Development of Aqueous Polyethylene Oxide Gel Inks for the Pharmaceutical Semisolid-Extrusion 3D Printing of Plant-Origin Flavonoids

1. Introduction

Flavonoids are phenolic substances which are very abundant in plants and represent more than 6000 compounds [1,2]. Due to their chemical structure, flavonoids have significant therapeutic properties, including antioxidant, antibacterial, antiviral, and anti-inflammatory effects [3]. Flavonoid-based natural and semi-synthetic drugs are widely represented in the pharmaceutical market for the treatment of e.g., venous insufficiency (Diosmin, Troxerutin, Hesperidin), gastric ulcer and gastritis (Eupatilin), and liver disorders (Silibinin) [4].

Rutin (3,3´,4´,5,7-pentahydroxyflavone-3-rhamnoglucoside), also known as quercetin-3-O-rutinoside and vitamin P, is a flavonol and one of the most common and wide-spread flavonoid [5]. Rutin is also an important nutrient component of plant-based food. First time, rutin was isolated from the common plant Rue (Ruta graveolens L.), from which it also received its name [3]. So far, it has been reported that more than 70 plant species contain rutin, including buckwheat (Fagopyrum esculentum Moench), Japanese pagoda tree (Sophora japonica L), American elderberry (Sambucus canadensis L.), apricot (Prunus armeniaca L.), orange (Citrus sinensis L. Osbeck), white mulberry (Morus alba L.), rhubarb (Rheum rhabarbarum L.), etc. [5,6,7]. Buckwheat is considered to be one of the most profitable sources of rutin [8]. The quantitative content of the compound can reach up to 18.67 mg/g (dry weight) [9].

Rutin is an odourless and yellowish-greenish pigment which exists in the form of needle crystals and is almost insoluble in water [3,10]. Due to its low solubility and low oral bioavailability, it has limitations in its use as a therapeutic agent [11,12]. To enhance the oral bioavailability of flavonoids, numerous formulation and delivery strategies have been introduced in the state-of-the-art literature, including structural transformation (glycosylation, prodrugs), nanotechnology-based carriers, crystals, micelles, carrier complexes, microspheres, self-micro-emulsifying drug delivery systems (DDSs), self-nanoemulsifying DDSs [12,13,14].

Numerous scientific papers have reported that rutin possesses a wide range of pharmacological activity [3,6,15,16]. Rutin has been shown to have a strong concentration-dependent antioxidant effect in various in vitro systems and the ability to inhibit lipid peroxidation. Moreover, rutin has shown to enhance antioxidant status in the liver, kidneys, and brain of rats with diabetes [6,17,18]. Rutin is also widely used in the capillary fragility treatment, vascular sealing and bleeding prevention, and as an antioxidant and anti-inflammatory agent in combination with ascorbic acid [10,19]. Many studies on rutin formulations have demonstrated the significant antimicrobial activity against different strains of bacteria, including Pseudomonas aeruginosa and Klebsiella pneumoniae [6,17,20]. It has been also reported that rutin is an effective antiviral agent, which has shown affinity against the parainfluenza-3 virus, avian influenza virus, HSV-1, and HSV-2 [21]. In addition, rutin is obviously a potent inhibitor of COVID-19 infection treatment [21]. Recently, the anti-proliferative and anti-apoptotic activity of rutin were studied with liquid crystalline nanoparticles loaded with rutin in an in-vitro model using a human lung epithelial carcinoma cell line [11]. The rutin dose of 50 mg/kg was found to decrease glucose levels by increasing insulin secretion in hyperglycemic rats [5].

Today, virtually all flavonoid(s) containing commercial pharmaceuticals are conventional “one size fits all” oral solid dosage forms, such as immediate-release tablets or hard capsules. Sometimes such traditional formulation approach, however, is not appropriate, and a customized drug therapy and DDSs are needed. Therefore, 3D printing technologies are interesting and promising techniques for preparing novel types of personalized dosage forms for flavonoids [22,23]. By using a science-based material(s) selection, formulation development and computer-assisted design (CAD), modern 3D printing technologies could enable preparing next-generation customized oral DDSs for therapeutic flavonoids. [23].

Semisolid extrusion (SSE) 3D printing is a printing technique widely used in pharmaceutical and biomedical applications [24]. SSE 3D printing exploits the sequential deposition of gel (or paste) layers to form the DDS or tissue engineering scaffold with the desired size and shape [24]. The present printing technique has proven efficient in delivering a precise and accurate personalized drug dose(s) and in preparing for example combinatorial DDSs with customized drug release properties [25]. Therefore, it holds considerable promises and potential in drug delivery applications. However, implementing SSE 3D printing technology in clinical practice requires new regulation and proper quality control approaches [26].

Viidik et al. [27] identified and optimized the critical material and process parameters of SSE 3D printing using aqueous polyethylene oxide (PEO) gels as a printing ink. More recently, PEO was successfully used as a printing gel ink former in the SSE 3D printing of novel DDSs of rosmarinic acid [28] and some plant extracts, such as eucalypt [29,30], chamomile [31], motherwort [32] and cranberry [33] extracts. Based on these results, PEO could be a feasible printing gel ink former for the SSE 3D printing of flavonoids, such as rutin (vitamin P).

The aim of the present study was to develop aqueous PEO gel inks loaded with rutin (vitamin P) as a model compound of flavonoids for SSE 3D printing and to investigate the physicochemical and pharmaceutical properties of the corresponding solid DDSs intended for the oral administration of flavonoids. The physical appearance, homogeneity, viscosity and SSE 3D printability of the rutin-loaded PEO gel inks were studied. The rutin content and in-vitro dissolution behavior of SSE 3D-printed oral DDSs were investigated.

2. Materials and Methods

2.1. Preparation of the PEO Gels

For preparing PEO gel inks, rutin (Nanjing NutriHerb BioTech Co, China, purity 95%), Tween 80 (Ferak Berlin GmbH, Germany), PEO (MW approx. 900,000, Sigma-Aldrich, USA), ethanol (Peenviinavabrik, Estonia), and purified water were used at different proportions. The 12% aqueous PEO gel was used as a semisolid ink base for the SSE 3D printing of rutin. First, 12% aqueous PEO gels were prepared, and then the mixture of rutin (0.5 g, 1.0 g and 1.5 g in 1 ml of ethanol) with Tween 80 (at the rutin-Tween 80 ratio of 1:1, 1:2, or 1:3) were loaded in the PEO gel, homogenized and kept at an ambient room temperature (22 ± 2 °C) for at least subsequent 12-14 hours [27].

2.2. Characterisation of the PEO Gels

The viscosity of PEO gels was studied with a Physica MCR 101 rheometer (Anton Paar, Austria) using a cone-plate geometry at 22 ± 2 °C. For measuring the gel viscosity, a rotational shear test at 0.060 1/s was used [29]. The gel homogeneity and structure were determined with an optical light microscope (Magtex-T Dual Illum., Medline Scientific, United Kingdom) equipped with a digital camera (Industrial Digital Camera UCMOS09000KPB (9.0 MP 1 / 2.4” APTNA CMOS sensor) [28,34].

2.3. SSE 3D Printing

The rutin-PEO gel inks were printed using a bench-top SSE 3D printing system (System 30 M, Hyrel 3D, USA) with a printing head consisting of a steel syringe connected to a blunt needle (Gauge, 21G). The printing plate temperature was kept at 30 °C. The printing head speed (rate) was 0.5 mm/s, and the printing speed was controlled by the software of an SSE 3D printer (Repetrel, Rev3.083_K, Hyrel 3D, USA). The model lattices were composed of a total of eight printed layers, and the round-shaped scaffolds consisted of five layers, which were generated with Autodesk 3ds Max Design 2017 software (Autodesk Inc., USA) [28,29]. Round-shaped special scaffolds (20 mm in diameter) were designed using a FreeCAD software (vers. 0.19 / release date 2021). The dimensions (size) of a square-shaped lattice were 30 x 30 x 0.5 mm. For verifying the printability of the gel inks, the weight and area of the 3D-printed lattices were determined. The surface area of the 3D-printed lattices was compared with the theoretical lattice area (324 mm²) [29,34]. The 3D-printed scaffolds were weighed with an analytical scale (Scaltec SBC 33, Scaltec, Germany), and photographed with an ImageJ (National Institute of Health, USA) image analysis software (version 1.51k).

2.4. Assay of Rutin Content by UV Spectrophotometry

The assay of rutin in the 3D-printed scaffolds was carried out by UV spectrophotometry after forming complexes with aluminium chloride [35]. The pre-weighed sample of a 3D-printed scaffold (100.00 mg) was dissolved in an aqueous isopropanol solution (50% V/V) R and analyzed according to a pharmacopeial method [35,36].

2.5. Assay of Rutin Content by HPLC

The quantitative content of rutin in the 3D-printed scaffolds was determined with a modified high-performance liquid chromatography (HPLC) method using Chromatograph Prominence Modular HPLC (Shimadzu, Japan) equipped with an online degassing unit (DGU-20ASR, 2 x Solvent Delivery Unit LC-20AD), photo-diode array detector (SPD-M20A), autosampler (Nexera X2 SIL-30AC), Phenomenex Luna 5 µm C18(2) 100Å LC Column (250 x 4,6 mm) and column oven (CTO-20AC). LabSolutions Version 5.97 SP1 (c) 2019 (Shimadzu Corporation, Japan) was used to control the process.

The pre-weighed sample of a 3D-printed scaffold (200.0 mg) was dissolved in an aqueous isopropanol solution (50% V/V) R and filled in with the same solvent to 100.0 mL. Total 1.0 mL of the solution was filtered through a 0.45 µm nylon syringe filter into a chromatography vial. As a standard solution, 50 mg of rutin standard (Thermo Scientific, USA) dissolved in 100.0 mL aqueous ethanol (50% V/V) R was used in the analysis. Phosphoric acid 1%, acetonitrile 25% and water 74% were used as a mobile phase. The flow rate was 1.0 mL/min. Spectrophotometric detection was carried out at the analytical wavelength of 330 nm. The injected volume was 10 µL, and the run time was 18 min.

2.6. Statistical Analysis

Statistical properties of random variables with n-dimensional normal distribution are given by their correlation matrices, which can be calculated from the original matrices. Statistical data assessment is reported as mean ± SEM. MS Excel (Microsoft Excel 2016, version 16.0, Microsoft Corporation, USA) was used for data analysis. The P values less than 0.05 were considered statistically significant [35,37].

3. Results

In the preliminary tests, we printed12 % aqueous PEO gels loaded with rutin (vitamin P) without any additional excipients. The results, however, were not satisfactory, since the homogeneity of gels was quite poor, and rutin in the final 3D-printed scaffolds partially oxidized during the manufacturing process, which was manifested with brownish-colored areas. Consequently, and the reasons mentioned above, eumulgin [28,29] or Tween 80 was added as a surface-active agent at a small concentration in the PEO printing gel ink to improve the homogeneity and printability of the gels. The inclusion of eumulgin in the PEO gel ink resulted in the 3D-printed scaffolds, which were readily removable from the printing plate surface, but sometimes it was even impossible to terminate a SSE 3D printing process. Next, we used Tween 80 at different concentrations (1, 3 and 5% w/w), and found that the most feasible concentrations of Tween 80 in the aqueous PEO gel inks were 3% and 5% (w/w). The corresponding PEO gel inks were homogeneous and showed a great printability without any printing interruptions or flaws. Therefore, the present Tween 80 containing PEO gel ink was chosen for the next experimental gel ink samples loaded with rutin (vitamin P).

Table 1 shows the compositions of the Tween 80 containing aqueous PEO gel inks loaded with rutin (vitamin P), which were prepared and used for SSE 3D printing experiments. For preparing such PEO gel inks, rutin was first dissolved in a small amount of ethanol, and then this ethanolic mixture was added by gently mixing in an aqueous PEO gel. Finally, Tween 80 was added in driblets into a PEO gel ink and mixed gradually to achieve a homogeneous gel. Tween 80 as a non-ionic surface-active agent improved the homogeneity of the rutin-loaded aqueous PEO gels and prevented the precipitation and oxidation of rutin.

Table 2 shows the viscosity of the experimental PEO gel inks loaded with rutin (vitamin P). The viscosity of the gel inks was relatively high, but only other hand very feasible for a SSE 3D printing. Decreasing the amount of rutin from 1.00 g to 0.50 g appeared to increase the viscosity of the Tween 80 containing PEO gels. However, the difference in the viscosity of the gel inks loaded with rutin was not statistically significant.

For investigating the homogeneity of the PEO gel inks loaded with rutin (vitamin P), the gels were studied under an optical light microscopy immediately after preparation. Figure 1 shows the optical light microscopy images of the experimental PEO gel inks at three different magnifications (5×, 20× and 50×). As seen in Figure 1, rutin was homogeneously dispersed in all Tween 80 containing aqueous PEO gels, thus suggesting the applicability of the present gels as a printing ink in SSE 3D printing. No cluster formation was observed in the gel inks. The PEO gel inks containing rutin (vitamin P) were yellow-to-brownish in color.

Figure 2 shows the physical appearance of the SSE 3D-printed lattices prepared by using the PEO gels loaded with rutin (vitamin P). The two upper photographs present the reference 3D-printed lattices prepared using a 12% PEO gel ink containing Tween 80 as surface active agent 3% or 5% (w/w) and without rutin. The photographs in the three lower rows of Figure 2 show the physical appearance of the SSE 3D-printed lattices (three parallel printings) loaded with rutin (vitamin P) (reference is also made to Table 1). As seen in Figure 2, all three PEO gel inks loaded with rutin at three different concentrations showed very good printability. The lattices containing rutin (vitamin P) were brownish-to-yellow in color.

The surface area and weight of the SSE 3D-printed lattices of rutin (vitamin P) are summarized in Table 3. The weight and surface area variation of the 3D-printed lattices was less than 12% and 10%, respectively.

We used also SSE 3D printing for preparing the round-shaped DDSs and scaffolds of rutin (vitamin P) intended for the oral administration (Table 4). The present scaffolds were uniform in color, size, weight and physical appearance. The weight variation of the 3D-printed lattices was less than 5%.

The content of rutin (%) in the SSE 3D-printed scaffolds was studied by means of UV spectrophotometry and HPLC, and the results are shown in Table 5 and Figure 3. As seen in Table 5, the content of rutin (vitamin P) in the SSE 3D-printed scaffolds was close to a theoretically calculated content value of the present flavonoid in the aqueous PEO gels, and the content uniformity was very good. The rutin assay results were also not dependent on the analytical method used. The content of the rutin determined by UV spectrophotometry and HPLC were comparable with the theoretical content value and the results obtained with each other.

4. Discussion

In the present study, we formulated and evaluated aqueous PEO gel inks loaded with plant-origin rutin (vitamin P) for SSE 3D printing. The PEO gel inks loaded with rutin and Tween 80 as a surface-active agent, were found to be homogeneous viscous gels with yеl low color. The addition of rutin in the aqueous PEO gels without any surface-active agent leads to the impaired homogeneity of the gels and to the color changes in the final 3D-printed scaffolds due to the oxidation of rutin. To avoid such drawbacks, surface-active agents, such as Emulgin and Tween 80 were used. When Emulgin was added in the PEO gel inks, we found that the 3D-printed scaffolds readily (too quickly) peeled off from the printing plate, which in some cases prevented the successful completion of the 3D printing. Therefore, Tween 80 was selected as a surface-active agent for the PEO gel inks. The use of Tween 80 enabled more homogeneous distribution of rutin (vitamin P) in the PEO gels. The addition of Tween 80 prevented also the oxidation of rutin and provided very good 3D printing results. Among the three concentration levels of Tween 80 studied, the use of the two highest concentrations of Tween 80 (3% and 5% w/w) resulted in a good stability and printability of a PEO gel. The PEO gel inks with the lowest concentration level of Tween 80 (1% w/w) were not homogeneous and stable. When preparing the PEO gels loaded with rutin, the order of mixing and adding ingredients was crucial. If the procedure was violated, it was impossible to obtain homogeneous gels. First, rutin was dissolved in a minimal volume of ethanol. This liquid volume needs to be taken into account and adjusted with the volume of water used in preparing the PEO gel inks for 3D printing. Then, the pre-calculated amount of Tween 80 was added in driblets to this solution and thoroughly mixed. The PEO gel inks should be prepared according to the standard procedure, considering the volume of ethanol used to dissolve the rutin [27]. Finally, the ethanolic rutin solution with Tween 80 was gradually added to the PEO gel while continuously stirring. This procedure ensured the preparation of homogeneous gels, which was confirmed by visual inspection and by means of optical light microscopy (Fig. 1).

The PEO gels loaded with rutin at all three concentrations and prepared as described in the previous paragraph, showed a good printability in a SSE 3D printing process (Fig. 2, Table 3). The use of PEO gels with the rutin concentration of 100 mg/ml led to the highest quality of printed scaffolds based on a visual inspection and surface area analysis. The results on the dissolution test in vitro showed that the SSE 3D-printed PEO scaffolds completely lost their shape and disintegrated within 15-20 minutes in purified water at a room temperature at 22 ± 2 °C. This suggests the immediate-release behavior of the present SSE 3D-printed DDSs and potential for oral administration. Further studies, however, are needed to verify the disintegration and drug release in vitro according to the European Pharmacopoeia (Ph.Eur.).

For the quantitative standardization of the present 3D-printed drug preparations, both HPLC and UV spectrophotometry were used as slightly modified Ph.Eur. analytical methods. Both analytical methods developed showed a good reproducibility in the content analysis of rutin (vitamin P) and their potential for the use in the standardization of the corresponding 3D-printed oral solid dosage forms of rutin was verified. The assay results of rutin obtained with UV spectrophotometry and HPLC were in line with the theoretical rutin content and comparable with each other. However, it should be noted that both of these analytical methods still need further validation for their intended use for the content analysis of the present SSE 3D-printed DDSs.

Since rutin (vitamin P) is an established and well-known representative of the plant-origin flavonoids, the SSE 3D-printed formulations (i.e., a printing gel inks) developed in our study, are most likely feasible also in the SSE 3D printing of other plant-origin flavonoids with a potential therapeutic value and their implementation into pharmaceutical practice.

5. Conclusions

Novel aqueous PEO gel ink compositions were developed for the pharmaceutical SSE 3D printing of plant-origin rutin (vitamin P). The present 3D-printed formulations and customized drug preparations could be used also for the oral administration of other plant-origin flavonoids with a potential therapeutic value. The PEO gel inks loaded with rutin and Tween 80 (as a surface-active agent) showed a good printability at the printing head speed (rate) levels of 0.5 mm/s. The most feasible aqueous PEO gel formulation for the SSE 3D printing of rutin (vitamin P) was composed of rutin 100 mg/ml and Tween 80 as a surface-active agent 50 mg/ml (dissolved in a 12% aqueous PEO gel). Further studies are needed to reveal potential physicochemical incompatibilities between the material components in the present PEO gel inks and to verify the in-vitro dissolution properties and storage stability of the SSE 3D-printed DDSs of rutin (vitamin P).