Near-Field Electrospinning and Melt Electrowriting of Biomedical Polymers-Progress and Limitations

Electrohydrodynamic Direct-Writing Micro/Nanofibrous Architectures: Principle, Materials, and Biomedical Applications

Electrohydrodynamic (EHD) direct-writing has recently gained attention as a highly promising additive manufacturing strategy for fabricating intricate micro/nanoscale architectures. This technique is particularly well-suited for mimicking the extracellular matrix (ECM) present in biological tissue, which serves a vital function in facilitating cell colonization, migration, and growth. The integration of EHD direct-writing with other techniques has been employed to enhance the biological performance of scaffolds, and significant advancements have been made in the development of tailored scaffold architectures and constituents to meet the specific requirements of various biomedical applications. Here, a comprehensive overview of EHD direct-writing is provided, including its underlying principles, demonstrated materials systems, and biomedical applications. A brief chronology of EHD direct-writing is provided, along with an examination of the observed phenomena that occur during the printing process. The impact of biomaterial selection and architectural topographic cues on biological performance is also highlighted. Finally, the major limitations associated with EHD direct-writing are discussed.

An Integrated Dual-Layer Heterogeneous Polycaprolactone Scaffold Promotes Oral Mucosal Wound Healing through Inhibiting Bacterial Adhesion and Mediating HGF-1 Behavior

Recently, the high incidence of oral mucosal defects and the subsequent functional impairments have attracted widespread attention. Controlling scaffold geometry pattern has been proposed as a strategy to promote cell behavior and facilitate soft tissue repair. In this study, we innovatively construct an integrated dual-layer heterogeneous polycaprolactone (PCL) scaffold using melt electrowriting (MEW) technology. The outer layer was disordered, while the inner layer featured oriented fiber patterns: parallel (P-par), rhombic (P-rhomb), and square (P-sq). Our findings revealed that the P-rhomb and P-sq scaffolds exhibited superior surface wettability, roughness, and tensile strength compared to the pure disordered PCL scaffolds (P) and P-par. Compared to the commercial collagen membranes, the outer layer of PCL can effectively inhibit bacterial adhesion and biofilm formation. Furthermore, the P-rhomb and P-sq groups demonstrated higher gene and protein expression levels related to cell adhesion and cell migration rates than did the P and P-par groups. Among them, P-sq plays an important role in inducing the differentiation of gingival fibroblasts into myofibroblasts rich in α-smooth muscle actin (α-SMA). Additionally, P-sq could reduce inflammation, promote epithelial regeneration, and accelerate wound healing when used in full-thickness oral mucosal defects in rabbits. Overall, the integrated dual-layer heterogeneous PCL scaffold fabricated by MEW technology effectively inhibited bacterial adhesion and guided tissue regeneration, offering advantages for clinical translation and large-scale production. This promising material holds important potential for treating full-thickness mucosal defects in a bacteria-rich oral environments.

Controlling Mechanical Properties of Medical-Grade Scaffolds through Electrospinning Parameter Selection

Electrospinning (ES) is a versatile process mode for creating fibrous materials with various structures that have broad applications ranging from regenerative medicine to tissue engineering and surgical mesh implants. The recent commercialization of this technology for implant use has driven the use of resorbable electrospun products. Resorbable electrospun meshes offer great promise as temporary implants that can utilize the layer upon layer method of additive manufacturing to incorporate porosity as a function of process parameters into a scaffold structure. The interconnected porosity and feature size known to ES have previously been observed to hold great potential for simulating the natural cellular environment of soft tissue. This microstructure, proper degradation kinetics, and mechanical properties combine to provide the design basis for artificial tissue structures that could aid in not only wound healing but also true tissue engineering and regenerative medicine. While current advancement in the field is understood to be limited by material properties, the importance of optimizing mechanical properties with currently available materials should not be overlooked. This work investigated the process parameter effects and interactions that control the structure-property relationship for a range of medical-grade aliphatic polyester materials with a range of intrinsic properties. An ε-caprolactone homopolymer (PCL), l-lactide homopolymer (PLLA), and Lactoflex, a copolymer with intermediate properties relative to the homopolymers, were characterized before, during, and after the additive manufacturing process. The interacting effects of process parameters, distance to collector, and dispensing rate were shown to produce variable-density, nonwoven scaffold structures. The resorbable mesh scaffolds of PLLA, PCL, and Lactoflex demonstrated a broad range of mechanical properties (approximately 1-10 MPa ultimate tensile strength and 5-390 MPa tensile modulus). Postprocessing of scaffolds demonstrated removal of solvents and preservation of micrometer-sized features. Resorbable polymers and electrospinning can produce scaffold materials with excellent features and offer tremendous potential in the field of implantable resorbable devices.

Electrowriting of SU-8 Microfibers

As microfiber-based additive manufacturing (AM) technologies, melt electrowriting (MEW) and solution electrowriting (SEW) have demonstrated efficacy with more biomedically relevant materials. By processing SU-8 resin using MEW and SEW techniques, a material with substantially different mechanical, thermal, and optical properties than that typically processed is introduced. SU-8 polymer is temperature sensitive and requires the devising of a specific heating protocol to be properly processed. Smooth-surfaced microfibers resulted from MEW of SU8 for a short period (from 30 to 90 min), which provides the greatest control and, thus, reproducibility of the printed microfibers. This investigation explores various parameters influencing the electrowriting process, printing conditions, and post-processing to optimize the fabrication of intricate 3D structures. This work demonstrates the controlled generation of straight filaments and complex multi-layered architectures, which were characterized by brightfield, darkfield, and scanning electron microscopy (SEM). This research opens new avenues for the design and development of 3D-printed photonic systems by leveraging the properties of SU-8 after both MEW and SEW processing.

Materials and Strategies to Enhance Melt Electrowriting Potential

Melt electrowriting (MEW) is an emerging additive manufacturing (AM) technology that enables the precise deposition of continuous polymeric microfibers, allowing for the creation of high-resolution constructs. In recent years, MEW has undergone a revolution, with the introduction of active properties or additional functionalities through novel polymer processing strategies, the incorporation of functional fillers, postprocessing, or the combination with other techniques. While extensively explored in biomedical applications, MEW's potential in other fields remains untapped. Thus, this review explores MEW's characteristics from a materials science perspective, emphasizing the diverse range of materials and composites processed by this technique and their current and potential applications. Additionally, the prospects offered by postprinting processing techniques are explored, together with the synergy achieved by combining melt electrowriting with other manufacturing methods. By highlighting the untapped potentials of MEW, this review aims to inspire research groups across various fields to leverage this technology for innovative endeavors.

Advanced Nanomaterials in Medical 3D Printing

3D printing is now recognized as a significant tool for medical research and clinical practice, leading to the emergence of medical 3D printing technology. It is essential to improve the properties of 3D-printed products to meet the demand for medical use. The core of generating qualified 3D printing products is to develop advanced materials and processes. Taking advantage of nanomaterials with tunable and distinct physical, chemical, and biological properties, integrating nanotechnology into 3D printing creates new opportunities for advancing medical 3D printing field. Recently, some attempts are made to improve medical 3D printing through nanotechnology, providing new insights into developing advanced medical 3D printing technology. With high-resolution 3D printing technology, nano-structures can be directly fabricated for medical applications. Incorporating nanomaterials into the 3D printing material system can improve the properties of the 3D-printed medical products. At the same time, nanomaterials can be used to expand novel medical 3D printing technologies. This review introduced the strategies and progresses of improving medical 3D printing through nanotechnology and discussed challenges in clinical translation.

Enhancing quality control in bioprinting through machine learning

Bioprinting technologies have been extensively studied in literature to fabricate three-dimensional constructs for tissue engineering applications. However, very few examples are currently available on clinical trials using bioprinted products, due to a combination of technological challenges (i.e. difficulties in replicating the native tissue complexity, long printing times, limited choice of printable biomaterials) and regulatory barriers (i.e. no clear indication on the product classification in the current regulatory framework). In particular, quality control (QC) solutions are needed at different stages of the bioprinting workflow (including pre-process optimization, in-process monitoring, and post-process assessment) to guarantee a repeatable product which is functional and safe for the patient. In this context, machine learning (ML) algorithms can be envisioned as a promising solution for the automatization of the quality assessment, reducing the inter-batch variability and thus potentially accelerating the product clinical translation and commercialization. In this review, we comprehensively analyse the main solutions that are being developed in the bioprinting literature on QC enabled by ML, evaluating different models from a technical perspective, including the amount and type of data used, the algorithms, and performance measures. Finally, we give a perspective view on current challenges and future research directions on using these technologies to enhance the quality assessment in bioprinting.

A direct-writing electrospinning system for designing complex architectures in tissue engineering

Recently, direct-writing electrospinning has been pursued to reach a higher accuracy and complexity in fiber scaffold fabrication compared to other extrusion techniques more frequently encountered in tissue engineering. However, to date, direct-writing electrospinning lacks a wide application to process materials such as nature-derived polymers, of huge importance in tissue engineering given their chemical properties similar to that of native tissues. In this work, a setup to perform direct-writing electrospinning was developed and demonstrated versatility and efficiency in obtaining submicrometric fibers and guiding their deposition along various types of paths and patterns, resulting in a user-friendly method to create structures closely resembling tissue architecture.

Review of Piezoelectrical Materials Potentially Useful for Peripheral Nerve Repair

It has increasingly been recognized that electrical currents play a pivotal role in cell migration and tissue repair, in a process named "galvanotaxis". In this review, we summarize the current evidence supporting the potential benefits of electric stimulation (ES) in the physiology of peripheral nerve repair (PNR). Moreover, we discuss the potential of piezoelectric materials in this context. The use of these materials has deserved great attention, as the movement of the body or of the external environment can be used to power internally the electrical properties of devices used for providing ES or acting as sensory receptors in artificial skin (e-skin). The fact that organic materials sustain spontaneous degradation inside the body means their piezoelectric effect is limited in duration. In the case of PNR, this is not necessarily problematic, as ES is only required during the regeneration period. Arguably, piezoelectric materials have the potential to revolutionize PNR with new biomedical devices that range from scaffolds and nerve-guiding conduits to sensory or efferent components of e-skin. However, much remains to be learned regarding piezoelectric materials, their use in manufacturing of biomedical devices, and their sterilization process, to fine-tune their safe, effective, and predictable in vivo application.

A novel polyhydroxyalkanoate/polyvinyl alcohol composite porous membrane via electrospinning and spin coating as potential application for chemotherapy and tissue engineering

Polyhydroxyalkanoate/polyvinyl alcohol (PHA/PVA) composite porous membranes were successfully prepared by coupling of electrospinning and spin‐coating. The resulting composite membranes were characterized by scanning electron microscope (SEM), FT‐IR spectrometer, x‐ray diffraction (XRD), contact angle tester and Brunner–Emmet–Teller (BET). It indicated that the PHA/PVA membrane belonged to a mesoporous material, which can be used as a drug delivery carrier for doxorubicin hydrochloride (DOX). In vitro drug release experiments showed that DOX loaded PHA/PVA composite membranes presented higher DOX release level in acidic environment than that in neutral environment since the degradation rate of the membranes under pH = 4 was significantly higher. And that, the DOX loaded membranes exhibited excellent performance for inhibiting the growth of Caco‐2 cells, which revealed the membranes' biomedical potential for chemotherapy of colon cancer. Meanwhile, in view of the good adhesion of the cells to the membranes, this novel mesoporous material was also perspective in tissue engineering.

Fibrous wearable and implantable bioelectronics

Fibrous wearable and implantable devices have emerged as a promising technology, offering a range of new solutions for minimally invasive monitoring of human health. Compared to traditional biomedical devices, fibers offer a possibility for a modular design compatible with large-scale manufacturing and a plethora of advantages including mechanical compliance, breathability, and biocompatibility. The new generation of fibrous biomedical devices can revolutionize easy-to-use and accessible health monitoring systems by serving as building blocks for most common wearables such as fabrics and clothes. Despite significant progress in the fabrication, materials, and application of fibrous biomedical devices, there is still a notable absence of a comprehensive and systematic review on the subject. This review paper provides an overview of recent advancements in the development of fibrous wearable and implantable electronics. We categorized these advancements into three main areas: manufacturing processes, platforms, and applications, outlining their respective merits and limitations. The paper concludes by discussing the outlook and challenges that lie ahead for fiber bioelectronics, providing a holistic view of its current stage of development.

Expanded Polytetrafluoroethylene Membranes for Vascular Stent Coating: Manufacturing, Biomedical and Surgical Applications, Innovations and Case Reports

Coated stents are defined as innovative stents surrounded by a thin polymer membrane based on polytetrafluoroethylene (PTFE)useful in the treatment of numerous vascular pathologies. Endovascular methodology involves the use of such devices to restore blood flow in small-, medium- and large-calibre arteries, both centrally and peripherally. These membranes cross the stent struts and act as a physical barrier to block the growth of intimal tissue in the lumen, preventing so-called intimal hyperplasia and late stent thrombosis. PTFE for vascular applications is known as expanded polytetrafluoroethylene (e-PTFE) and it can be rolled up to form a thin multilayer membrane expandable by 4 to 5 times its original diameter. This membrane plays an important role in initiating the restenotic process because wrapped graft stent could be used as the treatment option for trauma devices during emergency situations and to treat a number of pathological vascular disease. In this review, we will investigate the multidisciplinary techniques used for the production of e-PTFE membranes, the advantages and disadvantages of their use, the innovations and the results in biomedical and surgery field when used to cover graft stents.

PCL/Collagen/UA Composite Biomedical Dressing with Ordered Microfiberous Structure Fabricated by a 3D Near-Field Electrospinning Process

In this work, a functionalized polycaprolactone (PCL) composite fiber combining calf-type I collagen (CO) and natural drug usnic acid (UA) was prepared, in which UA was used as an antibacterial agent. Through 3D near-field electrospinning, the mixed solution was prepared into PCL/CO/UA composite fibers (PCUCF), which has a well-defined perfect arrangement structure. The influence of electrospinning process parameters on fiber diameter was investigated, the optimal electrospinning parameters were determined, and the electric field simulation was conducted to verify the optimal parameters. The addition of 20% collagen made the composite fiber have good hydrophilicity and water absorption property. In the presence of PCUCF, 1% UA content significantly inhibited the growth rate of Gram-positive and negative bacteria in the plate culture. The AC-PCUCF (after crosslinking PCUCF) prepared by crosslinking collagen with genipin showed stronger mechanical properties, water absorption property, thermal stability, and drug release performance. Cell proliferation experiments showed that PCUCF and AC-PCUCF had no cytotoxicity and could promote cell proliferation and adhesion. The results show that PCL/CO/UA composite fiber has potential application prospects in biomedical dressing.

Melt electrowriting of PLA, PCL, and composite PLA/PCL scaffolds for tissue engineering application

Fabrication of well-ordered and bio-mimetic scaffolds is one of the most important research lines in tissue engineering. Different techniques have been utilized to achieve this goal, however, each method has its own disadvantages. Recently, melt electrowriting (MEW) as a technique for fabrication of well-organized scaffolds has attracted the researchers' attention due to simultaneous use of principles of additive manufacturing and electrohydrodynamic phenomena. In previous research studies, polycaprolactone (PCL) has been mostly used in MEW process. PCL is a biocompatible polymer with characteristics that make it easy to fabricate well-arranged structures using MEW device. However, the mechanical properties of PCL are not favorable for applications like bone tissue engineering. Furthermore, it is of vital importance to demonstrate the capability of MEW technique for processing a broad range of polymers. To address aforementioned problems, in this study, three ten-layered box-structured well-ordered scaffolds, including neat PLA, neat PCL, and PLA/PCL composite are fabricated using an MEW device. Printing of the composite PLA/PCL scaffold using the MEW device is conducted in this study for the first time. The MEW device used in this study is a commercial fused deposition modeling (FDM) 3D printer which with some changes in its setup and configuration becomes prepared for being used as an MEW device. Since in most of previous studies, a setup has been designed and built for MEW process, the use of the FDM device can be considered as one of the novelties of this research. The printing parameters are adjusted in a way that scaffolds with nearly equal pore sizes in the range of 140 µm to 150 µm are fabricated. However, PCL fibers are mostly narrower (diameters in the range of 5 µm to 15 µm) than PLA fibers with diameters between 15 and 25 µm. Unlike the MEW process of PCL, accurate positioning of PLA fibers is difficult which can be due to higher viscosity of PLA melt compared to PCL melt. The printed composite PLA/PCL scaffold possesses a well-ordered box structure with improved mechanical properties and cell-scaffold interactions compared to both neat PLA and PCL scaffolds. Besides, the composite scaffold exhibits a higher swelling ratio than the neat PCL scaffold which can be related to the presence of less hydrophobic PLA fibers. This scaffold demonstrates an anisotropic behavior during uniaxial tensile test in which its Young's modulus, ultimate tensile stress, and strain to failure all depend on the direction of the applied tensile force. This anisotropy makes the composite PLA/PCL scaffold an exciting candidate for applications in heart tissue engineering. The results of in-vitro cell viability test using L929 mouse murine fibroblast and human umbilical vein endothelial (HUVEC) cells demonstrate that all of the printed scaffolds are biocompatible. In particular, the composite scaffold presents the highest cell viability value among the fabricated scaffolds. All in all, the composite PLA/PCL scaffold shows that it can be a promising substitution for neat PCL scaffold used in previous MEW studies.

Smart Film Actuators for Biomedical Applications

Taking inspiration from the extremely flexible motion abilities in natural organisms, soft actuators have emerged in the past few decades. Particularly, smart film actuators (SFAs) demonstrate unique superiority in easy fabrication, tailorable geometric configurations, and programmable 3D deformations. Thus, they are promising in many biomedical applications, such as soft robotics, tissue engineering, delivery system, and organ-on-a-chip. In this review, the latest achievements of SFAs applied in biomedical fields are summarized. The authors start by introducing the fabrication techniques of SFAs, then shift to the topology design of SFAs, followed by their material selections and distinct actuating mechanisms. After that, their biomedical applications are categorized in practical aspects. The challenges and prospects of this field are finally discussed. The authors believe that this review can boost the development of soft robotics, biomimetics, and human healthcare.

Recycling and Reutilizing Polymer Waste via Electrospun Micro/Nanofibers: A Review

The accumulation of plastic waste resulting from the increasing demand for non-degradable plastics has led to a global environmental crisis. The severe environmental and economic drawbacks of inefficient, expensive, and impractical traditional waste disposal methods, such as landfills, incineration, plastic recycling, and energy production, limit the expansion of their applications to solving the plastic waste problem. Finding novel ways to manage the large amount of disposed plastic waste is urgent. Until now, one of the most valuable strategies for the handling of plastic waste has been to reutilize the waste as raw material for the preparation of functional and high-value products. Electrospun micro/nanofibers have drawn much attention in recent years due to their advantages of small diameter, large specific area, and excellent physicochemical features. Thus, electrospinning recycled plastic waste into micro/nanofibers creates diverse opportunities to deal with the environmental issue caused by the growing accumulation of plastic waste. This paper presents a review of recycling and reutilizing polymer waste via electrospinning. Firstly, the advantages of the electrospinning approach to recycling plastic waste are summarized. Then, the studies of electrospun recycled plastic waste are concluded. Finally, the challenges and future perspectives of electrospun recycled plastic waste are provided. In conclusion, this paper aims to provide a comprehensive overview of electrospun recycled plastic waste for researchers to develop further studies.

Polymer-Based Constructs for Flexor Tendon Repair: A Review

A flexor tendon injury is acquired fast and is common for athletes, construction workers, and military personnel among others, treated in the emergency department. However, the healing of injured flexor tendons is stretched over a long period of up to 12 weeks, therefore, remaining a significant clinical problem. Postoperative complications, arising after traditional tendon repair strategies, include adhesion and tendon scar tissue formation, insufficient mechanical strength for early active mobilization, and infections. Various researchers have tried to develop innovative strategies for developing a polymer-based construct that minimalizes these postoperative complications, yet none are routinely used in clinical practice. Understanding the role such constructs play in tendon repair should enable a more targeted approach. This review mainly describes the polymer-based constructs that show promising results in solving these complications, in the hope that one day these will be used as a routine practice in flexor tendon repair, increasing the well-being of the patients. In addition, the review also focuses on the incorporation of active compounds in these constructs, to provide an enhanced healing environment for the flexor tendon.

Near-Field Electrospinning: Crucial Parameters, Challenges, and Applications

Near-field electrospinning (NFES) is a micro- or nanofiber production technology based on jetting molten polymer or polymer solution. Thanks to the programmable collector and nozzle movement, it can generate designed patterns in the presence of an electric field. Despite a few shortcomings of NFES, its high resolution, simplicity, precision, high throughput, reproducibility, and low costs have convinced researchers to employ it for various purposes. Furthermore, as the paradigm of fiber-based structures shifts from random textures toward delicate designs, NFES can bridge the gap between existing inefficient processes and aspired technologies for precise patterning. NFES facilitates the production of ultrafine nanofibers because it can be used to fabricate them in every laboratory. These robust fibers are convenient tools for small and additive manufacturing. As such, NFES is considered a potent additive fabrication technology that facilitates the production of complicated patterns as well. It is suggested that near-field electrospun fibers exhibit outstanding results in various applications, owing to their precise and controllable positioning. Meanwhile, the ongoing development of NFES has yet to reach its climax, making it attractive for further research. In this review, the basic principles of NFES, derivatives, limitations, and applications in nanomanufacturing, tissue engineering, microscale electronics, biosensors, and optics are presented.

Bamboo Charcoal/Poly(L-lactide) Fiber Webs Prepared Using Laser-Heated Melt Electrospinning

Although several studies have reported that the addition of bamboo charcoal (BC) to polylactide (PLA) enhances the properties of PLA, to date, no study has been reported on the fabrication of ultrafine BC/poly(L-lactide) (PLLA) webs via electrospinning. Therefore, ultrafine fiber webs of PLLA and BC/PLLA were prepared using PLLA and BC/PLLA raw fibers via a novel laser electrospinning method. Ultrafine PLLA and BC/PLLA fibers with average diameters of approximately 1 μm and coefficients of variation of 13–23 and 20–46% were obtained. Via wide-angle X-ray diffraction (WAXD) analysis, highly oriented crystals were detected in the raw fibers; however, WAXD patterns of both PLLA and BC/PLLA webs implied an amorphous structure of PLLA. Polarizing microscopy images revealed that the webs comprised ultrafine fibers with uniform diameters and wide variations in birefringence. Temperature-modulated differential scanning calorimetry measurements indicated that the degree of order of the crystals in the fibers was lower and the molecules in the fibers had higher mobilities than those in the raw fibers. Transmittance of BC/PLLA webs with an area density of 2.6 mg/cm² suggested that the addition of BC improved UV-shielding efficiencies.