Emulsion electrospun polylactic acid/ Apocynum venetum nanocellulose nanofiber membranes with controlled sea buckthorn extract release as a drug delivery system

Abstract

Prolonging the duration of drug action and reducing toxicity play a vital role in wound administration as they reduce the chance of infection and decrease complications and cost. This study reports the natural antioxidant procyanidins extracted from sea buckthorn (SBT) and laboratory-manufactured Apocynum venetum cellulose nanofiber as core drugs. The sustained-release nanofiber membrane was prepared by electrospinning on polylactic acid/polyvinyl pyrrolidone nanofibers. High-performance liquid chromatography-mass spectrometry was used to identify the phenolic compounds in SBT extracts and confirmed the presence of procyanidins with a content of 0.0345 mg/g. The nanofiber membrane was characterized through transmission electron microscopy, encapsulation efficiency, in vitro drug-release study and antioxidant assay. The results indicated that the extracted procyanidins were successfully encapsulated in the core–sheath structure nanofibers, and the encapsulation efficiency of nanofiber membranes reached 83.84%. In vitro measurements of the delivery showed this core–sheath structure could significantly alleviate the drug burst release, which is followed by a linear and smooth release within 30 hours. Further tests showed that the removal efficiency of 2,2-diphenyl-1-picrylhydrazyl reached 88.62%, indicating that the membranes had high antioxidant activity. This work implies that the combination of Apocynum venetum nanocellulose and emulsion electrospun fibers has promising potential applications in tissue engineering or drug delivery.

Keywords

sea buckthorn core–sheath structure antioxidant activity controlled-release delivery

Wound treatment is a major health issue, requiring wound dressings that can be effective for a long time with the reduction of drug resistance. Excessive and improper use of broad spectrum antibiotics exacerbates the problem of wound health.¹ To date, various types of wound dressings, namely general cotton dressings, electrospun nanofibers membranes and hydrogels, have been developed, loaded with single resistant synthetic drugs and with rapid release rates that limited dressing performance.² Therefore, the search for reducing drug resistance and tailored release kinetics plays an important role in wound healing. Recently, nanofibrous materials for loading and controlled-release therapeutic drugs have been studied due to the structural similarity of nanofibers to the extracellular matrix and the porous structure.³ Generally, the sustained-release method requires direct application or microencapsulation,⁴ followed by wrapping in nanofiber membranes. However, the weak interaction between microspheres and the polymer matrix often leads to separation from the electrospun nanofiber membranes, and the loading of drugs with different hydrophilic and hydrophobic properties on the nanofiber may also be limited.⁵ Emulsion electrospinning could enhance drug loading by forming a uniform emulsion to encapsulate drug molecules. The polymer is used as a core material for storing and protecting the shell layer of the nanofiber, and encapsulating the active substance in the core layer improves the drug loading. The medicine is released through the polymer nanopore penetration or released as the shell sheath polymer degrades, thereby slowly controlling the sustained release of the medicine. In addition to efficiency and continuous administration, such a delivery route also minimizes toxicity or side effects of the drug.⁶ Biopolymer fibers provide the drug with a sustained-release profile, which improves the therapeutic effect of the drug. At the same time, it is necessary to find natural drugs with low resistance to reduce damage to the human body.

Sea buckthorn (SBT) berries contain a range of healthful compounds, such as flavonoids, tannins and polyphenol procyanidins, so, the SBT extract shows a variety of medicinal effects.^7–9 Studies have reported that some molecules from medicinal plants possess a high antioxidative potential and are thought to have beneficial effects in diseases related to oxidative stress and free radicals, with a small chance of developing resistance.^9–11 Upadhyay et al.¹² reported that SBT leaf extract has significant healing potential in burn wounds and a positive influence on the different phases of wound repair in rats. Janeš and Glavač¹³ treated burn patients with a SBT pericarp oil dressing, whereas the control group was treated with petrolatum, and pericarp oil was proven to act as an antioxidant, anti-inflammatory and regenerative agent. Therefore, SBT with natural antioxidant procyanidins is preferred for effectively promoted wound healing and could be used potentially for bio-medical materials. Although there are various advantages of SBT berries extract in the bio-medical field, these also have some limitations, including dripping, inconvenience in application and poor permeation in the skin.¹⁴ In order to expand the application of SBT in the medical field, it is necessary to add nanocellulose, which has high biocompatibility and can enhance the sustained-release effect. Cellulose nanofiber (CNF) is an ideal drug carrier matrix due to its natural ultrastructure and high bioadhesiveness, which can improve the sustained-release effect.¹⁵ We have previously studied¹⁶ the different antibacterial effects of Apocynum venetum CNF and its convenience as medical dressing, which can make up for the deficiency of SBT and be used as drug carrier material for wound dressings. Moreover, the possibility of modifying the polymer solution used for electrospinning with bioactive substances or drugs make it an attractive method for wound dressings.^17,18 Therefore, it is necessary to add antioxidant natural components and CNF into the drug delivery system that are capable of sustained release to enhance the function of wound dressings.

In this study, after extracting procyanidins from SBT were selected as hydrophilic drugs, the core consists of a typical water-soluble polymer polyvinyl pyrrolidone (PVP) aqueous solution to encapsulate the drug and the sheath consists of polylactic acid (PLA) dissolved in chloroform. It is supposed that the water-soluble drug procyanidins and Apocynum venetum CNF are loaded into the PVP first, followed by incorporation into the PLA nanofiber membranes to form PLA/drug-loaded PVP nanofiber membranes. The release profiles of encapsulated drugs from the emulsion electrospun fibers are tested, which provides a theoretical basis for the further development and application of SBT procyanidins and plays a positive role in promoting the development of superior characteristic plant resources in western China.

Materials and methods

Materials and reagents

SBT (Hippophae rhamnoides L.) was supplied by Aksu Wushi County, Xinjiang, China. Ethanol, Span-80, PVP, N,N-dimethylformamide (DMF) (>99.7%), trichloromethane (TCM) (>99.7%), phosphate buffered solution (PBS), 2,2-diphenyl-1-picrylhydrazyl (DPPH), N-butyl alcohol, hydrochloric acid, 2,2-azino-di-(3-ethyl-benzothialozine-sulfonic acid) (ABTS), potassium persulfate, Trolox, 2,4,6-tri(2-pyridyl)-1,3,5-triazine (TPTZ), iron (III) chloride hexahydrate (FeCl₃·6H₂O) and ferrous sulfate (FeSO₄) were all purchased from Aladdin Reagent Co., Ltd, Shanghai, China. PLA (Wm = 80,000) was obtained from Shenzhen Esun Industrial Co., Ltd, China. Silica gel (200–300 mesh) was obtained from Dingkang Gel Co. (Qingdao, China). Moreover, all chemicals were employed as acquired without any further purification. Nanocelluloses derived from Apocynum straw (sCNF) and distilled water were made in the laboratory.

Extraction and purification procedures of procyanidins from sea buckthorn

The SBT raw material was cleared and milled by a grinding miller (DXF 06D, China), then passed through an 80-mesh sieve. Dried SBT powder (10.0 g) was placed into a 250 ml round-bottom glass flask equipped with a reflux system. The sample was extracted using a 100 ml ethanol solution (mass concentration of 70%) three times in total. The filtrate was mixed, and the extracts were obtained after evaporating the solvent at 50℃ under vacuum. Silica gel column chromatography was conducted as follows: the eluent used in this process was obtained by mixing the mobile phase (ethanol solution with mass concentration of 70%). The column (30 mm × 500 mm) was packed with 100 g silica gel pre-mixed with the eluent and the mixture was quickly poured into the column. When the elution was performed, the flow rate of the eluent was 3 ml/min and products were collected until completion. The purified SBT product was labeled as SBTP.

Preparation of water-in-oil emulsion and electrospinning

Our previous article¹⁶ compared the Apocynum phloem and the straw nanofiber membranes and found that straw as an agricultural waste has the same excellent properties as phloem, so we conducted follow-up research on that basis. Briefly, sCNF and SBTP were added into 10 ml of 5% (w/v) PVP solution under stirring at 700 r/min with a mechanical stirrer (ZNCL-DL140 * 140 4-link constant temperature magnetic stirrer) to prepare the water phase and 1.0 g Span 80 dissolved in 20 ml TCM/DMF (with the mass ratio of 4/1) as the oil phase. Then, 3 ml water phase was added and 1.2 g PLA was incorporated into the oil phase under stirring at 700 r/min for 4 h to obtain a uniform white emulsion; the ratios are shown in Table 1, marked as S1, S2 and S3, respectively. The nanofibrous membranes were collected on an aluminum foil by setting a distance of 15 cm between the collector and the needle tip, with an applied voltage of 15 kV and an emulsion feed rate of 1.2 ml/h. These settings are made with slight changes based on our previous article.¹⁶

Table 1.

Preparation parameters of nanofiber membranes with different proportions of purified sea buckthorn (SBTP)

	PVP content	sCNF content	Drug content (SBTP)	Oil phase (PLA)
S1	0.5 g	0.8 g	10%	1.2 g
S2	0.5 g	0.8 g	20%	1.2 g
S3	0.5 g	0.8 g	30%	1.2 g

PVP: polyvinyl pyrrolidone; sCNF: nanocelluloses derived from Apocynum straw; PLA: polylactic acid.

Characterization

Content of procyanidins and high-performance liquid chromatography analysis of SBTP

The high-performance liquid chromatography (HPLC) analysis of procyanidins was performed using a 1200 series liquid chromatograph manufactured by Agilent Technologies (Palo Alto, CA, USA). The mobile phase was a mixture of solvents: methanol:water:acetonitrile (75:5:20 v/v/v). The ﬂow rate of the mobile phase was 1 mL/min. The amount of injected solution was 10 µl. Identification of separated procyanidins was based on mass spectrometry (MS) spectra. The content of analyzed compounds was calculated based on the cyanidin 3-rutinoside calibration curve with a diode-array detector (DAD) detector set at a wavelength of 517 nm.

Thermal stability analysis

Bioactive compounds always have different activities under different temperature conditions. The objective is apparently to develop a drug-release system for the wound treatment of the human skin surface, while at the same time considering the different environmental changes of SBTP during preparation, storage and use. Therefore, thermal stability analysis was conducted. A certain amount of SBTP was completely dissolved in ethanol and placed into a crystallizer. The mixture was agitated under four different temperatures as follows: refrigerating temperature (4℃), room temperature (25℃), human skin temperature (33℃) and heating temperature (60℃) for 5 h and measured and analyzed every hour by ultraviolet (UV) spectrophotometry at 517 nm. The whole thermal stability experiment was conducted in the dark. The concentration changes were used to calculate the decomposition rates of SBTP.

Characterizations of emulsion nanofiber membranes

The morphologies of water/oil (W/O) emulsion nanofiber membranes were characterized using transmission electron microscopy (TEM, JEM-2100, Japan Electronics Co., Ltd (JEOL)) at 80 kV accelerating voltage. The samples were deposited with the electrospinning process onto glow-discharged carbon-coated grids (300-mesh copper). Image-Pro Plus software was used to measure the diameter of the nanofibers.

A total of 10 mg of each nanofiber mat sample was added to 10 ml of PBS (pH 7.4, 37℃) under stirring (500 r/min) for 24 h. Then the solution concentration in each sample was measured by UV spectrophotometry at 517 nm. The absorbance of the same sample was measured again after one month’s storage in dark room temperature. The encapsulation efficiency (EE) of SBTP in each type of as-prepared nanofiber membrane was calculated according to the equation below

Encapsulation efficiency (%) = (c_{x} / c_{a}) \times 100 %

(1)

where c_a is the theoretical concentration of the encapsulated SBTP and c_x is the SBTP concentration in the sample during measurement.

In vitro drug-release studies

The in vitro release behavior of the SBTP loaded nanofiber mats was evaluated using a PBS (pH 7.4) as the release medium. The 10 mg mats were immersed in 10 ml of PBS and incubated in an oven at 37℃ in capped glass flasks. At predefined time intervals, the release medium was completely removed and substituted with the same amount of fresh PBS. The SBTP concentration in each sample was measured by UV spectrophotometry at 517 nm. Drug release was expressed as the percent ratio of the cumulative mass of released SBTP and the initial amount loaded in the samples. Each experiment was carried out in triplicate with samples randomly taken from different sites of the electrospun mats.

Antioxidant assay

In this work, antioxidant activity of the as-loaded SBTP was measured by standard assays (DPPH, ferric reducing antioxidant power (FRAP), ABTS and scavenging activity against OH⁻). A total of 1.5 ml of the sample solution was added to 1.5 ml of 2 × 10^–4mol /l DPPH solution, which was rapidly mixed. The absorbance value was measured at 517 nm at room temperature after 30 min, recorded as A1. A total of 1.5 ml of anhydrous ethanol was added to 1.5 ml of 2 × 10^–4mol /l DPPH solution, which was rapidly mixed, and the absorbance value was measured at 517 nm after 30 min of standing in the dark at room temperature, recorded as A2. A total of 1.5 ml of anhydrous ethanol was added to 1.5 ml of the sample solution, which was rapidly mixed and left in the dark for 30 min at room temperature and absorbance values at 517 nm values were recorded as A3. The calculation formula of the DPPH radical-scavenging rate is

Clearancerate (%) = 100 % \times (A 2 + A 3 - A 1) / A 2

(2)

For FRAP the reagent included 300 mmol/l acetate buffer (2.04 g/l C₂H₃NaO₂ and 8 ml/l C₂H₄O₂, pH = 3.6); 10 mmol/l TPTZ solution in which 40 mmol/l HCl was used as the solvent; 20 mmol/l FeCl₃·6H₂O. The FRAP reagent was obtained by mixing 100 ml acetate buffer, 10 ml TPTZ solution and 10 ml FeCl₃·6H₂O solution. About 0.1 ml of different quantities of SBTP products or FeSO₄ solutions were added to 6 ml FRAP solution, and then the temperature of the mixture was fixed at 30℃ for 30 min. The absorbance of these samples was monitored at 593 nm until a constant reading was obtained.

The ABTS+ solution was prepared by the addition of 7 mM ABTS to 2.45 mM of potassium persulfate. This solution was mixed and allowed to stand in the dark for 12–16 h at room temperature in order to produce ABTS radical cations. The working solution of ABTS+ was obtained by the dilution of the previously made ABTS solution with ethanol until an absorbance of 0.70 at 734 nm was achieved. Each sample and Trolox standards were added to ABTS+ solution and the absorbance was read 10 min after mixing. The percentage of inhibition was calculated by the following formula

% inhibition = 100 \times (A_{0} - A) / A_{0}

(3)

where A₀ was the initial absorbance obtained by measuring the solvent absorbance and A was the final absorbance of each tested sample at 734 nm. The calibration curve between % inhibition and Trolox (100–2000 μM) solutions was then established. The radical-scavenging activity of each tested sample was expressed in Trolox equivalent antioxidant capacity (TEAC µmol Trolox/g).

One milliliter of salicylic acid-ethanol solution (4.4 mM) was mixed with 1 ml of 9 mM FeSO₄ solution. After that, 2 ml of sample solutions of different concentrations (20, 30, 40, 50 and 60 µg/ml) followed by 1 ml of 0.3% H₂O₂ solution were added. After the mixture solution was incubated in a 37℃ water bath for 30 min, its absorbance (A₁) at 510 nm was measured. The absorbance A₀ was obtained from the mixture without sample solution, while A₂ was obtained from the mixture without 0.3% H₂O₂.

Results and discussion

Analysis of procyanidins of SBT extracts and thermal stability

In this study, procyanidin A2 and B2 were selected as representative procyanidins in quantitative determination experiments. The chromatographic fingerprints of procyanidins in the SBTP samples were initially analyzed using HPLC and are presented in Figure 1. The identification of compounds was done by comparing the retention time with the standards. HPLC analysis revealed the presence of procyanidin A2 and B2 in SBTP. The chromatogram peaks of SBTP were noted at a retention time of 2.54 min and the corresponding peaks were found to be procyanidin A2 and B2. It can be seen that the peaks of procyanidins A2 and B2 as monomers appeared earlier, and the components all peak before 10 min, indicating that the oligomers are rich in content. The excimer ion peaks of the first-order mass spectrum corresponding to A2 and B2 were at m/z 579, which is a typical fragment of dimer with a procyanidin characteristic UV absorption spectrum. These results confirmed that SBT constitutes a source of various valuable procyanidins and that the extraction of commercially useful quantities of these compounds may be feasible. The standard curve of each compound was determined in the range of 0.0001–0.01 mg/ml. The regression equation was obtained from the standard curve: y = 1.143 x – 0.0399, R²= 0.9995. Standard curve analysis of concentrations of SBTP revealed that SBTP contained up to 0.0345 mg of the procyanidin B2 in 1 g of the extract.

Figure 1.

High-performance liquid chromatography and mass spectrometry of (a) procyanidin A2 and (b) procyanidin B2 in purified sea buckthorn.

Bioactive compounds always have different activities under different temperature conditions. The SBTP could encounter different ambient temperatures in the process of storage, preparation and use and wound healing may occur on the human skin surface. So, four different ambient temperatures (refrigerating temperature (4℃), room temperature (25℃), human skin temperature (33℃), heating temperature (60℃)) were investigated in the thermal stability analysis. As shown in Figure 2, 2.35%, 5.21%, 6.46% and 13.67% of SBTP was degraded when the temperatures were 4℃, 25℃, 33℃ and 60℃, respectively. It can be seen that at temperatures that are not heated, the loss of SBTP was degraded but no significant increase occurred, while the concentration loss rate increased significantly at high temperature and SBTP was inactivated. This phenomenon was possibly caused by the heat sensitivity of SBTP, and a large amount of heat accumulation will occur at the relatively high temperature, leading to the destruction and decomposition of B-type chains and the decrease of its concentration. In addition, the inactivation or decomposition could also be partially interpreted as the potential reactions between residual oxygen and SBTP because of the reducibility of phenolic groups in procyanidins.¹⁹ The antioxidant capacity of flavonoids is due to their -OH units, while the alteration in this ability is a result of three parameters: the temperature, chemical structure and concentration.²⁰ Since the concentration of SBTP changes, which indicated the temperature of whole purification should not exceed 33℃, the extract should be stored at below 4℃ or 25℃ and protected from light.

Figure 2.

Thermal stability of purified sea buckthorn.

Morphologies of W/O emulsion nanofiber membranes

Due to its poor stability and weak binding with PLA, SBTP was encapsulated in nanofibers by emulsion electrospinning to further test its performance. Figure 3 displays a picture of electrospun samples. Clearly, relatively uniform core–sheath structures without beads were formed during electrospinning. The micrograph depicts nanoscaled bicomponent fibers with a suggested core enriched with SBTP/sCNF (darker area) and PLA as the main sheath material (brighter area), and the boundary between the two was quite sharp. The average diameters of the cores were 528, 279 and 189 nm for S1, S2 and S3 fibers, respectively. The formation of core thickness is related to the concentration of the aqueous phase drug and morphology of sCNF. Apparently, increase of the content of SBTP, higher total volume ratio and larger difference in viscosity between the PVP/SBTP/sCNF aqueous droplets and the surrounding PLA polymer solution matrix leads to a higher degree of de-emulsification and concentration of the PVP/SBTP/sCNF droplets, resulting in a thinner core.²¹ After water phase evaporation, the elongated fibrillous sCNFs were able to orientate within the fiber on the wall of the cylinder, and our previous research also showed that sCNF can support the entire spinning network.²² So, the absorption of emulsion droplets and the different solvent evaporation rates of the solvent form a core–sheath structure. The drug can be encapsulated in the form of double-layer fibers to greatly increase the surface, which promotes the absorption of the drug in the human body. As shown in Figure 4, we made a schematic diagram to illustrate the formation of composite nanofibers in the electrospinning process. Because of the differences in vapor pressure, chloroform evaporates faster than water and the viscosity of the CHCl₃/PLA oily phase increases compared to the PVP/SBTP/sCNF water phase. This viscosity gradient from the outer layer to the inner layer causes the inward movement of the oval-shaped PVP/SBTP/sCNF water droplets and causes the agglomeration and aggregation of the aqueous phase droplets, thereby forming a double core–sheath structure.²³

Figure 3.

Transmission electron microscopy images of (a) S1, (b) S2 and (c) S3 and diameters of the nanofiber membranes.

Figure 4.

Schematic of the formation of composite nanofibers in the electrospinning process. PVP: polyvinyl pyrrolidone; sCNF: nanocelluloses derived from Apocynum straw; PLA: polylactic acid.

Encapsulation efficiency and storage stability

EE is an important feature for a drug delivery carrier, and it depends on the combination of drug/solvent/polymer of each particular system, where the hydrophilic/hydrophobic characteristics of each component play an important role.²⁴ The loading of SBTP on nanofiber membranes is shown in Figure 5. In the case of the emulsion solvent evaporation technique, as the SBTP content increased, S1, S2 and S3 nanofiber membranes had EE values of 75.85%, 79.64% and 83.84%, respectively, indicating that the emulsion-based electrospun fibers have a great potential to be used as a delivery vehicle. Similar results were reported by Tavassoli-Kafrani et al.,²⁵ who found an increase in the EE of both gelatin and gelatin-cross-linked nanofibers with an increasing amount of essential oil. This behavior might be related to the total drug content of emulsions. This was attributed to the SBTP/sCNF concentration, which could have modified the viscosity ratio of the dispersed to continuous phase, increasing the total drug concentration and leading to a higher viscosity of the aqueous phase of emulsions, which could minimize the internal circulation of SBTP/sCNF inside the droplets.^26,27 The uniformly structured sCNF is uniformly dispersed in the water phase to form a more stable emulsion compared to general emulsion electrospinning. That prevents its migration to the PLA surface, consequently improving SBTP/sCNF encapsulation. As expected, there was no significant change in the EE after a month, with an average decrease of only 1.78%. The results demonstrated an excellent storage stability of the encapsulated SBTP and a good barrier effect of the emulsion-based nanofiber membranes as a core–sheath structure. This change in stability could be attributed to its physical entrapment in the emulsion electrospun nanofiber, as well as the presence of natural antioxidants in the SBTP.

Figure 5.

Encapsulation efficiency and storage stability of the nanofiber membranes. SBTP: purified sea buckthorn.

In vitro drug-release studies

Figure 6 exhibits the SBTP cumulative release profile of membranes at various time intervals. All nanofiber membranes exhibited an initial high burst released in the first 8 h, and at 8–14 h there was a slightly slower release than at the beginning, but the remaining drug was released gradually during the following 14–30 h; the cumulative release percentages of S1, S2 and S3 at 30 h were 63.29%, 68.98% and 75.41%, respectively. It is likely that the burst release from the samples was due to the good solubility of SBTP in the phosphate buffer and the existence of some drug on the surface of fibers; the slow release in the second stage was the entanglement of sCNF to slow down the release rate, while the subsequent sustained release was due to the diﬀusion of SBTP molecules through the matrix of microspheres.²⁸ The sequential release capability of SBTP-produced membranes is suitable for wound healing, as an initial burst release is actually desirable, since primary sudden release can improve and accelerate treatment. However, the burst release must be controlled, so that, after this stage, a sufficient quantity of SBTP still remains in the membranes to maintain release rates enough for a satisfactory treatment time period.²⁹ It was expected that the SBTP immobilized by the emulsion method and entanglement of sCNF makes the drug diffusion path more complicated; after incubating into the PBS solution caused chain relaxation of the PLA matrix, the solution entered into the PLA matrix and because the fibrillar network of sCNF produces a tortuous path, it limits the large release of the drug, desorbed from the complex path of sCNF into the PLA matrix, presenting a more gradual and prolonged release, and the burst release will be efficiently suppressed. Then, the difference in drug concentration between the inside and outside of the slow-release membrane causes the drug to diffuse outward through the complex path of the sCNF, and the solution outside the membrane also dissolves and releases the SBTP into the inside, while the SBTP in the PLA matrix would further diffuse into the PBS.³⁰ Therefore, the release of SBTP from the nanofiber membranes exhibited a relatively slow manner. This is consistent with the variation trend of diameter in Figure 3. The higher the concentration, the smaller the core diameter and the slower the SBTP release.

Figure 6.

In vitro drug release of the nanofiber membranes.

Antioxidant activity

In the present study, the antioxidant activity of the emulsion nanofiber membranes was analyzed by performing assays such as DPPH, ABTS radical-scavenging activity, ferric reducing power assay and scavenging activity against OH⁻. These four tests gave a fairly comprehensive explanation that SBTP was reported as a potential candidate for the antioxidative effect. The values obtained for the different antioxidant activities of the nanofiber membranes are shown in Table 2.

Table 2.

Antioxidant properties of nanofiber membranes

	Against DPPH free radical (%)	Against OH⁻(%)	FRAP (mmol/g)	ABTS (%)
S1	79.53	81.34	12.79	52.01
S2	83.54	82.08	15.51	54.20
S3	88.62	84.39	19.09	63.29

DPPH: 2,2-diphenyl-1-picrylhydrazyl; FRAP: ferric reducing antioxidant power; ABTS: 2,2-azino-di-(3-ethyl-benzothialozine-sulphonic acid).

Overall, nanofiber membranes of bound SBTP showed a high antioxidant potential in all the four antioxidant assays. According to the data shown in Table 2, it can be noted that the radical-scavenging potential of the samples was the highest for the DPPH assay, up to 88.62%, against OH- in which the clearance rate was up to 84.39% and ABTS where the clearance was 63.29%, indicating that the nanofiber membranes have higher antioxidant activity. The strong antioxidant effect of SBTP is attributed to the extensive adjacent diphenol hydroxyl groups on the B ring, which reduce free radicals that induce an oxidative stress environment.³¹ Due to certain physiological activities, imbalance in the production of reactive oxygen species (ROS), which generates oxidative damage in lipids, proteins and nucleic acids and leads to additional tissue damage, excessive reactive oxygen species and other substances, will be produced that further delays the cell and tissue healing.³² SBTP has a higher content of antioxidant compounds release hydrogen ions, which then combine with free radicals, leading to the termination of free radical chain reactions and the promotion of normal cell activity and antioxidant capacity.³³ In addition, dihydroxyphenol groups enable SBTP to fully bind to metal ion (Fe³⁺, Cu²⁺, Al³⁺) proteins, prevent the activity of metal ion catalyzing free radical reactions and facilitate the release of new growth factors to promote antioxidant capacity.³⁴ Further, the antioxidant activity of the films improves with the addition of SBTP, which may be useful for certain commercial applications, such as wound dressings.

Conclusions

We have successfully extracted procyanidins from SBT and constructed a drug delivery system, nanofiber membranes encapsulated with SBTP and Apocynum venetum sCNF through emulsion electrospinning. The content and purity of SBTP were calculated and confirmed by HPLC analysis, which revealed that SBT contains procyanidins. The thermal stability of SBTP was explored, which showed good stability under refrigeration or room temperature, and laid a good foundation for subsequent reprocessing. In this bilayer nanostructure, the nanofiber membranes were composed of smooth and uniform fibers with a core–sheath structure, with the SBTP/sCNF-loaded PVP water phase being the core and PLA being the shell, with over 83.84% EE. What is more, the nanofiber membranes showed great antioxidant ability, with a DPPH clearance rate of up to 88.62%. Through embedding SBTP by adding sCNF, the nanofiber membranes performed a prolonged and sustained release of SBTP, in which the cumulative release reached a maximum of 75.41%; it displayed profiles with a high initial release, followed by a slow diffusional phase, resulting in a promoting eﬀect on cell proliferation and wound healing. To our knowledge, this is the first study describing and characterizing the incorporation of Apocynum venetum sCNF into SBTP with demonstrated antioxidant activity. sCNF acts as a fortifier inducing water phase forming and enhancing the formation of the core–sheath structure and coating of active molecules. These results should be useful for the development of an eﬀective delivery system for SBTP, which demonstrated that the SBTP/sCNF-added emulsion electrospun nanofiber membranes had a potential application as a drug-release device as well as for wound healing.

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 Innovation Research Program for Xinjiang Graduate Students (XJ2019G074), the Development and Application Innovation Team of Xinjiang Special Textile Materials and The National Natural Science Foundation of China (Grant No. 51763022) and the Construction and analysis of comprehensive evaluation system for acid-proof and moisture-permeable nanocomposite fabric (XJEDU2018Y006).

ORCID iDs

Lu Wang

Qingle Zhang

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