Rapid synchronized fabrication of vascularized thermosets and composites

Frontal Polymerization for the Rapid Obtainment of Polymer Composites

Among the different polymerization techniques, frontal polymerization (FP) has gained high interest from the scientific community because of its peculiar characteristics: in particular, compared to classic polymerization reactions, FP allows for a better exploitation of the heat of polymerization involved, without requiring any external energy input apart from an initial photo or thermal ignition that triggers the reaction. The latter usually propagates in a few tenths of seconds or (at most) minutes through a hot self-sustaining polymerization front, giving rise to the formation of fully cured thermosetting networks or thermoplastic polymers. Furthermore, different polymerization mechanisms can be involved in FP reactions, comprising cationic or anionic, ring-opening metathesis, and free-radical polymerization, among others. Further, it is possible to run FP reactions in bulk, in solution, or even using solid monomers if they are melted at the temperature of the front, notwithstanding the possibility of using reactive systems containing fillers or fiber/fabric reinforcements. In this context, the use of FP is becoming very important also for the design and production of advanced (nano)composite materials, saving processing time and achieving the completeness of the curing reaction, even in the presence of high filler/reinforcement loadings. Therefore, this mini-review aims to provide the reader with the basics of FP and its main peculiarities, even in the context of preparing high-performing composites. In this respect, some recent case studies witnessing the potentialities of frontal polymerization for the design of advanced (nano)composite systems will be elucidated. Finally, some perspectives about possible future developments will be proposed.

Using Data Science Tools to Reveal and Understand Subtle Relationships of Inhibitor Structure in Frontal Ring-Opening Metathesis Polymerization

The rate of frontal ring-opening metathesis polymerization (FROMP) using the Grubbs generation II catalyst is impacted by both the concentration and choice of monomers and inhibitors, usually organophosphorus derivatives. Herein we report a data-science-driven workflow to evaluate how these factors impact both the rate of FROMP and how long the formulation of the mixture is stable (pot life). Using this workflow, we built a classification model using a single-node decision tree to determine how a simple phosphine structural descriptor (Vbur-near) can bin long versus short pot life. Additionally, we applied a nonlinear kernel ridge regression model to predict how the inhibitor and selection/concentration of comonomers impact the FROMP rate. The analysis provides selection criteria for material network structures that span from highly cross-linked thermosets to non-cross-linked thermoplastics as well as degradable and nondegradable materials.

Unraveling Reactivity Differences: Room-Temperature Ring-Opening Metathesis Polymerization (ROMP) versus Frontal ROMP

In this study, we explore the distinct reactivity patterns between frontal ring-opening metathesis polymerization (FROMP) and room-temperature solventless ring-opening metathesis polymerization (ROMP). Despite their shared mechanism, we find that FROMP is less sensitive to inhibitor concentration than room-temperature ROMP. By increasing the initiator-to-monomer ratio for a fixed inhibitor/initiator quantity, we find reduction in the ROMP background reactivity at room temperature (i.e., increased resin pot life). At elevated temperatures where inhibitor dissociation prevails, accelerated frontal polymerization rates are observed because of the concentrated presence of the initiator. Surprisingly, the strategy of employing higher initiator loading enhances both pot life and front speeds, which leads to FROMP rates exceeding prior reported values by over 5 times. This counterintuitive behavior is attributed to an increase in the proximity of the inhibitor to the initiator within the bulk resin and to whether the temperature favors coordination or dissociation of the inhibitor. A rapid method was developed for assessing resin pot life, and a straightforward model for active initiator behavior was established. Modified resin systems enabled direct ink writing of robust thermoset structures at rates much faster than previously possible.

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.

Sustainable mechanochemical growth of double-network hydrogels supported by vascular-like perfusion

Double-network (DN) gels are unique mechanochemical materials owing to their structures that can be dynamically remodelled during use. The mechanical energy applied to DN gels is efficiently transferred to the chemical bonds of the brittle network, generating mechanoradicals that initiate the polymerisation of pre-loaded monomers, thereby remodelling the materials. To attain continuous remodelling or growth in response to repetitive mechanical stimuli, a sustainable supply of chemical reagents to such dynamic materials is essential. In this study, inspired by the vascular perfusion transporting nutrients to cells, we constructed a circulatory system for a continuous supply of chemicals to channel-containing DN hydrogels (c-DN gels). The perfusion of monomer solutions through the channel and permeability of the c-DN gels not only replenishes the monomers consumed by the polymerisation but also replenishes the water loss caused by the surface evaporation of hydrogel, thereby freeing the mechanochemical process of DN gels from the constraints of the underwater environment. The facile chemical supply enabled us to modulate the mechanical enhancement of the c-DN gel and attain muscle-like strengthening under repeated mechanical training in deoxygenated air. We also studied the kinetics of polymer growth and strengthening and deciphered unique features of mechanochemical reaction in DN gels including the extremely long-living radicals and delayed mechanical strengthening.

Configuration-independent thermal invariants under flow reversal in thin vascular systems

Modulating temperature fields is indispensable for advancing modern technologies: space probes, electronic packing, and implantable medical devices, to name a few. Bio-inspired thermal regulation achieved via fluid flow within a network of embedded vesicles is notably desirable for slender synthetic material systems. This far-reaching study-availing theory, numerics, and experiments-reveals a counter-intuitive yet fundamental property of vascular-based fluid-flow-engendered thermal regulation. For such thin systems, the mean surface temperature and the outlet temperature-consequently, the heat extracted by the flowing fluid (coolant)-are invariant under flow reversal (i.e. swapping the inlet and outlet). Despite markedly different temperature fields under flow reversal, our newfound invariance-a discovery-holds for anisotropic thermal conductivity, any inlet and ambient temperatures, transient and steady-state responses, irregular domains, and arbitrary internal vascular topologies, including those with branching. The reported configuration-independent result benefits thermal regulation designers. For instance, the flexibility in the coolant's inlet location eases coordination challenges between electronics and various delivery systems in microfluidic devices without compromising performance (e.g. soft implantable coolers for pain management). Last but not least, the invariance offers an innovative way to verify computer codes, especially when analytical solutions are unavailable for intricate domain and vascular configurations.

Rapid Preparation of Superabsorbent Self-Healing Hydrogels by Frontal Polymerization

Hydrogels have received increasing interest owing to their excellent physicochemical properties and wide applications. In this paper, we report the rapid fabrication of new hydrogels possessing a super water swelling capacity and self-healing ability using a fast, energy-efficient, and convenient method of frontal polymerization (FP). Self-sustained copolymerization of acrylamide (AM), 3-[Dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]azaniumyl]propane-1-sulfonate (SBMA), and acrylic acid (AA) within 10 min via FP yielded highly transparent and stretchable poly(AM-co-SBMA-co-AA) hydrogels. Thermogravimetric analysis and Fourier transform infrared spectroscopy confirmed the successful fabrication of poly(AM-co-SBMA-co-AA) hydrogels with a single copolymer composition without branched polymers. The effect of monomer ratio on FP features as well as porous morphology, swelling behavior, and self-healing performance of the hydrogels were systematically investigated, showing that the properties of the hydrogels could be tuned by adjusting the chemical composition. The resulting hydrogels were superabsorbent and sensitive to pH, exhibiting a high swelling ratio of up to 11,802% in water and 13,588% in an alkaline environment. The rheological data revealed a stable gel network. These hydrogels also had a favorable self-healing ability with a healing efficiency of up to 95%. This work contributes a simple and efficient method for the rapid preparation of superabsorbent and self-healing hydrogels.

Frontal Polymerizations: From Chemical Perspectives to Macroscopic Properties and Applications

The synthesis and processing of most thermoplastics and thermoset polymeric materials rely on energy-inefficient and environmentally burdensome manufacturing methods. Frontal polymerization is an attractive, scalable alternative due to its exploitation of polymerization heat that is generally wasted and unutilized. The only external energy needed for frontal polymerization is an initial thermal (or photo) stimulus that locally ignites the reaction. The subsequent reaction exothermicity provides local heating; the transport of this thermal energy to neighboring monomers in either a liquid or gel-like state results in a self-perpetuating reaction zone that provides fully cured thermosets and thermoplastics. Propagation of this polymerization front continues through the unreacted monomer media until either all reactants are consumed or sufficient heat loss stalls further reaction. Several different polymerization mechanisms support frontal processes, including free-radical, cat- or anionic, amine-cure epoxides, and ring-opening metathesis polymerization. The choice of monomer, initiator/catalyst, and additives dictates how fast the polymer front traverses the reactant medium, as well as the maximum temperature achievable. Numerous applications of frontally generated materials exist, ranging from porous substrate reinforcement to fabrication of patterned composites. In this review, we examine in detail the physical and chemical phenomena that govern frontal polymerization, as well as outline the existing applications.

Frontal polymerization-triggered simultaneous ring-opening metathesis polymerization and cross metathesis affords anisotropic macroporous dicyclopentadiene cellulose nanocrystal foam

Multifunctionality and effectiveness of macroporous solid foams in extreme environments have captivated the attention of both academia and industries. The most recent rapid, energy-efficient strategy to manufacture solid foams with directionality is the frontal polymerization (FP) of dicyclopentadiene (DCPD). However, there still remains the need for a time efficient one-pot approach to induce anisotropic macroporosity in DCPD foams. Here we show a rapid production of cellular solids by frontally polymerizing a mixture of DCPD monomer and allyl-functionalized cellulose nanocrystals (ACs). Our results demonstrate a clear correlation between increasing % allylation and AC wt%, and the formed pore architectures. Especially, we show enhanced front velocity (v_f) and reduced reaction initiation time (t_init) by introducing an optimal amount of 2 wt% AC. Conclusively, the small- and wide-angle X-ray scattering (SAXS, WAXS) analyses reveal that the incorporation of 2 wt% AC affects the crystal structure of FP-mediated DCPD/AC foams and enhances their oxidation resistance.

Switching Frontal Polymerization Mechanisms: FROMP and FRaP

Two frontal polymerization (FP) mechanisms, frontal ring-opening metathesis polymerization (FROMP) of dicyclopentadiene and frontal radical polymerization (FRaP) of benzyl acrylate and hexanediol diacrylate, were combined for rapid manufacturing of welded thermoset materials. Leveraging the immiscibility of the two different FP resins, welded thermosets and gradient foams of varying composition were achieved by switching of FP mechanisms. The adhesion strength of the welded thermoset materials differed depending on the originating mechanism. Finally, welded thermoset foams of varying porosity and homogeneity were generated through initiation from the bottom of the two resins.

Anisotropic Foams via Frontal Polymerization

The properties of foams, an important class of cellular solids, are most sensitive to the volume fraction and openness of its elementary compartments; size, shape, orientation, and the interconnectedness of the cells are other important design attributes. Control of these morphological traits would allow the tailored fabrication of useful materials. While approaches like ice templating have produced foams with elongated cells, there is a need for rapid, versatile, and energy-efficient methods that also control the local order and macroscopic alignment of cellular elements. Here, a fast and convenient method is described to obtain anisotropic structural foams using frontal polymerization. Foams are fabricated by curing mixtures of dicyclopentadiene and a blowing agent via frontal ring-opening metathesis polymerization (FROMP). The materials are characterized using microcomputed tomography (micro-CT) and an image analysis protocol to quantify the morphological characteristics. The cellular structure, porosity, and hardness of the foams change with blowing agent, concentration, and resin viscosity. Moreover, a full factorial combination of variables is used to correlate each parameter with the structure of the obtained foams. The results demonstrate the controlled production of foams with specific morphologies using the simple and efficient method of frontal polymerization.

Quantum Dots-Loaded Self-Healing Gels for Versatile Fluorescent Assembly

From the perspective of applied science, methods that allow the simple construction of versatile quantum dots (QDs)-loaded gels are highly desirable. In this work, we report the self-healing assembly methods for various fluorescent QDs-loaded gels. Firstly, we employed horizontal frontal polymerization (FP) to fabricate self-healing gels within several minutes using a rapid and energy-saving means of preparation. The as-prepared gels showed pH sensitivity, satisfactory mechanical properties and excellent self-healing properties and the healing efficiency reached 90%. The integration of the QDs with the gels allowed the generation of fluorescent composites, which were successfully applied to an LED device. In addition, by using the self-healing QDs-loaded gels as building blocks, the self-healing assembly method was used to construct complex structures with different fluorescence, which could then be used for sensing and encoding. This work offers a new perspective on constructing various fluorescent assemblies by self-healing assembly, and it might stimulate the future application of self-healing gels in a self-healing assembly fashion.

Progress of 3D Bioprinting in Organ Manufacturing

Three-dimensional (3D) bioprinting is a family of rapid prototyping technologies, which assemble biomaterials, including cells and bioactive agents, under the control of a computer-aided design model in a layer-by-layer fashion. It has great potential in organ manufacturing areas with the combination of biology, polymers, chemistry, engineering, medicine, and mechanics. At present, 3D bioprinting technologies can be used to successfully print living tissues and organs, including blood vessels, skin, bones, cartilage, kidney, heart, and liver. The unique advantages of 3D bioprinting technologies for organ manufacturing have improved the traditional medical level significantly. In this article, we summarize the latest research progress of polymers in bioartificial organ 3D printing areas. The important characteristics of the printable polymers and the typical 3D bioprinting technologies for several complex bioartificial organs, such as the heart, liver, nerve, and skin, are introduced.