Recent Progress in Electrospun Nanofibres: Reinforcement Effect and Mechanical Performance

Composite materials are becoming increasingly important as structural materials for aeronautical and space engineering, naval, automotive, and civil engineering, sporting goods, and other consumer products. Fiber-based reinforcement represents one of the most effective manufacturing strategies for enhancing the mechanical strength and other properties of composite materials. Electrospinning has gained widespread interest in the last two decades because of its ability to fabricate continuous ultrafine nanofibers with unique characteristics. The impact of electrospinning on fiber synthesis and processing, characterization, and applications in drug delivery, nanofiltration, tissue scaffolding, and electronics has been extensively studied in the past. In this article, the authors have focused on a comprehensive review of the mechanical performance and properties of electrospun nanofibers as potential reinforcements as well as their advanced nanocomposites

Sustainable Robots 4D Printing

Nature frequently serves as an inspiration for modern robotics innovations that emphasize secure human–machine interaction. However, the advantages of increased automation and digital technology integration conflict with the global environmental objectives. Accordingly, biodegradable soft robots have been proposed for a range of intelligent applications. Biodegradability provides soft robotics with an extraordinary functional advantage for operations involving intelligent shape transformation in response to external stimuli such as heat, pH, and light. Soft robot fabrication using conventional manufacturing techniques is inflexible, time-consuming, and labor-intensive. Recent advances in 3D and 4D printing of soft materials and multi-materials have become the key to enabling the direct manufacture of soft robotics with complex designs and functions. This review comprises a detailed survey of 3D and 4D printing advances in biodegradable soft sensors and actuators (BSSA), which serve as the most prominent parts of each robotic system. In addition, a concise overview of biodegradable materials for the fabrication of 3D-printed flexible devices with medical along with industrial applications is provided. A complete summary of current additive manufacturing techniques for BSSA is discussed in depth. Moreover, the concept of biodegradable 4D-printed soft actuators and sensors and biohybrid soft robots is reviewed

Design, Synthesis and Study of Thermomechanically Active Polymer Networks Based on Latent Crosslinking of Semicrystalline Polymers

Demand has arisen rapidly for smart materials in the world of the need to develop and understand new functional products like plastics, rubber, adhesives, fibers, and coatings. Such products are essentially composed of polymers, large molecules of high molecular weight with homogeneous or various repeating units, which researchers term “macromolecules” that engender specific structural, morphological, and physical and mechanical properties. Those polymers with the capacity to change their configuration in accordance with environmental alteration are specifically referred to as shape memory polymers (SMPs), attracting much interest of study both academically and industrially. Herein, this dissertation aims at design, fabrication, and characterization of novel crosslinkable semicrystalline polymeric materials utilizing different techniques and mechanisms in order to explore their special thermomechanical features as well as the possibilities for potential industrial application based on shape memory (SM) effects. Key aspects include use of modern polymer synthesis to tailor thermal and shape memory properties and the adoption of electrospinning processing techniques to form continuous, fine fibers that allow unique molecular modifications, study of enzymatic degradation behavior involving physical form and microstructural state, and unprecedented approaches of making new kinds of shape memory assisted self-healing (SMASH) materials and thermal-responsive self-reversible actuators that require no human intervention. In the following is described the dissertation scope and organization. Chapter 1 goes over background relating to material science within the scope of SM material, self-healing (SH) material, and actuators. Chapter 2 outlines research conducted to achieve new compositions of matter and post-synthesis process, along with supporting characterization for the development of novel SMP materials with featuring tunable reversible actuation capability under ambient stimulus. We prepared a family of crosslinkable (unsaturated), semicrystalline cyclooctene (CO)-based copolymers with varying second monomer and composition via ring opening metathesis polymerization (ROMP). The unsaturation enables covalent crosslinking of polymer chains, in the presence of select thermal initiator through compression molding, allowing subsequent formation of a temperature-responsive network that shows a reversible two-way shape memory (2WSM) effect, indicative of crystallization-induced elongation upon cooling and melting-induced contraction upon heating when a constant, external stress is applied. Molecular, thermomechanical, and SM experiments were performed to investigate and tune the reversible actuation of aforementioned copolymers for the purpose of yielding quantitative guidelines for tailoring material and actuation performance through variations in composition and process. Chapter 3 seeks a latent-crosslinkable, mechanically flexible, fully thermoplastic shape memory polymer. Towards this end, we have developed a simple but effective macromolecular design that includes pendent crosslinking sites via the chain extender of a polyurethane architecture bearing semicrystalline poly(ε-caprolactone) (PCL) soft segment. This new composition was used to prepare fibrous mats by electrospinning and films by solvent casting, each containing thermal initiators for chemical crosslinking. Relevant to medical applications, in vitro enzymatic degradation experiments were carried out to understand the effect of crosslinking state and crystalline structure on degradation behavior of the materials. Chapter 4 builds upon the results of Chapter 3, reporting on the design, fabrication and characterization of a novel, electrospun SMASH polymer blend that incorporates the aforementioned latent-crosslinkable polyurethane. This unique blend system has been unprecedentedly developed by employing a solution in which crosslinkable polyurethane and linear polyurethane are mixed homogeneously for electrospinning. After preparing a family of blends with varying compositions, comprehensive characterizations and various healing tests were done to determine optimal healing performance. Further, the effect of different damage types and molecular anisotropy (nanofibers aligned in high speeds during electrospinning process) were studied for their effect on healing performance. Chapter 5 continues along the line of Chapter 3, presenting the fabrication and testing of novel, electrospun SMP composites that were designed to exploit molecular and geometric anisotropy in reversible actuation under external stress-free condition upon change in ambient temperature. More specifically, the SMP composites consist of two electrospinnable constituents, one being the aforementioned latent crosslinkable polyurethane that serves to shape fixing and recovery (SM properties), and the other being a thermoplastic elastomer known as Pellethane that provides the internal stress field needed for 2WSM to occur. Multiple designs were developed and investigated in this chapter, in particular, including uniaxial actuator, bending actuator, and twisting actuator along with their bench demonstration of self-reversible actuation. Chapter 6 discusses the overall dissertation conclusions, followed descriptions of suggestions for future work, some of which are sub-sectioned at the end of this dissertation

New trends in 4D printing: A critical review

In a variety of industries, Additive Manufacturing has revolutionized the whole design-fabrication cycle. Traditional 3D printing is typically employed to produce static components, which are not able to fulfill the dynamic structures requirements and relevant applications such as soft grippers, self-assembly systems, and smart actuators. To address this limitation, an innovative technology has emerged and is called “4D printing”. It processes smart materials by using 3D printing for fabricating smart structures that can be reconfigured by applying different inputs such as heat, humidity, magnetic, electricity, light etc. At present, 4D printing is still a growing technology and it presents numerous challenges regarding materials, design, simulation, fabrication processes, applied strategies and reversibility. In this work a critical review about 4D printing technologies, materials and applications is discussed

Feasibility of Thermoplastic Extrusion Welding as a Joining Method For Vacuum-Assisted Additively Manufactured Tooling

In recent years, additive manufacturing (AM) has been successfully utilized for the production of large-scale composite tooling. Within these endeavors, however, limited research has focused on joining methods between printed sections. This work evaluates the feasibility of thermoplastic extrusion welding as a joining method for additively manufactured tooling structures. This joining method was assessed based on industry specifications of conventional thermoset tooling for wind blade manufacturing utilizing the vacuum-assisted resin transfer molding (VARTM) process. The specifications include requirements for the mechanical strength, vacuum integrity, roughness, and hardness of the tool surface. The feasibility of this welded polymer joint was demonstrated through subscale testing of 1” thick, welded, AM high-impact polystyrene (HIPS) plates. It was found that thermoplastic extrusion welds within AM components can maintain vacuum integrity at 20℃ with proper surface preparation and without a surface coating. This met the industry vacuum leakage specification of 10 millibar over 30 minutes with an average loss of 6.61 mbar over 30 minutes through the welded AM plate and bag system. Although beyond the industry specification, the vacuum leakage was further tested to evaluate performance at an infusion temperature of 80℃. At elevated temperature, the joint and plate lost approximately 26 mbar over 30 minutes. The surface finish was compared with hardness and roughness testing of the welded and machined AM surfaces, showing a decrease in hardness and roughness in the surface of the weld at both temperatures. Standardized ASTM mechanical testing of welded specimens showed an average comparative tensile strength of 80% of the base AM HIPS material. With the addition of undersurface reinforcement within the mold and a surface coating, extrusion welding shows promise for joining large-scale AM tool sections in a manufacturing environment

Using feedback control to actively regulate the healing rate of a self-healing process subjected to low cycle dynamic stress

Intrinsic and extrinsic self-healing approaches through which materials can be healed generally suffer from several problems. One key problem is that to ensure effective healing and to minimise the propagation of a fault, the healing rate needs to be matched to the damage rate. This requirement is usually not met with passive approaches. An alternative to passive healing is active self-healing, whereby the healing mechanism and in particular the healing rate, is controlled in the face of uncertainty and varying conditions. Active self-healing takes advantage of sensing and added external energy to achieve a desired healing rate. To demonstrate active self-healing, an electrochemical material based on the principles of piezoelectricity and electrolysis is modelled and adaptive feedback control is implemented. The adaptive feedback control compensates for the insufficient piezo-induced voltage and guarantees a response that meets the desired healing rate. Importantly, fault propagation can be eliminated or minimised by attaining a match between the healing and damage rate quicker than can be achieved with the equivalent passive system. The desired healing rate is a function of the fault propagation and is assumed known in this paper, but can be estimated in practice through established prognostic techniques

Design and characterization of thermally-induced shape memory polymers

Les polymères à mémoire de forme (SMP) sont des matériaux intelligents qui peuvent récupérer leur forme permanente à partir d'une forme temporaire lorsqu'ils sont exposés à un stimulus externe. Ils ont attiré beaucoup d’attention en raison de leurs propriétés uniques. Par rapport aux SMPs doubles, les SMPs complexes ayant des mémoires triples, multiples ou bidirectionnelles sont plus attirants en raison de leurs propriétés distinctes. Les SMPs multiples peuvent mémoriser trois formes ou plus, tandis que les SMPs bidirectionnels peuvent basculer entre deux formes distinctes. L'objectif principal de cette thèse est de concevoir des SMPs complexes par des méthodes simples pour des applications particulièrement en biomédecine. Deux systèmes SMP complexes biodégradables à base de bio-composés ou de monomères synthétiques ont été synthétisés. Pour les SMPs de mémorises multiples, nous avons synthétisé une série de copolymères statistiques avec des groupes pendants d'acide cholique en utilisant une méthode de polymérisation radicalaire simple. Ces copolymères ont une température de transition vitreuse (Tg) réglable, selon le ratio de comonomères, qui montrent à la fois des effets mémoires à double et triple états (PME). Les rapports entre la fixité et la récupération de la mémoire de forme double ou triple peuvent être améliorés par l'incorporation d'un groupe cinnamate dans les copolymères afin de permettre la photo-réticulation du polymère. Les polymères réticulés présentent des rapports de récupération améliorés pour la mémoire de forme double et triple et présentent même des PME quadruples. Le degré de réticulation affecte également les propriétés de mémoire de forme. Les meilleurs comportements de mémoire de forme dans ce genre de polymères ont été obtenus avec une réticulation de 2,2% molaire des monomères. Les SMP doubles, triples et multiples sont généralement SMP unidirectionnels et leurs transformations de formes sont irréversibles. Les SMP réversibles bidirectionnels (2W-SMP) peuvent basculer automatiquement entre deux formes distinctes lorsqu'elles sont exposées à deux stimuli externes différents. Cependant, la température d'actionnement (TA) de 2W-SMP est une valeur fixe telle que déterminée par la température de fusion (Tm) du segment actuateur du réseau polymère. Dans cette étude, une série de copolymères statistiques contenant ε-caprolactone (CL) et ω-pentadécalactone (PDL) ont été synthétisés par polymérisation par ouverture de cycle avec un catalyseur de lipase B de Candida antarctica (CALB). Les segments polymères de ces deux monomères sont co-cristallisables et la Tm des copolymères peut être adapte en ajustant le rapport molaire des comonomères. Après irradiation pour la réticulation de thiol-ène, le réseau de polymères a montré des 2W-PME dans des conditions avec ou sans tension, avec un changement de forme absolu de 13,2%. Des mouvements réversibles comme flexion-extension et enroulement-déroulement ont été observés pour le réseau de polymère. La TA de 2W-SMP sous conditions sans stress peut être facilement contrôlée en sélectionnant un ou deux prépolymères comme segments du réseau polymère. Le changement de l’élongation absolue des 2W-SMP est augmenté sous les conditions avec ou sans stress, mais le changement d’élongation relative est réduit avec l'augmentation de tension sous condition de stress. L'évolution de la microstructure de 2W-SMPs sous condition sans stress a également été conçu. Les 2W-SMPs sont souvent actionnés thermiquement, mais le chauffage indirect est souhaitable pour certaines applications. Une série de 2W-SMPs composites actionnés par la lumière a ainsi été préparée par l’incorporation de nanosphères de PDA dans les réseaux polymères contenant le CL et le PDL. Les nanosphères de PDA ont un effet photothermique très prononcé qui peut convertir l'énergie lumineuse en chaleur. La température de l'échantillon augmente selon l’intensité lumineuse et du contenu en nanosphères de PDA. Ces composites polymères présentent d'excellentes 2W-PME sensibles à la lumière sous condition sans stress avec un changement d'angle réversible de 45° lorsque la lumière est allumée puis éteinte. L'échantillon avec un contenu plus élevé en nanosphères de PDA ou sous une intensité lumineuse plus forte cause une ouverture d’angle plus rapide. Un micro-robot qui peut marcher sur une piste en dents de scie lorsque la lumière est allumée et éteinte a également été conçu arec un composite 2W-SMPs.Shape memory polymers (SMPs) are smart materials that can recover the permanent shape from a temporary shape when exposed to an external stimulus. They have drawn much attention due to their unique properties. In comparison to dual SMPs, complex SMPs with triple, multiple and two-way shape memories are more attractive due to their versatile properties. Triple and multiple SMPs could memorize three and more shapes, while two-way SMPs may automatically switch forth and back between two distinct shapes. The main purpose of this thesis is to design simple complex SMPs for their applications especially in biomedicine. Two biodegradable complex SMP systems based on bio-compounds or synthetic monomers have been synthesized. For multiple SMPs, we synthesized a series of random copolymers with pendent cholic acid groups by the use of a simple free radical polymerization method. Such copolymers have a glass transition temperature (Tg) range tunable by varying the monomers ratios, allowing the dual and triple shape memory effects (SMEs). The fixity and recovery ratios of dual and triple shape memory may be further improved by the incorporation of cinnamate groups into copolymers to enable a photo-cross-linking of the terpolymer. The cross-linked polymers show much improved recovery ratios for both dual and triple shape memory and even exhibit quadruple SMEs. The degree of cross-linking also affects the shape memory properties. The best shape memory behaviors were obtained with a 2.2 mol% cross-linking of the total monomers in the terpolymer. Dual, triple and multiple SMPs are usually one-way SMPs, and their shape transformations are irreversible. Two-way reversible SMPs (2W-SMPs) may switch between two different shapes automatically when they are exposed to two reverse external stimuli. However, the actuation temperature (TA) of 2W-SMP is at a fixed value as it is determined by the melting temperature (Tm) of the actuator segment of the polymer network. In this study, a series of random copolymers containing ε-caprolactone (CL) and ω-pentadecalactone (PDL) were synthesized through ring opening polymerization catalyzed by Candida antarctica lipase B (CALB). The polymers segments made of these co-monomers are co-crystallizable and the Tms of the copolymers may be changed by adjusting the molar ratio of the co-monomers. Upon light irradiation which induces thiol-ene cross-linking, the polymer network showed 2W-SMEs under both stress-free and stress conditions with the absolute shape change up to 13.2%. Bending-unbending and coiling-uncoiling reversible shape motions were observed for the polymer network. The TA of the 2W-SMP under stress-free conditions may be tuned by simply selecting one or two different prepolymers as segments in the polymer network. The absolute strain change of 2W-SMPs increased under both stress-free and stress conditions, but the relative strain change reduced with increasing tensile stress under stress conditions. The evolution of the microstructure of 2W-SMPs under stress-free conditions was also elaborated. The 2W-SMPs are often actuated thermally, but indirect heating is desirable for certain applications because of its convenience. Thus, a series of light-actuated 2W-SMP composites were prepared via the incorporation of tiny amounts of polydopamine (PDA) nanospheres into polymer networks made of CL and PDL. PDA nanospheres have a strong photothermal effect which may convert energy of light into heat. The temperature change of the sample is larger with higher light intensity and higher content of PDA nanospheres. Such polymer composites exhibited excellent light-responsive 2W-SMEs under stress-free conditions with a reversible angle change of 45° when the light was turned on and off. The sample with a higher content of PDA nanospheres or under a stronger light led to a faster angle opening. A micro-robot is also designed and made of the 2W-SMP composite, which may walk on a home-made track with right triangle sawteeth when the light is turned on and off

Overview on lightweight multifunctional materials

Lightweight multifunctional materials represent an increasing field in materials science and engineering based on their technological applications in a wide variety of areas ranging from sensors and actuators, materials for structural and environmental applications, energy generation and storage, or biomedicine, among others. This chapter presents an overview on the main types, preparation techniques and applications of the most relevant lightweight multifunctional materials, as well as on relevant materials to be applied and/or implemented in lightweight structures.FCT (Fundação para a Ciência e Tecnologia) for financial support under the framework of Strategic Funding grants UID/CTM/50025/2019, UID/FIS/04650/2019; and supported by FEDER funds through the COMPETE 2020 Programme under the project number PTDC/FIS-MAC/28157/2017 and POCI-01-0145- FEDER-007688. The authors also thank the FCT for financial support under grants SFRH/BPD/112547/2015 (C.M.C.) and SFRH/BPD/110914/2015 (P. C.). Financial support from the Basque Government under the ELKARTEK, HAZITEK and PIBA (PIBA-2018-06

Design and Fabrication of Soft 3D Printed Actuators: Expanding Soft Robotics Applications

Soft pneumatic actuators are ideal for soft robotic applications due to their innate compliance and high power-weight ratios. Presently, the majority of soft pneumatic actuators are used to create bending motions, with very few able to produce significant linear movements. Fewer can actively produce strains in multiple directions. The further development of these actuators is limited by their fabrication methods, specifically the lack of suitable stretchable materials for 3D printing. In this thesis, a new highly elastic resin for digital light projection 3D printers, designated ElastAMBER, is developed and evaluated, which shows improvements over previously synthesised elastic resins. It is prepared from a di-functional polyether urethane acrylate oligomer and a blend of two different diluent monomers. ElastAMBER exhibits a viscosity of 1000 mPa.s at 40 °C, allowing easy printing at near room temperatures. The 3D-printed components present an elastomeric behaviour with a maximum extension ratio of 4.02 ± 0.06, an ultimate tensile strength of (1.23 ± 0.09) MPa, low hysteresis, and negligible viscoelastic relaxation

Self-healing mechanisms for 3D-printed polymeric structures: from lab to reality

Existing self-healing mechanisms are still very far from full-scale implementation, and most published work has only demonstrated damage cure at the laboratory level. Their rheological nature makes the mechanisms for damage cure difficult to implement, as the component or structure is expected to continue performing its function. In most cases, a molecular bond level chemical reaction is required for complete healing with external stimulations such as heating, light and temperature change. Such requirements of external stimulations and reactions make the existing self-healing mechanism almost impossible to implement in 3D printed products, particularly in critical applications. In this paper, a conceptual description of the self-healing phenomenon in polymeric structures is provided. This is followed by how the concept of self-healing is motivated by the observation of nature. Next, the requirements of self-healing in modern polymeric structures and components are described. The existing self-healing mechanisms for 3D printed polymeric structures are also detailed, with a special emphasis on their working principles and advantages of the self-healing mechanism. A critical discussion on the challenges and limitations in the existing working principles is provided at the end. A novel self-healing idea is also proposed. Its ability to address current challenges is assessed in the conclusion