Three-dimensional microvascular fiber-reinforced composites

Synthesis and characterisation of microcapsules for self-healing dental resin composites

AIM

The purpose of this study was to i) synthesise TEGDMA-DHEPT microcapsules in a laboratory setting; ii) characterise the resultant microcapsules for quality measures.

MATERIALS & METHODS

Microcapsules were prepared by in situ polymerization of PUF shells. Microcapsules characterisation include size analysis, optical and SEM microscopy to measure the diameter and analyse the morphology of PUF microcapsules. FT-IR spectrometer evaluated microcapsules and benzyl peroxide catalyst polymerization independently.

RESULT

Average diameter of TEGDMA-DHEPT microcapsules was 120 ± 45 μm (n: 100). SEM imaging of the capsular shell revealed a smooth outer surface with deposits of PUF nanoparticles that facilitate resin matrix retention to the microcapsules upon composite fracture. FT-IR spectra showed that microcapsules crushed with BPO catalyst had degree of conversion reached to 60.3%.

CONCLUSION

TEGDMA-DHEPT microcapsules were synthesised according to the selected parameters. The synthesised microcapsules have a self-healing potential when embedded into dental resin composite as will be demonstrated in our future work.

Review on Frontal Polymerization Behavior for Thermosetting Resins: Materials, Modeling and Application

The traditional curing methods for thermosetting resins are energy-inefficient and environmentally unfriendly. Frontal polymerization (FP) is a self-sustaining process relying on the exothermic heat of polymerization. During FP, the external energy input (such as UV light input or heating) is only required at the initial stage to trigger a localized reaction front. FP is regarded as the rapid and energy-efficient manufacturing of polymers. The precise control of FP is essential for several manufacturing technologies, such as 3D printing, depending on the materials and the coupling of thermal transfer and polymerization. In this review, recent progress on the materials, modeling, and application of FP for thermosetting resins are presented. First, the effects of resin formulations and mixed fillers on FP behavior are discussed. Then, the basic mathematical model and reaction-thermal transfer model of FP are introduced. After that, recent developments in FP-based manufacturing applications are introduced in detail. Finally, this review outlines a roadmap for future research in this field.

Self-Healing Polymeric Soft Actuators

Natural evolution has provided multicellular organisms with sophisticated functionalities and repair mechanisms for surviving and preserve their functions after an injury and/or infection. In this context, biological systems have inspired material scientists over decades to design and fabricate both self-healing polymeric materials and soft actuators with remarkable performance. The latter are capable of modifying their shape in response to environmental changes, such as temperature, pH, light, electrical/magnetic field, chemical additives, etc. In this review, we focus on the fusion of both types of materials, affording new systems with the potential to revolutionize almost every aspect of our modern life, from healthcare to environmental remediation and energy. The integration of stimuli-triggered self-healing properties into polymeric soft actuators endow environmental friendliness, cost-saving, enhanced safety, and lifespan of functional materials. We discuss the details of the most remarkable examples of self-healing soft actuators that display a macroscopic movement under specific stimuli. The discussion includes key experimental data, potential limitations, and mechanistic insights. Finally, we include a general table providing at first glance information about the nature of the external stimuli, conditions for self-healing and actuation, key information about the driving forces behind both phenomena, and the most important features of the achieved movement.

Self-Healing of Polymers and Polymer Composites

This review is devoted to the description of methods for the self-healing of polymers, polymer composites, and coatings. The self-healing of damages that occur during the operation of the corresponding structures makes it possible to extend the service life of the latter, and in this case, the problem of saving non-renewable resources is simultaneously solved. Two strategies are considered: (a) creating reversible crosslinks in the thermoplastic and (b) introducing a healing agent into cracks. Bond exchange reactions in network polymers (a) proceed as a dissociative process, in which crosslinks are split into their constituent reactive fragments with subsequent regeneration, or as an associative process, the limiting stage of which is the interaction of the reactive end group and the crosslink. The latter process is implemented in vitrimers. Strategy (b) is associated with the use of containers (hollow glass fibers, capsules, microvessels) that burst under the action of a crack. Particular attention is paid to self-healing processes in metallopolymer systems.

Prolonged in situ self-healing in structural composites via thermo-reversible entanglement

Natural processes continuously degrade a material's performance throughout its life cycle. An emerging class of synthetic self-healing polymers and composites possess property-retaining functions with the promise of longer lifetimes. But sustained in-service repair of structural fiber-reinforced composites remains unfulfilled due to material heterogeneity and thermodynamic barriers in commonly cross-linked polymer-matrix constituents. Overcoming these inherent challenges for mechanical self-recovery is vital to extend in-service operation and attain widespread adoption of such bioinspired structural materials. Here we transcend existing obstacles and report a fiber-composite capable of minute-scale and prolonged in situ healing - 100 cycles: an order of magnitude higher than prior studies. By 3D printing a mendable thermoplastic onto woven glass/carbon fiber reinforcement and co-laminating with electrically resistive heater interlayers, we achieve in situ thermal remending of internal delamination via dynamic bond re-association. Full fracture recovery occurs below the glass-transition temperature of the thermoset epoxy-matrix composite, thus preserving stiffness during and after repair. A discovery of chemically driven improvement in thermal remending of glass- over carbon-fiber composites is also revealed. The marked lifetime extension offered by this self-healing strategy mitigates costly maintenance, facilitates repair of difficult-to-access structures (e.g., wind-turbine blades), and reduces part replacement, thereby benefiting economy and environment.

Strategies to design extrinsic stimuli-responsive dental polymers capable of autorepairing

Objectives

For many years, the requirements for dental polymers were limited to inertially filling the cavity and restoring form, function, and esthetics. Inorganic filler systems were widely enhanced to maximize the mechanical properties and optimize finishing and polishing procedures. The development of alternative photoinitiator systems also improved the carbon-carbon double bond conversion, increasing biocompatibility, wear, and stain resistance. However, despite laudable progress, the clinical life span of dental restorations is still limited, and their replacement is the most common procedure in dental offices worldwide. In the last few years, the development of materials with the potential to adapt to physiological stimuli has emerged as a key step to elevating dental polymers to a higher excellence level. In this context, using polymeric networks with self-healing properties that allow for the control of the propagation of microcracks is an appealing strategy to boost the lifetime of dental restorations. This review aims to report the current state-of-the-art of extrinsic self-healing dental polymers and provide insights to open new avenues for further developments. General classification of the self-healing polymeric systems focusing on the current extrinsic strategies used to inhibit microcracks propagation in dental polymers and recover their structural integrity and toughness are presented.

Search Strategy

An electronic search was perfomed using PubMed, Google Scholar, and Scopus databases. Only studies published in English on extrinsic self-healing polymeric systems were included.

Overall Conclusions

Self-healing materials are still in their infancy in dentistry, and the future possibilities are almost limitless. Although the mouth is a unique environment and the restorative materials have to survive chemical, physical, and mechanical challenges, which limits the use of some strategies that might compromise their physicochemical performance, there are countless untapped opportunities to overcome the challenges of the current systems and advance the field.

Solid-Liquid Host-Guest Composites: The Marriage of Porous Solids and Functional Liquids

Composite materials can provide remarkable improvements over the individual constituents. Especially, with a liquid component introduced into a solid porous host, solid-liquid host-guest composites have recently come to the forefront with exceptional functions that promise them for a wealth of applications. Combining the unprecedented dynamic, transparent, omniphobic, self-healing, diffusive and adaptive nature of functional liquid with inherent solid host's property, solid-liquid host-guest composites can realize the ease of fabrication, long-term stability, and a broad spectrum of enhanced properties, which cannot be fully met by conventional solid-solid composites or liquid-liquid composites. This review presents the state-of-the-art progress in solid-liquid host-guest composites. Initially, the concept, classification, design strategy, as well as fabrication methods as a path forward to develop the composites are unraveled, and further it is elaborated on how the functionality of porous solid and functional liquid can be harnessed to create composites with a broad range of unique properties, especially, the optical, thermal, electric, mechanical, sorption, and separation properties. With these fascinating properties, a myriad of emerging applications such as optical devices, thermal management, electromagnetic-interference shielding, soft electronics, gas capture and release, and multiphase separations are touched upon, inspiring more frontier researches in materials science, interfacial chemistry, membrane science, engineering, and multidisciplinary. Finally, this review provides the perspective on the future directions of solid-liquid host-guest composites and assesses the challenges and opportunities ahead.

Clip Chemistry: Diverse (Bio)(macro)molecular and Material Function through Breaking Covalent Bonds

In the two decades since the introduction of the "click chemistry" concept, the toolbox of "click reactions" has continually expanded, enabling chemists, materials scientists, and biologists to rapidly and selectively build complexity for their applications of interest. Similarly, selective and efficient covalent bond breaking reactions have provided and will continue to provide transformative advances. Here, we review key examples and applications of efficient, selective covalent bond cleavage reactions, which we refer to herein as "clip reactions." The strategic application of clip reactions offers opportunities to tailor the compositions and structures of complex (bio)(macro)molecular systems with exquisite control. Working in concert, click chemistry and clip chemistry offer scientists and engineers powerful methods to address next-generation challenges across the chemical sciences.

X-ray computed tomography evaluations of additive manufactured multimaterial composites

Additive Manufacturing (AM) often produces complex engineered structures by precisely distributing materials in a layer-by-layer fashion. Multimaterial AM is a particularly flexible technique able to combine a range of hard and soft materials to produce designed composites. Critically, the design of AM multimaterial structures requires the development of precise three-dimensional (3D) computed aided design (CAD) files. While such digital design is heavily used, techniques able to validate the physically manufactured composite against the digital design from which it is generated are lacking for AM, especially as any evaluations must be able to distinguish material variation across the 3D space. Nowadays, there is a growing interest in volumetric tools that can provide topological information hidden by the surface of shaped materials. So far, technologies such as Optical microscopy (OM), Scanning Electron Microscopy (SEM), and Coordinate Measuring Machine (CMM) have paved the way into the metrology field to measure the external geometry of physical objects. Currently, alongside conventional metrology tools, X-ray computed tomography (XCT) is emerging to measure the subsurface of the objects but maintaining the integrity of the probed samples. Thereby, the volumetric nature of the XCT investigations and its associated imaging techniques, ensure 3D quantitative measurements comparable to the output data from 2D metrology tools, but above all, supply the missing subsurface description for an exhaustive metrology study. The reward associated with XCT applied to multimaterial AM is a map reflecting the fabricated distribution of materials following CAD, with the benefits of better understanding the mechanical interplay within phases, hence, describing the hidden processes as well as the changes in phases due to a range of mechanical or chemical phenomena. In this study, a nondestructive approach using X-ray computed tomography (XCT) is used to fully evaluate the 3D distribution of multimaterials from an AM process. Specifically, two diverse hard and soft materials are alternatively produced in the form of a fibre embedded in a matrix via ink-jet printing. XCT coupled with imaging evaluation were able to distinguish between the differing materials and, importantly, to demonstrate a reduction in the expected fabricated volumes when compared to the respective CAD designs. LAY DESCRIPTION: Additive Manufacturing (AM) has recently become important in producing complex engineered structures. Using 3D CAD files and/or reconstructed data sets from imaging, hard and soft materials are manufactured independently or in combination, according to geometrical features and shapes in the input data. However, the evaluation of the resultant manufactured parts in comparison with the original 3D drawing is currently lacking. In this sense, X-ray computed tomography (XCT) provides an important metrology tool for mono and multimaterial AM. In this work a volumetric metrology investigation is proposed using higher resolution XCT to provide 3D information comparable to that of the 3D CAD drawings. A commercial high-resolution multijetting material printer (ProJet 5500X, 3D Systems, USA) is used to manufacture single fibre composites, through a complementary deposition of photo sensible polymers. Hard and soft plastics are produced using a UV curable step, resulting in materials of similar attenuation under an X-ray probe. A critical aim of the evaluations is the potential for XCT to distinguish between different UV curable 3D printing materials.

Rapid synchronized fabrication of vascularized thermosets and composites

Bioinspired vascular networks transport heat and mass in hydrogels, microfluidic devices, self-healing and self-cooling structures, filters, and flow batteries. Lengthy, multistep fabrication processes involving solvents, external heat, and vacuum hinder large-scale application of vascular networks in structural materials. Here, we report the rapid (seconds to minutes), scalable, and synchronized fabrication of vascular thermosets and fiber-reinforced composites under ambient conditions. The exothermic frontal polymerization (FP) of a liquid or gelled resin facilitates coordinated depolymerization of an embedded sacrificial template to create host structures with high-fidelity interconnected microchannels. The chemical energy released during matrix polymerization eliminates the need for a sustained external heat source and greatly reduces external energy consumption for processing. Programming the rate of depolymerization of the sacrificial thermoplastic to match the kinetics of FP has the potential to significantly expedite the fabrication of vascular structures with extended lifetimes, microreactors, and imaging phantoms for understanding capillary flow in biological systems.

Bioinspired vascular networks transport heat and mass in multifunctional materials but lengthy multistep fabrication processes hinder large-scale application of structural vascular materials. Here, the authors report rapid, scalable, and synchronized fabrication of vascular thermosets and fiberreinforced composites under ambient conditions.

Extremely Tough, Puncture-Resistant, Transparent, and Photoluminescent Polyurethane Elastomers for Crack Self-Diagnose and Healing Tracking

Ensuring material performance reliability and lifetime is crucial for practical operations. Small cracks on the material surface are often detrimental to its safe operation. This study describes the development of a hydrogen bond-rich puncture-resistant polyurethane elastomer with supertoughness. The as-prepared polyurethane transparent films feature high tensile break strength (57.4 MPa) and great toughness (228 MJ m^-3). Additionally, a facile, low-cost, crack self-diagnostic approach through photoluminescence using a small luminous pen is reported. The materials efficiently achieved self-healing at 90 °C after the crack formation. The change of fluorescence intensity on the crack can be used to track the self-healing process. Therefore, this work provides a guideline for the material design of supertough, puncture-resistant, transparent, and healable elastomers and a crack self-diagnosis and healing approach.

Construction and application of textile-based tissue engineering scaffolds: a review

Tissue engineering (TE) provides a practicable method for tissue and organ repair or substitution. As the most important component of TE, a scaffold plays a critical role in providing a growing environment for cell proliferation and functional differentiation as well as good mechanical support. And the restorative effects are greatly dependent upon the nature of the scaffold including the composition, morphology, structure, and mechanical performance. Medical textiles have been widely employed in the clinic for a long time and are being extensively investigated as TE scaffolds. However, unfortunately, the advantages of textile technology cannot be fully exploited in tissue regeneration due to the ignoring of the diversity of fabric structures. Therefore, this review focuses on textile-based scaffolds, emphasizing the significance of the fabric design and the resultant characteristics of cell behavior and extracellular matrix reconstruction. The structure and mechanical behavior of the fabrics constructed by various textile techniques for different tissue repairs are summarized. Furthermore, the prospect of structural design in the TE scaffold preparation was anticipated, including profiled fibers and some unique and complex textile structures. Hopefully, the readers of this review would appreciate the importance of structural design of the scaffold and the usefulness of textile-based TE scaffolds in tissue regeneration.

Rapid Degradation of Poly(lactic acid) with Organometallic Catalysts

Poly(lactic acid) (PLA) is an effective sacrificial material for the creation of vascular networks in thermoset polymers and composites. The high thermal stability of PLA limits its applications as an embedded sacrificial template in high-temperature-resistant thermoset matrices. Here, we demonstrate faster and more efficient PLA degradation at temperatures lower than previously reported using two organometallic catalysts: tin(II) oxalate (Sn(Oxa)) and tin(II) acetate (Sn(Ac)₂). We process Sn(Oxa) by two separate methods to obtain a significant difference in the specific surface area (SSA) of the catalyst particles and compare PLA degradation performance in a thermogravimetric analysis (TGA) instrument. Changing the SSA of Sn(Oxa) by a factor of ∼20 reduces the PLA degradation onset temperature by 37 °C. The total degradation time of PLA films also decreases after blending with Sn(Oxa) having a higher SSA. We also find Sn(Ac)₂ lowers the degradation onset of PLA by 53 °C compared to Sn(Oxa) with a similar SSA. In addition, Sn(Ac)₂ decreases the time for complete degradation of PLA films by an order of magnitude compared to Sn(Oxa) at 200 °C. Films with a significantly lower Sn(Ac)₂ concentration compared to Sn(Oxa) degrade much faster at lower temperatures up to 160 °C. Finally, PLA films with different loadings of Sn(Ac)₂ are embedded in an epoxy thermoset matrix and subsequently vascularized at elevated temperatures in a vacuum oven. Microchannel formation is observed at 170 °C using Sn(Ac)₂, reducing the temperature required for vaporization of embedded sacrificial polymer compared to Sn(Oxa) catalyst. Sn(Ac)₂ can potentially reduce the energy, time, and amount of catalyst required for degrading PLA into volatile products for sacrificial applications.

Replication of a Printed Volatile Mold: a novel microfabrication method for advanced microfluidic systems

A novel and simple method to fabricate microchannels is reported based on an inkjet printing of a volatile solid mold. A liquid ink -1,6 hexanediol- ejected from a piezoelectric nozzle is instantaneously frozen when touching a cooled substrate. The created mold is then poured with PDMS. Once the PDMS is crosslinked, the ink is sublimated and the device is ready. With this approach it is possible to make microchannels on different nature surfaces such as glass, paper, uncross-linked PDMS layer or non planar substrates. The versatility of this method is illustrated by printing channels directly on commercial electrodes and measuring the channel capacitance. Moreover, millimetric height microfluidic systems are easily produced (aspect ratio [Formula: see text] 25) as well as 3D structures such as bridges. To demonstrate, we have fabricated a combinatorial microfluidic system which makes 6 mixtures from 4 initial solutions without any stacking and tedious alignment procedure.

Leveraging Viscous Peeling to Create and Activate Soft Actuators and Microfluidic Devices

The research fields of microfluidics and soft robotics both involve complex small-scale internal channel networks, embedded within a solid structure. This study examines leveraging viscous peeling as a mechanism to create and activate soft actuators and microchannel networks, including complex elements such as valves, without the need for fabrication of structures with micron-scale internal cavities. We consider configurations composed of an internal slender structure embedded within another elastic solid. Pressurized viscous fluid is introduced into the interface between the two solids, thus peeling the two elastic structures and creating internal cavities. Since the gap between the solids is determined by the externally applied pressure, the characteristic size of the fluid network may vary with time and be much smaller than the resolution of the fabrication method. This study presents a model for the highly nonlinear elastic-viscous dynamics governing the flow and deformation of such configurations. Fabrication and experimental demonstrations of micron-scale valves and channel networks created from millimeter scale structures are presented, as well as the transient dynamics of viscous peeling-based soft actuators. The experimental data are compared with the suggested model, showing very good agreement.

Synthesis of UV-Responsive Self-Healing Microcapsules and Their Potential Application in Aerospace Coatings

Advanced polymer composite coatings in the spacecraft are threatened by harsh space environment factors, such as strong UV radiation, atomic oxygen, thermal cycles, space debris, etc. Their service life can be drastically shortened by the unavoidable formation of cracks caused by these factors (especially strong and abundant UV radiation) during long-term flight. Herein, a UV-responsive microcapsule-based coating is developed for in-orbit damage repairing. UV-responsive microcapsules of which the inner polymeric shell can be degraded rapidly by the outer pure TiO₂ shell under UV radiation are produced by UV-initiated polymerization of Pickering emulsions and subsequently embedded into silicon resin matrices. When damaged, some microcapsules will be ruptured under the stimulus of external force, afterward the unbroken ones around the scratched areas will be degraded by UV radiation, as a result, encapsulated healing agents can be released and finally repair cracks. In this system, UV-responsive microcapsules can release more agents more effectively due to the dual release mode, compared with the traditional crack-repairing system. Moreover, the damage of UV radiation in space can be transferred into the favorable ones, which makes it have a potential application in aerospace coatings.

Direct-write and sacrifice-based techniques for vasculatures

Fabricating biomimetic vasculatures is considered one of the greatest challenges in tissue regeneration due to their complex structures across various length scales. Many strategies have been investigated on how to fabricate tissue-engineering vasculatures (TEVs), including vascular-like and vascularized structures that can replace their native counterparts. The advancement of additive manufacturing (AM) technologies has enabled a wide range of fabrication techniques that can directly-write TEVs with complex and delicate structures. Meanwhile, sacrifice-based techniques, which rely on the removal of encapsulated sacrificial templates to form desired cavity-like structures, have also been widely studied. This review will specifically focus on the two most promising methods in these recently developed technologies, which are the direct-write method and the sacrifice-based method. The performance, advantages, and shortcomings of each technique are analyzed and compared. In the discussion, we list current challenges in this field and present our vision of next-generation TEVs technologies. Perspectives on future research in this field are given at the end.

Self-Reporting Fiber-Reinforced Composites That Mimic the Ability of Biological Materials to Sense and Report Damage

Sensing of damage, deformation, and mechanical forces is of vital importance in many applications of fiber-reinforced polymer composites, as it allows the structural health and integrity of composite components to be monitored and microdamage to be detected before it leads to catastrophic material failure. Bioinspired and biomimetic approaches to self-sensing and self-reporting materials are reviewed. Examples include bruising coatings and bleeding composites based on dye-filled microcapsules, hollow fibers, and vascular networks. Force-induced changes in color, fluorescence, or luminescence are achieved by mechanochromic epoxy resins, or by mechanophores and force-responsive proteins located at the interface of glass/carbon fibers and polymers. Composites can also feel strain, stress, and damage through embedded optical and electrical sensors, such as fiber Bragg grating sensors, or by resistance measurements of dispersed carbon fibers and carbon nanotubes. Bioinspired composites with the ability to show autonomously if and where they have been damaged lead to a multitude of opportunities for aerospace, automotive, civil engineering, and wind-turbine applications. They range from safety features for the detection of barely visible impact damage, to the real-time monitoring of deformation of load-bearing components.

Effect of 3D-Printed Microvascular Network Design on the Self-Healing Behavior of Cross-Linked Polymers

This article describes the manufacturing procedure and the characterization of self-healing polymers based on embedded microvascular networks. The samples were realized by resin casting into water-soluble PVA molds, fabricated via 3D printing. This technology allowed us to exploit the 3D printers' ability to produce complex structures with high resolution for the creation of independent microchannels networks. The two reacting components of a two-part resin could be stored separately within the microstructure. The materials' self-healing ability resulted from their reaction when severe damage caused the healing liquids to leak out, wetting the sample cross section and diffusing one into the other. The mechanical properties of healed samples were investigated by means of uniaxial tensile tests and compared to those of undamaged samples. The effect of microchannel density and different network designs on self-healing efficiency was determined. The different microstructures used were characterized using computerized X-ray microtomography. The versatility of the fabrication technique presented in this work allows conversion of any water-resistant resin into a fully functional self-healing polymeric composite.

Polymers with autonomous life-cycle control

The lifetime of man-made materials is controlled largely by the wear and tear of everyday use, environmental stress and unexpected damage, which ultimately lead to failure and disposal. Smart materials that mimic the ability of living systems to autonomously protect, report, heal and even regenerate in response to damage could increase the lifetime, safety and sustainability of many manufactured items. There are several approaches to achieving these functions using polymer-based materials, but making them work in highly variable, real-world situations is proving challenging.

An easily fabricated three-dimensional threaded lemniscate-shaped micromixer for a wide range of flow rates

Mixing fluid samples or reactants is a paramount function in the fields of micro total analysis system (μTAS) and microchemical processing. However, rapid and efficient fluid mixing is difficult to achieve inside microchannels because of the difficulty of diffusive mass transfer in the laminar regime of the typical microfluidic flows. It has been well recorded that the mixing efficiency can be boosted by migrating from two-dimensional (2D) to three-dimensional (3D) geometries. Although several 3D chaotic mixers have been designed, most of them offer a high mixing efficiency only in a very limited range of Reynolds numbers (Re). In this work, we developed a 3D fine-threaded lemniscate-shaped micromixer whose maximum numerical and empirical efficiency is around 97% and 93%, respectively, and maintains its high performance (i.e., >90%) over a wide range of 1 < Re < 1000 which meets the requirements of both the μTAS and microchemical process applications. The 3D micromixer was designed based on two distinct mixing strategies, namely, the inducing of chaotic advection by the presence of Dean flow and diffusive mixing through thread-like grooves around the curved body of the mixers. First, a set of numerical simulations was performed to study the physics of the flow and to determine the essential geometrical parameters of the mixers. Second, a simple and cost-effective method was exploited to fabricate the convoluted structure of the micromixers through the removal of a 3D-printed wax structure from a block of cured polydimethylsiloxane. Finally, the fabricated mixers with different threads were tested using a fluorescent microscope demonstrating a good agreement with the results of the numerical simulation. We envisage that the strategy used in this work would expand the scope of the micromixer technology by broadening the range of efficient working flow rate and providing an easy way to the fabrication of 3D convoluted microstructures.

Sustainable self-healing at ultra-low temperatures in structural composites incorporating hollow vessels and heating elements

Self-healing composites are able to restore their properties automatically. Impressive healing efficiencies can be achieved when conditions are favourable. On the other hand, healing might not be possible under adverse circumstances such as very low ambient temperature. Here, we report a structural composite able to maintain its temperature to provide a sustainable self-healing capability-similar to that in the natural world where some animals keep a constant body temperature to allow enzymes to stay active. The composite embeds three-dimensional hollow vessels with the purpose of delivering and releasing healing agents, and a porous conductive element to provide heat internally to defrost and promote healing reactions. A healing efficiency over 100% at around -60°C was obtained. The effects of the sheets on the interlaminar and tensile properties have been investigated experimentally. The proposed technique can be implemented in a majority of extrinsic self-healing composites to enable automatic recovery at ultra-low temperatures.

3D printing of liquid metals as fugitive inks for fabrication of 3D microfluidic channels

This paper demonstrates a simple method to fabricate 3D microchannels and microvasculature at room temperature by direct-writing liquid metal as a sacrificial template. The formation of a surface oxide skin on the low-viscosity liquid metal stabilizes the shape of the printed metal for planar and out-of-plane structures. The printed structures can be embedded in a variety of soft (e.g. elastomeric) and rigid (e.g. thermoset) polymers. Both acid and electrochemical reduction are capable of removing the oxide skin that forms on the metal, which destabilizes the ink so that it withdraws from the encapsulating material due to capillary forces, resulting in nearly full recovery of the fugitive ink at room temperature. Whereas conventional fabrication procedures typically confine microchannels to 2D planes, the geometry of the printed microchannels can be varied from a simple 2D network to complex 3D architectures without using lithography. The method produces robust monolithic structures without the need for any bonding or assembling techniques that often limit the materials of construction of conventional microchannels. Removing select portions of the metal leaves behind 3D metal features that can be used as antennas, interconnects, or electrodes for interfacing with lab-on-a-chip devices. This paper describes the capabilities and limitations of this simple process.

Design and development of self-healing dental composites

The purpose of this project is to design and develop a clinically applicable self-healing dental composite (SHDC). The value of resin-based dental restorations could be improved by increasing their service lives. One way to improve longevity is to obturate micro-cracks that form during or after the composite hardens in the dental cavity. Toward this end, we introduce here a new type of SHDC made with contemporary dental components plus two additional ingredients: a healing powder (HP, strongtium fluoroaluminosilicate particles) and a healing liquid (HL, aqueous solutions of polyacrylic acids) that is enclosed within silica microcapsules. As micro-cracks develop, they will break the microcapsules in their propagation path, thereby releasing HL. This liquid will then react with particles of HP exposed by the crack formation, forming an insoluble reaction product that fills and seals the cracks. The key factors to achieve this self-healing of cracks are discussed. The elastic modulus of a SHDC appeared to be satisfactory. The healing process was confirmed by means of mechanical, morphological, and chemical methods. The SHDC restored micro-cracks without external intervention, thereby showing potential for increasing the service lives of dental restorations. Importantly, this SHDC contains only clinically-tested, biocompatible materials, making it readily applicable.

Three-Dimensional Growth of Li2S in Lithium-Sulfur Batteries Promoted by a Redox Mediator

During the discharge of a lithium-sulfur (Li-S) battery, an electronically insulating 2D layer of Li2S is electrodeposited onto the current collector. Once the current collector is enveloped, the overpotential of the cell increases, and its discharge is arrested, often before reaching the full capacity of the active material. Guided by a new computational platform known as the Electrolyte Genome, we advance and apply benzo[ghi]peryleneimide (BPI) as a redox mediator for the reduction of dissolved polysulfides to Li2S. With BPI present, we show that it is now possible to electrodeposit Li2S as porous, 3D deposits onto carbon current collectors during cell discharge. As a result, sulfur utilization improved 220% due to a 6-fold increase in Li2S formation. To understand the growth mechanism, electrodeposition of Li2S was carried out under both galvanostatic and potentiostatic control. The observed kinetics under potentiostatic control were modeled using modified Avrami phase transformation kinetics, which showed that BPI slows the impingement of insulating Li2S islands on carbon. Conceptually, the pairing of conductive carbons with BPI can be viewed as a vascular approach to the design of current collectors for energy storage devices: here, conductive carbon "arteries" dominate long-range electron transport, while BPI "capillaries" mediate short-range transport and electron transfer between the storage materials and the carbon electrode.

High-Resolution Printing of 3D Structures Using an Electrohydrodynamic Inkjet with Multiple Functional Inks

Electrohydrodynamic-inkjet-printed high-resolution complex 3D structures with multiple functional inks are demonstrated. Printed 3D structures can have a variety of fine patterns, such as vertical or helix-shaped pillars and straight or rounded walls, with high aspect ratios (greater than ≈50) and narrow diameters (≈0.7 μm). Furthermore, the formation of freestanding, bridge-like Ag wire structures on plastic substrates suggests substantial potentials as high-precision, flexible 3D interconnects.

Biomimetic Self-Healing

Self-healing is a natural process common to all living organisms which provides increased longevity and the ability to adapt to changes in the environment. Inspired by this fitness-enhancing functionality, which was tuned by billions of years of evolution, scientists and engineers have been incorporating self-healing capabilities into synthetic materials. By mimicking mechanically triggered chemistry as well as the storage and delivery of liquid reagents, new materials have been developed with extended longevity that are capable of restoring mechanical integrity and additional functions after being damaged. This Review describes the fundamental steps in this new field of science, which combines chemistry, physics, materials science, and mechanical engineering.

Annealing free, clean graphene transfer using alternative polymer scaffolds

We examine the transfer of graphene grown by chemical vapor deposition (CVD) with polymer scaffolds of poly(methyl methacrylate) (PMMA), poly(lactic acid) (PLA), poly(phthalaldehyde) (PPA), and poly(bisphenol A carbonate) (PC). We find that optimally reactive PC scaffolds provide the cleanest graphene transfers without any annealing, after extensive comparison with optical microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Comparatively, films transferred with PLA, PPA, PMMA/PC, and PMMA have a two-fold higher roughness and a five-fold higher chemical doping. Using PC scaffolds, we demonstrate the clean transfer of CVD multilayer graphene, fluorinated graphene, and hexagonal boron nitride. Our annealing free, PC transfers enable the use of atomically-clean nanomaterials in biomolecule encapsulation and flexible electronic applications.

Rapid stiffening of a microfluidic endoskeleton via frontal polymerization

Materials capable of rapidly modifying their physical properties in response to stimuli are desirable for on-demand deployment and adaptive engineering structures. Frontal polymerization is a self-propagating reaction that can quickly transform liquid reactants into solid products. In this contribution, we demonstrate that frontal polymerization enables facile, rapid stiffening of a vascular network embedded in a flexible matrix. Systematic variation of the vascular architecture shows that polymerization fronts in a polydimethylsiloxane (PDMS) matrix are self-propagating in channels as small as 838 μm and even when curves, branch points and converging geometries are present. After polymerization the composite structure was dramatically stiffened (up to 18 times the original Young's modulus) based on tensile testing results. This work demonstrates the use of frontal polymerization as an efficient methodology for transforming flexible materials into functional supports or surfaces through endoskeletal stiffening.

A pressure-tolerant polymer microfluidic device fabricated by the simultaneous solidification-bonding method and flash chemistry application

Pressure-tolerant polymer-glass microfluidic reactors with excellent bonding strength have been fabricated by the simultaneous solidification-bonding (SSB) method, in which a viscous and reactive matrix polymer was cast on the glass substrate with pre-patterned wax as a sacrificial template. Elaborate interfacial chemistry between the matrix polymer and the functionalized glass surface was designed to achieve simultaneous solidification and chemical bonding under UV or/and mild thermal conditions (<200 °C with no pressure). Highly pressure-tolerant microchannels were obtained by complete removal of the liquid wax template at 80 °C. Versatility was demonstrated by fabricating microreactors from various polymers with different interfacial chemistry, which were all stable at 1000 psi with the highest burst pressure of 2000 psi. In particular, the fluoropolymer-glass microreactor can withstand a burst pressure that is two orders of magnitude higher than that of the microchannel made by the conventional method. Finally, the polymer-glass microfluidic device was used for the synthesis of a natural product, tryptanthrin, by flash chemistry under high pressure induced conditions (synthetic yield: 90%, flow rate: 10.5 mL min(-1), reaction time: 14 ms). The transparent microfluidic device can be used as a useful platform for miniaturizing spectroscopic tools for chemical analysis studies under high pressure conditions.

Engineering interconnected 3D vascular networks in hydrogels using molded sodium alginate lattice as the sacrificial template

Engineering 3D perfusable vascular networks in vitro and reproducing the physiological environment of blood vessels is very challenging for tissue engineering and investigation of blood vessel function. Here, we engineer interconnected 3D microfluidic vascular networks in hydrogels using molded sodium alginate lattice as sacrificial templates. The sacrificial templates are rapidly replicated in polydimethylsiloxane (PDMS) microfluidic chips via Ca⁺²-crosslinking and then fully encapsulated in hydrogels. Interconnected channels with well controlled size and morphology are obtained by dissolving the monolayer or multilayer templates with EDTA solution. The human umbilical vein endothelial cells (HUVECs) are cultured on the channel linings and proliferated to form vascular lumens. The strong cell adhesion capability and adaptive response to shear stress demonstrate the excellent cytocompatibility of both the template and template-sacrificing process. Furthermore, the barrier function of the endothelial layer is characterized and the results show that a confluent endothelial monolayer is fully developed. Taken together, we develop a facile and rapid approach to engineer a vascular model that could be potentially used in physiological studies of vascular functions and vascular tissue engineering.

Buried centimeter-long micro- and nanochannel arrays in porous silicon and glass

We developed a simple process to fabricate deeply buried micro- and nanoscale channels in glass and porous silicon from bulk silicon using a combination of ion beam irradiation, electrochemical anodization and high temperature oxidation. The depth, width and length of these structures can be controllably varied and we successfully fabricated an array of centimeter-long buried micro- and nanochannels. This process allows densely packed, arbitrary-shaped channel geometries with micro- to nanoscale dimensions to be produced in a three-dimensional multilevel architecture, providing a route to fabricate complex devices for use in nanofluidics and lab-on-a-chip systems. We demonstrate the integration of these channels with large reservoirs for DNA linearization in high aspect ratio nanochannels.

Anisotropic Janus Si nanopillar arrays as a microfluidic one-way valve for gas-liquid separation

In this paper, we demonstrate a facile strategy for the fabrication of a one-way valve for microfluidic (MF) systems. The micro-valve was fabricated by embedding arrays of Janus Si elliptical pillars (Si-EPAs) with anisotropic wettability into a MF channel fabricated in poly(dimethylsiloxane) (PDMS). Two sides of the Janus pillar are functionalized with molecules with distinct surface energies. The ability of the Janus pillar array to act as a valve was proved by investigating the flow behaviour of water in a T-shaped microchannel at different flow rates and pressures. In addition, the one-way valve was used to achieve gas-liquid separation. We believe that the Janus Si-EPAs modified by specific surface functionalization provide a new strategy to control the flow and motion of fluids in MF channels.

Three-dimensional conformal coatings through the entrapment of polymer membrane precursors

We report a technique to coat polymers onto 3D surfaces distinct from traditional spray, spin, or dip coating. In our technique, the surface of a template structure composed of poly(lactic acid) swells and entraps a soluble polymer precursor. Once entrapped, the precursor is cured, resulting in a thin, conformal membrane. The thickness of each coating depends on the coating solution composition, residence time, and template size. Thicknesses ranged from 400 nm to 4 μm within the experimental conditions we explored. The coating method was compatible with a range of polymers. Complicated 3D structures and microstructures of 10 μm thickness and separation were coated using this technique. The templates can also be selectively removed, leaving behind a hollow membrane structure in the shape of the original printed, extruded, or microporous template structures. This technique may be useful in applications that benefit from three-dimensional membrane topologies, including catalysis, separations, and potentially tissue engineering.

Structural reinforcement of microvascular networks using electrostatic layer-by-layer assembly with halloysite nanotubes

We demonstrate a method for tailoring local mechanical properties near channel surfaces of vascular structural polymers in order to achieve high structural performance in microvascular systems. While synthetic vascularized materials have been created by a variety of manufacturing techniques, unreinforced microchannels act as stress concentrators and lead to the initiation of premature failure. Taking inspiration from biological tissues such as dentin and bone, these mechanical deficiencies can be mitigated by complex hierarchical structural features near to channel surfaces. By employing electrostatic layer-by-layer assembly (ELbL) to deposit films containing halloysite nanotubes onto scaffold surfaces followed by matrix infiltration and scaffold removal, we are able to controllably deposit nanoscale reinforcement onto 200 micron diameter channel surface interiors in microvascular networks. High resolution strain measurements on reinforced networks under load verify that the halloysite reduces strain concentrations and improves mechanical performance.