Electrospun Polyurethane-Based Biomaterials

Polyurethanes (PU) are particularly interesting polymers because they can be synthesized through combinations of many different monomers that allow for detailed regulation of their physicochemical properties [Amalvy et al., 2020]. They are thermoplastic polymers with very diverse and broad application areas such as flexible foams (sponges), non-swelling elastomers, adhesives, coatings, integral products containing both elastomer and foam structures, rigid foams (insulation materials), non-swelling wood-like products, wound dressing materials, tissue engineering applications and medical materials. PUs are synthesized by polymerization of diisocyanates with polyols (Figure 1). Due to the breadth of application areas, high biocompatibility, mechanical properties and applicability, they have been the subject of numerous research areas and are among the most commonly used polymers for producing biomaterials through various methods. One of these methods is nanofiber production through the electrospinning method. [caption id="attachment_119588" align="aligncenter"] Figure 1. PU general synthesis scheme[/caption] [caption id="attachment_119589" align="aligncenter"] Figure 2. A simple electrospinning apparatus [Karayeğen, 2016, Li et al., 2009].[/caption]  The electrospinning method enables the production of a wide variety of PU-based nanofiber materials and simultaneously creates new research topics in many new applications due to ease of production. In this article, various biomaterial application areas of PU-based nanofiber materials have been compiled.

1. Electrospinning Applications in Polyurethane-Based Biomaterials

PU holds a very important place in biomaterial research due to its mechanical strength, elasticity, permeability and high biocompatibility. In electrospinning applications, PU-based materials are quite attractive and have very broad application areas. In this context, the diversity of work conducted is quite extensive. For example; in their study, Amalvy and colleagues carried out production of electrospun fibers from bio-based PU aqueous dispersions with polyethylene oxide (PEO). PU fibers were obtained only when PEO was added. In the absence of PEO, the dispersion underwent electrospray and particle formation was observed. It was confirmed that PEO changed the rheology of the dispersion and helped PU particles combine. After removal of PEO by immersing PU fibers in water, they retained their diameters and homogeneous structures, indicating that the distribution of both polymers within the fibers was uniform. Scanning Electron Microscope (SEM) images of the fibers obtained in the study are shown in Figure 3. This study provided both an understanding of the role of PEO and a new method for producing environmentally friendly bio-based PU microfibers from aqueous dispersions [Amalvy et al., 2020]. [caption id="attachment_119591" align="aligncenter"] Figure 3. (A) PEO fibers, (b) electrosprayed PU particles, (c) PEO-PU3, (d) PU3, (e) PEO-PU4, (f) PU4, (g) PEO-PU5 and (h) PU5 fibers SEM images [Amalvy et al., 2020].[/caption]  In their study, Lee and colleagues examined the production of a new polyketone nanofiber-reinforced UV-cured polyuretane-acrylate (PUA) nanocomposite. An aliphatic polyketone nanofibrous mat was efficiently prepared by electrospinning using a mixture of methylene chloride and trifluoroacetic acid solvent. The use of polymer nanofibers enables small pore size, high aspect ratio, molecular orientation and excellent mechanical performance. A new, highly transparent and flexible composite was produced based on casting, electrospinning and UV-curing processes. This method enabled significant improvement of mechanical properties using very small amounts of polyketone nanofiber. The yield strength and Young's modulus of the nanocomposite were improved to 61% and 60%, respectively, compared to UV-cured polyurethane acrylate film. The reinforcing effect of the nanofibers was achieved without compromising the transparency and flexibility of the PUA. In addition to the morphologies of the nanofibers and fiber/resin interface, the properties of the nanocomposite films were also examined. SEM images of the obtained fibers are shown in Figure 4. The effective use of nanofibers as reinforcement and preparation of a nanocomposite, as well as its characterization compared to a UV-cured PUA film, have been discussed [Lee et al., 2020]. [caption id="attachment_119594" align="aligncenter"] Figure 4. A) PUA film and B) PK/PUA nanocomposite film surface morphologies, C) PUA film and D) PK/PUA nanocomposite film fracture cross-section morphologies [Lee et al., 2020].[/caption]  In their study, Liping Wang and colleagues developed composite pH-sensitive sensors for the combination of conductive and flexible materials by transforming them into coaxial electrospun core-shell fibers of polyaniline (PANI) and PU. PU improved the mechanical properties of the sensors. The mechanical properties and conductivity of the sensor could be modified by adjusting the PANI and PU ratio. In buffer solutions, the sensor operated linearly in the pH range of 2-7 at a sensitivity of -60 mV/pH via chronopotentiometry, and pH changes below 0.2 could be detected. It was verified that the sensor could resist the effects of temperature changes, sensing time and sensor deformation and provided stability in pH detection. Moreover, our sensor adhered to the skin and showed its response to the pH of small amounts of sweat on the skin surface, demonstrating the potential for wearable devices (Figure 5) [Wang et al., 2020]. [caption id="attachment_119598" align="aligncenter"] Figure 5. (a) Cell viability of sensors, (b) Photograph of three-electrode chip attached to the arm surface and structure of the three-electrode chip, (c) Chronopotentiometric signal in detection of sweat pH on skin surface [Wang et al., 2020].[/caption]  In their study, Faiza Sharif and colleagues enriched hydroxyapatite (HA) with various additives to improve the biological and mechanical properties of skeletal tissue. Additive-enriched bioactive HA was incorporated into PU and polylactic acid (PLA) polymeric matrices for the production of osteogenic membranes. Strontium (Sr), magnesium (Mg), orthosilicate (SiO4) and carbonate (CO3 2-) were used as additives and PU-PLA-Sr-HA, PU-PLA-Mg-HA, PU-PLA-Si-HA and PU-PLA-C-HA composites were produced. Cell proliferation and calcium accumulation were evaluated in vivo on the prepared osteogenic membranes using the MC3T3 preosteoblast cell line. It was confirmed that PU-PLA-Sr-HA membranes exhibited fiber morphology with pores distributed homogeneously along the fiber surface. The tensile strength of membranes containing additive-enriched HA was observed to be much higher compared to those without additives. Cell viability testing proved that all prepared membranes demonstrated very high biocompatibility, and it was observed that PU-PLA-C-HA and PU-PLA-Sr-HA membranes in particular showed higher cell proliferation. SEM images of cells attached to the surface of the produced membranes are shown in Figure 6. As a result, it was predicted that electrospun fibrous membranes with additive-enriched HA would exhibit improved osteogenic properties and would therefore be excellent candidates for bone and dental regenerative applications [Sharif et al., 2020]. [caption id="attachment_119601" align="aligncenter"] Figure 6. SEM micrographs showing cells attached to the membrane surface [Sharif et al., 2020].[/caption]  In their study, Ni Li and colleagues prepared fluorine-free PU/silicon dioxide (SiO2) nanofibrous mats by electrospinning and hydro thermal treatments of nanofibrous mats and laminated fabrics (Figure 7). The developed mats were then laminated onto polyester woven fabrics to examine their practical applications in the textile field. The surface morphology, pore size, water contact angle and hydrostatic pressure were examined to investigate the waterproofing and breathability of the PU/SiO2. Results showed that when the tetraethoxysilane concentration in the electrospinning solution was 6% by weight, the fiber mat exhibited desired morphology, mechanical properties and hydrophobicity. At this point, the SiO2 nanoparticles were distributed homogeneously on and within the PU nanofibers and the fibers exhibited more uniform diameter. The produced composite laminated fabrics demonstrated a hydrostatic pressure value of 23.5 kPa with a water vapor transmission rate of 5.19 kg/m2/day. The results showed that the electrospun mats and laminated fabric demonstrated promising applications as separation membranes, filters, outdoor sports apparel and protective clothing [Li et al., 2020]. [caption id="attachment_119602" align="aligncenter"] Figure 7. Schematic representation for preparation of PU/SiO2 hybrid electrospun (electrospun) fibers [Li et al., 2020].[/caption]Electrochemical actuators made from conductive polymers are one of the most important types of artificial muscles that have attracted considerable attention in recent years. However, these artificial muscles suffer from potential drop along their length, which weakens their performance in applications requiring longer muscles. In their study, Hossein Fashandi and colleagues aimed to improve the performance of such artificial muscles by overcoming potential drop along the length. This was achieved by creating a full-length highly conductive nanofibrous layer through the copper electrospinning method. A schematic representation and SEM images of the intersection points of copper-electrolysis PU nanofibers are shown in Figure 8. PU/copper/polypyrrole (PU/Cu/PPy) nanofibrous artificial muscles were produced by combining electrospinning, electroplating and electrochemical polymerization (Figure 9). Cu electroplating was observed to affect many properties of the PU nanofiber layer including nanofiber surface morphology, thermal, mechanical and electrical properties. The average diameter of PU nanofibers was measured as 306 ± 48 nm and this was observed to increase significantly with increasing Cu electroplating time, and the initial temperature of thermal decomposition of nanofibers increased from 249 to 302°C with 20 s Cu electroplating. Electrochemical polymerization of pyrrole on the surface of metallized nanofibers increased the average diameter of the produced nanofibers to 1030±102 nm. Providing electrical continuity throughout the entire length of the artificial muscle resulted in improved electroactive properties, and therefore a final bending activation of 88° was observed for a cyclic potential between -0.8 V and 0.5 V at a scan rate of 5 mV/s. As a result, it is predicted that the nanofibrous artificial muscle produced by the proposed method has enormous potential for use in practical applications [Fakhrali et al., 2020]. [caption id="attachment_119622" align="aligncenter"] Figure 9. SEM image of PU/Cu/PPy nanofibers [Fakhrali et al., 2020].[/caption]  In their study, Ateş and colleagues carried out synthesis and application of biocompatible steviol glycoside-based PU/polycaprolactone (PU/PCL) fibers as a potential wound dressing material that could be used to close non-healing wounds through the electrospinning method. During electrospinning, steviol glycoside-based PU structures were used in formation of mixtures with PCL for ease of production. Steviol glycosides are a natural abundant and easily accessible source as the main component of wound dressing material due to their free hydroxyl groups, high biocompatibility and hydrophilicity. The structure of steviol glycosides consists of saccharide units and free OH groups. Thus, steviol glycosides serve as a cross-linker within the PU structure and provide mechanical strength. PU structures containing steviol glycosides were synthesized through solution polymerization technique with hexamethylene diisocyanate, lactose and polyethylene glycol-200 (PEG-200). PCL was added to the prepared PUs in a 1:2 ratio and a nanofiber structure was formed. After the prepared wound dressing material was characterized by various analyses, the swelling degree, water content and oxygen permeability of the steviol glycoside-based PU/PCL material were determined. SEM images of the produced PU/PCL fiber materials are shown in Figure 10. In the biocompatibility test, the cell viability value of PU/PCL fiber materials was determined as 86.9% in the indirect cytotoxicity test, and cell adhesion on hybrid PU/PCL fibers was morphologically confirmed. Steviol glycoside-based PU/PCL wound dressing material was produced easily and at low cost. As a result, the wound dressing materials obtained with high biocompatibility and low cost will be an effective and rapid method in the healing of open wounds in diabetic patients [Ateş et al., 2020]. [caption id="attachment_119643" align="aligncenter"] Figure 10. SEM images of stevia glycoside-based PU/PCL fibers [Ateş et al., 2020].[/caption]  The development of bio-fabrication techniques has led to advances in the field of tissue engineering, particularly in the production of tissue replacements that mimic the physiological complexity of natural tissues. Transplantation is an effective clinical strategy for millions of cardiovascular disease patients worldwide. Vascular grafts are used not only in the treatment of cardiovascular diseases but also in other clinical applications such as dialysis, pediatric cardiac surgery and mesenteric ischemia treatment. The success of vascular scaffolds and blood vessel transfers depends largely on their structure and mechanical properties.

In their study, Caiping Su and colleagues developed a new method to produce a multi-layer tube-shaped PU-based scaffold through an innovative 3D printing method to meet design requirements with high precision. It was predicted that the combination of composite materials and organized layers could solve existing limitations of tube-shaped scaffolds and that production of a four-layer composite tube structure would be possible. In scaffolds produced with organized fibers in a four-layer tube shape (FLTS); two middle layers mimic the tunica media of natural blood vessels and provide the required mechanical strength, which was confirmed. Schematic representations of natural blood vessels and small-diameter FLTS are shown in Figure 11. Four layers with differently oriented fibers were produced by the electrospinning method using a mandrel collector.

The aim was not only to organize using topographic cues that train endothelial cells (EC), but also to increase their proliferation and adhesion and improve the mechanical properties of the FLTS. The developed FLTS were electrospun with thermoplastic PU (TPU)/PCL/PEG, a completely random fiber layer (RLTS) to produce FLTS. SEM images of the obtained nanofibers are shown in Figure 12. The surface wettability of TPU/PCL/PEG FLTS was tested by water contact angle analysis, and when compared with RLTS, FLTS demonstrated excellent mechanical properties with higher circumferential and longitudinal tensile properties. Additionally, the high viability of human umbilical vein endothelial cells (HUVECs) on FLTS demonstrated the biocompatibility of tubular structure scaffolds compared to RLTS. It was confirmed that the organized and random composite structure of the FLTS was useful for promoting HUVEC growth, and cell adhesion and proliferation on these scaffolds were superior to RLTS. In light of these results, the FLTS produced showed high application potential in vascular tissue regeneration and clinical arterial replacements [Su et al., 2020]. [caption id="attachment_119646" align="aligncenter"] Figure 11. (a) Natural blood vessel and (b) Schematic diagram of small-diameter FLTS. FLTS production: (c) Inner layer, (d) Mid-inner layer, (e) Mid-outer layer and (f) Outer layer [Su et al., 2020].[/caption][caption id="attachment_119648" align="aligncenter"] Figure 12. SEM images and structure of electrospun TPU/PCL/PEG FLTS: (a) Inner layer, (b) Mid-inner layer, (c) Mid-outer layer, (d) Outer layer, (e) Cross-sections of tubular scaffold, (f) Four layers and (g) Magnified area showing delamination-free structure (h) Tubular scaffold [Su et al., 2020].[/caption]

Skeletal muscles are considered the best actuators available in nature owing to their hierarchically multi-scaled fibrous structures that can enhance their strength and contraction performance. In recent years, with the growth of soft robotics and tissue engineering research areas, many biomimetic soft actuators and scaffolds have been designed inspired by biological skeletal muscle. In their study, Andrea Zucchelli and colleagues used the electrospinning method to develop a hierarchically organized nanofibrous structure that resembles the morphology and passive biomechanical properties of skeletal muscles.

A low-modulus PU was used to mimic the passive properties of muscle. Several electrospun structures with different three-dimensional internal arrangements of nanofibers (mats, bundles and muscle-like tissues) were produced. A schematic representation of the electrospinning setups and sample preparation is shown in Figure 13. Thermal characterization through thermogravimetric and differential scanning calorimetry analysis investigated the physicochemical properties of the material. Multi-scaled morphological similarities with the biological counterpart were confirmed by SEM analysis (Figure 14). Tensile tests conducted on different electrospun samples showed that the muscle-like device exhibited slightly higher strength and stiffness compared to those of skeletal muscle. Additionally, mathematical models of the mechanical behavior of the nanofibrous structures were successfully developed, enabling better investigation of the relationships between the structure and mechanics of the samples. The highly promising results proved the suitability of the hierarchical electrospun nanofibrous structure in this study for applications in regenerative medicine and when combined with active materials would provide suitable use in soft actuators for the robotics field [Zucchelli et al., 2020]. [caption id="attachment_119650" align="aligncenter"] Figure 13. Schematic representation of electrospinning setups and sample preparation. (A) Production of PU random mats and bundles: (AI) random nanofibers, production on a low-speed rotating drum collector[/caption]   . (AII) Mats were then removed and cut into rectangular samples or cut into strips, wrapped and ultimately pulled from the drum producing ring-shaped random nanofibrous bundles. (B) Production of PU aligned mats and bundles: (BI) aligned nanofibers were electrospun on a high-speed rotating drum collector. (BII) Mats were then removed and cut into rectangular samples or cut into strips, wrapped and ultimately pulled from the drum producing ring-shaped bundles consisting of axially aligned nanofibers. (C) HNES preparation: (CI) Two aligned bundles bent and twisted in the middle and (CII) fixed to a custom electrospinning rotating apparatus with a flat collector at the back; PU during electrospinning (CIII, CIV) varying stasis and rotation periods, (CV) an external epimysium-like membrane was produced in the final HNES [Zucchelli et al., 2020].   [caption id="attachment_119656" align="aligncenter"] Figure 14. SEM analysis on electrospun samples. (A) Mat of random fibers on PE support paper (scale bar=20 mm); (B) Random fiber mat removed from PE support paper (scale bar=20 mm); (C) Mat of aligned fibers on PE support paper (scale bar=20 mm); (D) Mat of aligned fibers removed from PE support paper (scale bar=20 mm); (EI) Random fiber bundle (scale bar=100 mm); (EII) Random fiber bundle - magnification (scale bar=20 mm); (FI) Aligned fiber bundle (scale bar=100 mm); (FII) Aligned fiber bundle - magnification (scale bar=20 mm); (GI) HNES-Partial cross-section (scale bar=150 mm); (GII) HNES - Outer membrane of random fibers (scale bar=20 mm); (GIII) HNES-Cross-section of one of the inner bundles of aligned fibers (scale bar=100 mm) [Zucchelli et al., 2020].[/caption]  Nanomaterial structures contribute significantly to TEVS by virtue of their ability to mimic the nano-scale dimensions of the natural extracellular matrix (ECM) and the existing mechanical match between the vascular graft and natural blood vessels. In their study, Davod Mohebbi-Kalhori and colleagues aimed to develop and mechanically improve triad-hybrid nano-fibered scaffolds with different polyethylene terephthalate (PET), PU and PCL composite ratios. The morphological, biological, mechanical and biomechanical properties of clean and hybrid structures were examined using SEM, DSC, FTIR, tensile, compliance, burst pressure and MTT tests, and samples were implanted subcutaneously in rats to explore the in vivo immune response. SEM images of the obtained hybrid structures are shown in Figure 15. Results observed that changes in fiber diameter and porosity in the triad-hybrid electrospun scaffold ranged from 388 ± 88 to 547 ± 89 nm and 56.60% ± 2.06 to 75.00% ± 1.94%, respectively, and the range of strain and strength changes in the scaffolds were confirmed to be in the ranges of 2.7 ± 0.44 to 5.27 ± 0.83 MPa and 2.68 ± 0.19 to 10.03 ± 0.75 MPa, respectively. Additionally, the compliance and burst pressure of the structures were determined as 4.05 ± 0.21 to 7.09 ± 0.49 and 1623 ± 329 to 2560 ± 121 mmHg, respectively. According to MTT testing, high cell viability was observed in triad-hybrid structures with high PET percentage compared to PU. Based on the findings obtained, it was predicted that the PET/PU/PCL triad-hybrid vascular scaffold has sufficient potential for use in vascular tissue engineering applications [Mohebbi-Kalhori et al., 2020] [caption id="attachment_119663" align="aligncenter"] Figure 15. Electrospun fiber morphology of clean and triad hybrid tube-shaped scaffolds: (A) PET, (B) PU, (C) PCL, (D) PET50/PU25/PCL25, (F) PET25/PU25/PCL50, (G) PET33/PU33/PCL33 and (H) electrospun vascular scaffolds with 5 mm inner diameter. [Mohebbi-Kalhori et al., 2020][/caption]  In biomaterial research, the development of new treatment methods and updating of existing methods are very important. Due to their high biocompatibility, mechanical strength, ease of functionalization with different properties and physicochemical characteristics, polyurethanes are the subject of extensive research in the scientific community. The electrospinning method is a practical and very low-cost method that enables the development of many different biomaterials in various fields. Therefore, many PU-based composite biomaterials have been produced through the electrospinning method and what more can be developed in this field constitutes a very current research area. In this sense, the electrospinning method holds great importance as a practical, low-cost, applicable and current method in biomaterial production.

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  Seda Kolak İnönü Üniversitesi Fen Edebiyat Fakültesi Kimya Bölümü Dr. Ahmet Ulu İnönü Üniversitesi Fen Edebiyat Fakültesi Kimya Bölümü Prof. Dr. Burhan Ateş İnönü Üniversitesi Fen Edebiyat Fakültesi Kimya Bölümü