1085.7 cm⁻¹ indicated the presence of (C-H), (-CH2) and (-CH2-COO-), respectively. Moreover, the modified PGA (M-PGA1 and M-PGA2) showed the blue-shift phenomenon compared with non-modified PGA (PGA1 and PGA2). In summary, these findings illustrate that surface modification improved the polarities of molecular bonds for both PLA and PGA, which may be beneficial to the increase of surface hydrophilicity.

Figure 3.

FT-IR analysis of PLA and PGA monofilament samples.

Evaluation of pH

As shown in Table 2, both the PLA and PGA groups presented weak alkalinity after the ammonia treatment, but the modified PGA samples showed slightly higher pH values than the modified PLA samples.

Table 2.

pH values of PLA and PGA monofilament samples after ammonia solution treatment

Sample	Modified PLA		Modified PGA
Sample	M-PLA 1	M-PLA 2	M-PGA 1	M-PGA 2
pH value	11.7	10.5	12.1	12.8

Weight and diameter

The weights and diameters of PLA and PGA were shown in Figure 4. In case of the weights of samples, both modified PLA (M-PLA1 and M-PLA2) and modified PGA (M-PGA1 and M-PGA2) were observed to have smaller weight values compared to that of unmodified PLA (PLA1 and PLA2) and unmodified PGA (PGA1 and PGA2). The weight of sample PLA decreased larger than that of sample PGA for the analysis of Figure 4. To regard with the diameter of samples, sample PLA1 showed slightly larger diameter values compared to that of sample M-PLA1, while the similar regular was observed for the diameters of PGA (sample PGA1>sample M-PGA1). The reason was that the ammonium hydroxide treatment caused some surface corrosions on the PLA and PGA, which caused the slightly decreased of their weights and diameters.

Figure 4.

Weights and diameters of PLA and PGA; Polylactic acid (PLA) and polyglycolic acid (PGA).

Surface hydrophilicity

Figure 5 shows the surface hydrophilicity of the PLA and PGA groups. The contact angle values of the modified samples were smaller than those of the unmodified ones. In detail, sample M-PLA1 (84.2° ± 1.5°) presented much smaller contact angle values than PLA 1 (118.5° ± 3.2°), while the contact angle values of M-PLA2 (79.2° ± 2.5°) were also much smaller compared with PLA 2 (127.3° ± 1.8°). As for the PGA group, the contact angle value of sample PGA 1 was the largest (104.7° ± 1.6°), followed by samples M-PGA1 and PGA2 which had contact angle values of 81.4° ± 3.7° and 126.8° ± 2.4°, respectively. M-PGA2 exhibited the smallest contact angle value, at 75.9° ± 1.4°.

Figure 5.

Contact angle values of PLA and PGA monofilament samples.

The modified PGA group (M-PGA1 and M-PGA2) exhibited greater surface hydrophilicity than the modified PLA group (M-PLA1 and M-PLA2). The samples with larger diameters (PLA 2/M-PLA2 and PGA 2/M-PGA2) (see Figure 4) presented greater reduction in contact angle values after modification than samples with smaller diameters (PLA 1/M-PLA1 and PGA 1/M-PGA1). These findings illustrate that both the PLA and PGA groups presented better surface hydrophilicity after ammonium hydroxide modification, which was consistent with the increase of surface roughness observed from the analysis of their SEM images.^39,40 In summary, the increase of surface roughness and hydrophilic or polar groups were the main factors for improvement of modified PLA and PGA monofilaments as ACET materials.

Mechanical properties of prepared samples

Tensile properties

The tensile properties of the PLA and PGA monofilaments before and after ammonia treatment were examined and compared, respectively. As shown in Figure 6, in the case of tensile strength, unmodified PLA samples differed from PGA samples, while the modified samples had similar tensile strength values compared with their unmodified counterparts. A similar pattern was found for the breaking elongation values of the PLA and PGA groups. For instance, the breaking elongation of sample PLA 2 (61.35 ± 1.06%) was greater than that of sample PGA 2 (48.52 ± 1.28%), while it was similar to the modified sample M-PLA 2 (57.36 ± 1.84%). In addition, PLA 1 (56.24 ± 3.29%) presented a slightly larger breaking elongation value than that of M-PLA 1 (53.26 ± 4.23%). The modified samples exhibited smaller breaking elongation values compared with non-modified ones, which was caused by the surface alkaline corrosion from the ammonium hydroxide solution, and the PLA group exhibited greater tensile strength and breaking elongation values than the PGA group. The reason may be that the chemical bonds, such as covalent bonds and hydrogen bonds between macromolecules, are slightly destroyed, causing the decrease of elongation at break for both PLA and PGA monofilaments. Thus, both the PLA and PGA groups could retain the majority of their tensile properties after modification. In the authors’ present studies, tensile strength values of more than 20 cN/dtex, and tensile elongation of more than 15%, are deemed suitable for ACET.

Figure 6.

Tensile properties values of PLA and PGA monofilament samples.

Flexibility

The flexibility values of the PLA and PGA groups are shown in Figure 7. It can be seen that bending rigidity was adopted to evaluate the flexibility of the samples according to the cantilever beam method and calculation formula.^41,42 In detail, the bending stiffness values of the modified PLA monofilaments (M-PLA 1, M-PLA 2) exhibited little difference from those of the unmodified PLA monofilaments (PLA 1, PLA 2), and a similar pattern was observed among the PGA group; PGA monofilaments changed little in their bending stiffness values compared with unmodified ones.

Figure 7.

Bending rigidity of PLA and PGA monofilament samples.

This phenomenon could be explained by the observation that ammonium hydroxide treatment had little effect on the flexibility of the PLA and PGA monofilaments, and this was similar to tensile property; the etching action of the treatment only applied on the surface of the samples, not causing further damage to the modified monofilaments. In summary, both the PLA and PGA groups changed little in their bending stiffness values after modification, but the PLA group presented greater rigidity than the PGA group. Samples with large diameters (PLA 2 and PGA 2) exhibited similar changes of bending stiffness to samples with small diameters (PLA 1 and PGA 1) after modification.^43,44

Swelling behavior

Figure 8 shows the swelling behaviors of the PLA and PGA groups. The PGA monofilaments presented better swelling behavior than the PLA monofilaments. For example, sample PGA 1 (12.35 ± 1.84%) had larger swelling ratio values compared with sample PLA 1 (6.19 ± 0.82%), and the swelling ratio of M-PGA 1 (23.18 ± 2.57%) was larger than that of M-PLA 1 (17.62 ± 3.46%). Moreover, the modified samples had larger swelling ratio values than the unmodified samples. This illustrates that the ammonia modification was beneficial for the swelling behavior of the PLA and PGA monofilaments.

Figure 8.

Swelling behavior of PLA and PGA monofilament samples.

The reason for the increased swelling may be that the surface roughness and hydrophilicity of the PLA and PGA samples were enhanced after modification, and the surface energy of the monofilaments was improved, further increasing their water absorption capacity, and further causing the enhancement of the swelling ratio for both PLA and PGA groups.⁴⁵

Cell compatibility

Cytocompatibility characterization

The cell viability of the PLA and PGA samples is shown in Figure 9. Cell viability was determined by incubating C2C12 rat skeletal muscle cells with the media of PLA and PGA groups’ leach liquors for 48 h. All the samples were observed to present no toxicity (more than 75% of cells being viable). The unmodifed samples (PLA 1 = 81.3 ± 1.8%, PGA 1 = 87.2 ± 1.5%) showed less cell viability than the modified samples (M-PLA 1 = 92.5 ± 2.4%, M-PGA 1 = 94.7 ± 3.8%). The reason may be that the ammonium hydroxide treatment was beneficial for the removal of internal impurities and undesirable products in the degradation process. Moreover, the diameter of PLA and PGA monofilaments had little effect on their cytocompatibility before and after ammonia modification. The modified samples showed higher cell viability values than unmodified samples, which may be caused by the disinfection and impurity removal functions of the ammonium hydroxide treatment.

Figure 9.

Cell viability of PLA and PGA monofilament samples.

Cell attachment and cell morphology

As shown in Figure 10, the modified PLA monofilaments (M-PLA1 and M-PLA2) exhibited larger cell attachment ratio values than the unmodified PLA monofilaments (PLA 1 and PLA 2). As for the PGA group, sample M-PGA2 presented the largest cell attachment ratio, which was different from that of sample PGA 2, and the cell attachment ratio of sample M-PGA 1 was much larger compared with that of sample PGA 1. These findings reveal that ammonium hydroxide modification could effectively improve the cell adhesion ability of PLA and PGA materials. The reason is that the surface roughness of PLA and PGA is increased by the corrosion of the alkaline liquid, further causing the increase of specific surface area, which is beneficial for the improvement of cell attachment ability for PLA and PGA monofilaments. Furthermore, the increase of polar chemical bonds will also enhance the surface attachment capacity for PLA and PGA monofilaments.

Figure 10.

Cell attachment of PLA and PGA monofilament samples.

Figure 11 shows optical micrographs of the C2C12 cells attached to the PLA and PGA groups. The unmodified PLA and PGA monofilaments were observed to have relatively few C2C12 cells on their surfaces, and there was little difference between the unmodified PLA and PGA cultures after 48 h. As for the modified samples, more C2C12 cells could be clearly seen on the surfaces of both PLA and PGA groups, and the cell growth around the modified monofilaments was better than around the unmodified ones. These findings reveal that ammonium hydroxide treatment effectively enhanced the cell attachment ability for both the PLA and PGA groups, and the PGA group presented better cell attachment ratios than the PLA group.

Figure 11.

Cell morphology of modified and unmodified PLA and PGA monofilaments samples.

Conclusion

In this work, characterizations of surface modified PLA and PGA monofilaments were fully evaluated using a series of testing technologies. The modified samples exhibited greater surface roughness, and some grooves and peels were observed on the SEM images. As for the XPS measurements, chemical bonds such as C=O and O-C=O increased, while the C-C bond decreased slightly, indicating that the surface polarity of samples increased after modification. Modified samples tended to have weak alkalinity, according to the results of pH tests. The modified samples had little change in their tensile and flexibility properties, but their swelling behaviors were greatly improved. Samples M-PLA 2 (20.13% ± 2.45%) and M-PGA 2 (28.51% ± 1.38%) presented the greatest swelling ratio values among their groups, respectively. Moreover, all the prepared samples were proven to be non-toxic, and the modification was observed to improve cell viability. The ammonium hydroxide treatment effectively enhanced the cell attachment ability of the samples, which may be caused by the increase of surface roughness and hydrophilicity. The modified PGA samples exhibited larger cell attachment ratios compared with the PLA samples, which seems to have more outstanding potentiality in clinical applications.

In summary, ammonia surface modification is a novel approach for improving the hydrophilicity and cytocompatibility of PLA and PGA, while retaining other excellent characteristics of these materials after modification. The surface modified PLA and PGA monofilaments had comprehensively better properties, so that they can directly replace the traditional ACET materials, or have the potential to be braided into multifilament sutures or threads for ACET in future applications.

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 is Sponsored by Shanghai Sailing Program (20YF1401000), the Fundamental Research Funds for the Central Universities, the Initial Research Funds for Young Teachers of Donghua University, Engineering Research Center of Technical Textiles, Ministry of Education, Donghua University and the 111 Project (BP0719035).

ORCID iDs

Shaoju Fu

Peihua Zhang

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