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9УДК 547.995.15: 544.35.03
Снетков П.П., Захарова К.С., Тянутова М.И., Морозкина С.Н., Олехнович Р.О., Успенская М.В.
ИССЛЕДОВАНИЕ ФИЗИЧЕСКИХ СВОЙСТВ ПРЯДИЛЬНЫХ ВОДНО-ОРГАНИЧЕСКИХ РАСТВОРОВ
ГИАЛУРОНОВОЙ КИСЛОТЫ
Университет ИТМО, Кронверкский пр., д.49, литер А, Санкт-Петербург, 197101, Россия. E-mail: ppsnetkov@itmo. ru
Гиалуроновая кислота (ГК) - природный полимер, встречающийся в разнообразных тканях и биологических жидкостях. Благодаря своим уникальным свойствам, таким как биодеградируемость и биосовместимость, гиалуроновая кислота может применяться в производстве полимерных нановолокон для тканевой инженерии и раневых повязок. Для получения таких на-но- и микроструктур может быть использован метод электроспиннинга. Этот метод позволяет получать частицы и волокна из раствора полимера под действием высокого напряжения. Однако стабильность процесса зависит от множества параметров, включая физические характеристики полимерного раствора. В данной статье рассматриваются физические свойства водно-органических прядильных растворов гиалуроновой кислоты с учетом их пригодности для электроспиннинга.
Ключевые слова: биополимер, гиалуроновая кислота, диметилсульфоксид, заживление ран, нановолокна, тканевая инженерия, электроспиннинг.
DOI 10.36807/1998-9849-2020-55-81-16-21
Introduction
Hyaluronic acid was originally described by Karl Meyer and John Palmer in 1934. The name "hyaluronic acid" was derived from "hyalos" (glass-like) and "uronic acid" [1]. HA consists from regularly repeating residues of D-glucuronic acid and N-acetyl-D-glucosamine that form linear molecule [2]. Containing both polar and non-polar segments HA can exist in different conformational states while interacting with various chemical substances and with its own chains [3].
In view of its biocompatibility, HA has plenty of applications and continues to attract the research interest. It is utilized in arthrology for viscosupplementation, in drug delivery as a carrier for hydrophobic drugs, in ophthalmology and dentistry and as a food supplement [4-8]. HA can be utilized in a various form, such as gel, powder, nanoparticles, non-woven micro- and nanofibers. Nano-fibers obtained from HA have potential application in wound healing and tissue engineering.
Electrospinning is one of the ways of producing nano- and microfibers and it is notable for its hardware simplicity, high energy efficiency, wide universality to the materials being obtained, and flexibility in controlling pro-
Petr P. Snetkov, Kseniia S. Zakharova, Maria I Tyanutova., Svetlana N. Morozkina, Roman O. Olekhnovich, Mayya V. Uspenskaya
THE STUDY OF THE
PHYSICAL PROPERTIES OF
AQUEOUS-ORGANIC
HYALURONAN
ELECTROSPINNING
SOLUTIONS
ITMO University, Kronverkskiy prospekt, 49A, St. Petersburg, 197101, Russia. E-mail: [email protected]
Hyaluronic acid (HA) is a naturally occurring polymer that is found in variety of tissues and biological fluids. Due to its unique properties such as biodegradability and biocompatibility, HA can be applied in fabrication of nanofibers for tissue engineering and wound healing. Electrospinning as one of the methods of creating such structures allows to obtain fibers and particles with nano- and microscale from polymer solutions utilizing the high voltage power supply. However, the electrospinning process directly depends upon different parameters, including physical characteristics of the polymer solutions. This paper reviews physicalprop-erties of aqueous-organic HA solutions considering their suttability for electrospinning.
Keywords: biopolymer, dimethyl sulfoxide, electrospinning, hyaluronic acid, nanofiber, tissue engineering, wound healing.
Дата поступления - 4 июля 2020 года
cess parameters [9]. Required polymer solution is loaded into syringe with metallic needle connected to a high voltage power supply. During the high voltage supply like charges are formed, and as a result of Coulomb electrostatic interactions the polymer solution is drawn in a thin stream. The droplet at the end of the needle creates a continuous, stationary, accelerating and thinning jet in a cone shape, which is called Taylor Cone. As the jet flows to the plate, solvent is evaporated, and a solidified polymer fiber is collected [10, 11]. Despite the seeming simplicity, there are a number of technological parameters and solution characteristics that must be taken into consideration (Table 1) [9, 12-14].
The values of all these parameters must be in a certain range for a stable and successful electrospinning process. Thus, a low viscosity of the solution does not lead to the formation of a stable jet, while high viscosity results in the clogging of the feed systems of the electrospinning device [12]. High surface tension, high electrical conductivity, and low solvent evaporability are the limitations of HA aqueous solutions that make electrospinning difficult to accomplish. To facilitate the nanofibers obtain-ment, organic co-solvents can be utilized. Yang Lui et al.
[15] conducted electrospinning of HA from solution composed of deionized water, dimethyl formamide (DMF) and formic acid. As formic acid was introduced in a solution, it was found to disrupt hydrogen bonds formed by HA with water, thereby changing the conformation of HA and increasing viscosity of the solution. DMF was used to decrease surface tension and electrical conductivity. These parameters together determine the voltage applied to the solution. Respectively, lowering the voltage, the probability of electrical breakdown can be reduced [9].
Table 1. Variable parameters of electrospinning
Solution parameters Technological parameters Environment Parameters
Viscosity Applied voltage Temperature
Concentration Collector type Humidity
Polymer's molecular weight Distance between the collector and the spinneret Air pressure
Electrical conductivity Feeding rate
Surface tension Needle size
Rotation speed of the drum/rod collector
Another method to avoid the restrictions generated by HA is to add second polymer to the solution. Li Junxing et al. [16] successfully prepared solutions of HA and gelatin in water/DMF solvents system. The addition of gelatin allows to decrease in surface tension and electrical conductivity of the solutions and to obtain fibers with larger diameter. Viscosity of the solution was not affected by gelatin thus leading to stable process of electrospinning.
Note, that not only additional biopolymers could be utilized to improving the electrospinning process. Also, the water-soluble modifying polymers named carrying polymers are used [13]. The most usable polymers are polyethylene oxide (PEO) and polyvinyl alcohol (PVA). However, in some cases the quantity of HA is less than the additional polymer content which provides no undertaking therapeutic effects of materials obtained.
It this paper dimethyl sulfoxide (DMSO) was used as co-solvent [17]. DMSO is a polar aprotic solvent that is commonly used in cryobiology as a cryoprotectant. It has less toxic effect than other widely used solvents, such as dimethylacetamide, and DMF [18]. DMSO is well compatible in water. Interestingly, its solutions properties show deviation from theoretical assumption due to formation of 1DMSO-2H2O clusters [19]. Hydrogen bonds formed between water and DMSO are stronger than the ones formed between molecules of water, which explains increasing viscosity of aqueous solution of DMSO. However, in presence of DMSO, water-water bond has greater lifetime than in water alone, supporting the concept that the distortion of water molecules is related to the growth of the local rigidity [19,20]. With this in mind, it can be predicted that conformation of HA in solution with presence of DMSO is affected. This may influence the parameters of the polymer solution.
The physical characteristics of the polymer solution play a significant role in the stability of the electro-spinning process. Knowing the solution parameters indicated in the Table 1, the choice of technical process parameters becomes more evident.
Materials
Hyaluronic acid was obtained from Bloomage Freda Biopharm Co., Ltd (China). Molecular weight 1.29
MDa, glucuronic acid content 45 %, protein content 0.05 %. Dimethyl sulfoxide (DMSO, 99.5% ACS) was purchased from JSC EKOS-1 (Russian Federation). Distilled water was obtained from distillation unit was utilized as solvent. All materials were used without any further purification.
Solutions preparation
The required amount of distilled water was put in the chemical glass, and then hyaluronic acid was added. After the hyaluronic acid was completely dissolved (~1 hour), the required amount of DMSO was added and solution was left on the magnetic stirrer for 24 hours until a homogeneous mixture was obtained. Eight concentrations of HA were selected ranging from 0.25 wt. % to 2.00 wt. % at a pitch of 0.25. Each concentration was prepared in four different binary solvent systems: 100/0, 75/25, 50/50, 25/75 H2O/DMSO (v/v). Solutions with 1.75 wt. % and 2.00 wt. % HA in system 25/75 had failed to be prepared because of the HA partial solubility even after 24 hours of stirring.
Experimental methods
For studying density of HA solutions Mettler Toledo Densito 30PX (USA) density meter was used. The samples were put to analysis through the external syringe to prevent air bubbles forming in the solution. Surface tension was studied using pendant drop tensiometry with drop shape analyzer DSA 100 by KRUSS (Germany). Electrical conductivity of solutions was measured by InoLab Cond 720 (USA). Dynamic viscosity was determined by Anton Paar Physica MCR-502 rheometer (Austria) in two modes: changing shear rate with constant temperature and changing temperature with constant shear rate.
Results and discussion
Density measurements are necessary for subsequent surface tension analysis. Utilized density meter based on the method of determination of the oscillation frequency of the tube with the sample. For this reason, the results drastically depend on the quality of the solutions prepared and could be impaired by air bubbles. Thus, the solutions prepared could not be measured straight after their mixing.
The results of density measurements at 25.0 ± 1.0 °C are demonstrated in Fig. 1. It is obvious, that density has moderate increase with increase of polymer concentration and the DMSO content. Note that the density of distilled water at 25.0 °C is equal to 0.9970 g/cm3, and the DMSO density at the same temperature is equal to 1.1010 g/cm3 [21].
In general, the characteristic curve the dependence obtained has a linear character is corresponded to previous studies [22,23]. Note that in the mentioned papers organic solvent was not utilized.
Surface tension plays an important role in the electrospinning process. Initially, after supplementation of the high electric voltage, an electrical like charges are prompted on the surface of the drop of the polymer solution. This charge leads to the overcome of the surface tension forces and the alteration of drop shape from spherical to conical. After the excess of the electric field degree over the specified critical value, the electrostatic forces surpass the polymer solution surface tension and the dispersal of polymer solution is started [24].
Fig. 1. Dependence of the density from hyaluronan concentration and H2O:DMSO volume ratio at 25.0 ± 1.0 °C.
Unfortunately, the vast majority of scientific studies in the area of hyaluronic acid electrospun nanofibers did not focus on the physical characteristics of the solutions. The authors, in general, established the fiber obtaining and their characterization as the main goal, but missed the fact that the physical properties could influence the nanofiber properties.
Fig. 2 shows the dependence of the surface tension from hyaluronan concentration and H2O:DMSO volume ratio at 25.0 ± 1.0 °C. The decrease in the surface tension is observable with an increase of both polymer concentration and DMSO content. Note, that our results are correlated to [25]. Rapid changes in the surface tension are not identified because of shorting of the concentration in which the entanglement process of polymer chains is started.
0,25 0,50 0,75 1,00 1,25 1,50 1,75 Hyaluronan concentration, % (w/w) Fig. 2. Dependence of the surface tension from hyaluronan concentration and H2O:DMSO volume ratio at 25.0 ± 1.0 °C.
Electrical conductivity is also a key parameter of the polymer solution for electrospinning. HA as a natural polysaccharide and polyelectrolyte could be named as electro-active polymer [26]. Water solutions based on it show high level of electrical conductivity that could hinder the electrospinning process, as was mentioned above. It is known that the solutions of charged polymers could possess the electrolytic conductivity [27].
The relationship between the electrical conductivity of solution and hyaluronan concentration and H2O:DMSO volume ratio at 25.0 ± 1.0 °C is demonstrated in Fig. 3.
0,25 0,50 0,75 1,00 1,25 1,50 1,75 Hyaluronan concentration, % (w/w) , 3. Dependence of the electrical conductivity from hyaluronan concentration and H2O:DMSO volume ratio at 25.0 ± 1.0 °C.
Obviously, that addition of polymer leads to increasing of ions content and to increase of conductivity, consequently. By contrast, increase in content of DMSO which has even lower electrical conductivity than distilled water (2E-9 S/m) [28] in binary system results in decrease of the electrical conductivity of solution.
On the one hand, the polymer solutions must have a certain value of electrical conductivity to allow the charge transfer from the spinneret. On the other hand, high level of electrical conductivity combined with low evaporability of water-based solutions could result in creation of electrical breakdown between the electrodes using the wet fiber as a conductor. For this reason, the knowledge of such parameter is necessary for subsequent elaboration of nanofibrous materials for tissue engineering and wound healing.
Dynamic viscosity of the polymer solution is a technological parameter, which could demonstrate the possibility of solution utilizing in view of thin transporting systems and needles. It is known that hyaluronic acid could form the tertiary helicoid structure [29-33]. Moreover, in HA aqueous solutions with concentration from 1.0 wt. % the polymer chains start to entangle each other leading to rapid increasing of the viscosity [3]. Note, that high level of solution viscosity hinders the transportability of solution which results in impossibility of electrospinning process.
It is known that dynamic viscosity of HA solutions depends on shear rate and is in accordance with nonlinear law. It is typical behavior of non-Newtonian solutions [34]. As shown in Fig. 4 the dynamic viscosity of HA solution with concentration 0.25 wt.% at 25 °C has the increasing tendency while DMSO content rises, however with rising of shear rate this difference is smoothed. The viscosity curves with higher HA concentrations and temperatures have similar character.
Fig. 4. Dependence of the dynamic viscosity of HA solutions (0.25 wt.%) from shear rate and H2O:DMSO volume ratio.
Temperature dependence of dynamic viscosity has inverse relationship from shear rate (see Fig. 5). Note, that solutions with higher HA concentration demonstrate the same behavior.
Q.
E
1000 800
600400-
200-
-H?0:DMS0 (100:0)
-HjO DMSO (75:25)
-HjO:DMSO (50:50)
-H 0:DMS0 (25:75)
10 15 20 25 30 35 40 45 50 Temperature, С
55
Fig. 5.
Dependence of the dynamic viscosity of HA solutions (0.25 wt.%) from temperature.
The increase in dynamic viscosity with an increase in the proportion of organic solvent can be explained in several ways. HA forms hydrogen bonds, such as intramolecular, intermolecular and with water acting as linkage. After DMSO addition the forming of stronger hydrogen bonds is happen [20,35,36].
This fact leads, in general, to an increase in the rigidity of the system and an increase in dynamic viscosity. Interestingly, viscosity of solutions with equal volumes of water and DMSO is much higher than viscosity of other systems containing organic solvent. This can be explained by the fact that during mixing DMSO and water the summary volume of system decreases, which results in increase of polymer concentration [20].
Conclusion
This paper demonstrates the results of a study of the physical properties of hyaluronic acid solutions designed to obtain nano- and microfibers via electrospinning. Note, that the presence of residual quantity of DMSO in resulting fibrous materials could attach healing properties to the obtained materials.
It is shown, that with increase of the DMSO content in binary solution, an increase of density and dynamic viscosity is happen. Meanwhile, surface tension and electrical conductivity decrease with raise the DMSO volume fraction.
The data obtained can be used by researchers in the process of formulating biopolymer solutions for obtaining of fibers and materials based on it for the development of modern scaffolds and wound dressings based not only on hyaluronic acid, but also on other polysaccharides.
Заключение
Статья отражает результаты исследования физических характеристик растворов гиалуроновой кислоты, предназначенных для получения нано- и микроволокон с помощью метода электроспиннинга. Отметим, что присутствие остаточного количества ДМСО в полученных волокнистых материалах может придавать им дополнительные противовоспалительные и анальгезирующие свойства.
Показано, что с увеличением содержания ДМСО в бинарном растворе происходит увеличение плотности и динамической вязкости. Между тем, поверхностное натяжение и электропроводность уменьшаются с увеличением объемной доли ДМСО.
Полученные данные могут быть использованы исследователями в процессе составления биополимерных растворов для получения волокон и материалов на их основе для разработки современных клеточных каркасов и раневых покрытий не только на основе ги-алуроновой кислоты, но и других полисахаридов.
Acknowledgment
This work was financially supported by Government of Russian Federation (Grant 08-08) and by RFBR (project number19-33-90098).
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Сведения об авторах
Snetkov Petr Petrovich, PhD Student, [email protected]; Снетков Петр Петрович, аспирант
Zakharova Kseniia Sergeevna, Graduate student, [email protected]; Захарова Ксения Сергеевна, магистрант Tyanutova Maria Ivanovna, PhD Student, [email protected]; Тянутова Мария Ивановна, аспирант Morozkina Svetlana Nikolaevna, PhD (Chem.), Associate professor, Lecturer, [email protected]; Морозкина Светлана Николаевна, канд. хим. наук, доцент преподаватель
Oiekhnovich Roman Olegovich, PhD (Eng.), [email protected]; Олехнович Роман Олегович, канд. техн. наук, доцент Uspenskaya Mayya Valerievna, D.Sci. (Eng.), Professor, [email protected]; Успенская Майя Валерьевна, д-р техн. наук, профессор
