3D printing dielectric elastomers for advanced functional structures: A mini-review

Three-dimensional (3D) printed bionic products play an important role in intelligent robotics, microelectronics, and polymers. The printing and manufacturing process of 3D printers is conducive to obtaining soft structures that meet specific requirements, and saves time and cost. Soft intelligent robotics, an emerging research field, has always been developed based on soft materials and actuators with their biological properties. This article reviews the current understanding of 3D bioprinting technologies for dielectric elastomers (DEs), DE actuators (DEAs) and soft robots, such as inkjet, extrusion, laser-induced and stereolithography bioprinting. 3D printers for fabricating soft materials are presented and classified. The approaches to exploit 3D bioprinters for DEs/DEAs are as follows: (1) 3D printing DEAs utilize ionic hydrogel–elastomer hybrids that are analogous to human muscles, and the DEAs usually have flexible structures and large deformations with multiple functionalities. (2) An electrohydrodynamic (EHD) 3D printer confers high printing resolution and high production efficiency, which offers advantages such as full automation and flexible design. The optimal printing conditions are mainly determined by the effects of printing voltages and ink properties, which are related to the formation of the liquid cone and the printed line width. Furthermore, the advantages of 3D bioprinting technologies have accelerated their development and applications.     

Experimental Design in the Preparation of Modified HEMA-Based Superporous Hydrogels in an Aqueous Medium

Due to a relatively large number of excipients and their concentrations, which can be used effectively in the preparation of superporous hydrogels, an experimental design based on the Taguchi matrix has proven to be a very valuable tool in screening and narrowing down the final formulation. In this study, the effect of starting materials, their concentrations as well as the starting reaction temperature, were examined in the preparation of superporous hydrogels based on hydroxyethyl methacrylate. A large number of possible formulations and conditions might lead to the production of a reasonable hydrogel network, but some formulations produce stronger or faster swelling superporous hydrogels than others. The final properties of the superporous hydrogels depend upon the events that occur during formation of the gel, including the presence of atmospheric oxygen, which is responsible for the inhibition period seen at the start of the reaction, and also including the change in temperature at which the reaction starts. These events can be largely affected by the choice of ingredients used in the reaction. For this study, eight variables were chosen, and their effects were examined using a Taguchi matrix. The parameters examined were the maximum temperature during the reaction, the time corresponding to the maximum temperature, and the reaction yield which is represented by the weight of the dry final SPH.     

Crops: a green approach toward self-assembled soft materials

To date, a wide range of industrial materials such as solvents, fuels, synthetic fibers, and chemical products are being manufactured from petroleum resources. However, rapid depletion of fossil and petroleum resources is encouraging current and future chemists to orient their research toward designing safer chemicals, products, and processes from renewable feedstock with an increased awareness of environmental and industrial impact. Advances in genetics, biotechnology, process chemistry, and engineering are leading to a new manufacturing concept for converting renewable biomass to valuable fuels and products, generally known as the biorefinery concept. The swift integration of crop-based materials synthesis and biorefinery manufacturing technologies offers the potential for new advances in sustainable energy alternatives and biomaterials that will lead to a new manufacturing paradigm. This Account presents a novel and emerging concept of generating various forms of soft materials from crops (an alternate feedstock). In future research, developing biobased soft materials will be a fascinating yet demanding practice, which will have direct impact on industrial applications as an economically viable alternative. Here we discuss some remarkable examples of glycolipids generated from industrial byproducts such as cashew nut shell liquid, which upon self-assembly produced soft nanoarchitectures including lipid nanotubes, twisted/helical nanofibers, low-molecular-weight gels, and liquid crystals. Synthetic methods applied to a "chiral pool" of carbohydrates using the selectivity of enzyme catalysis yield amphiphilic products derived from biobased feedstock including amygdalin, trehalose, and vitamin C. This has been achieved with a lipase-mediated regioselective synthetic procedure to obtain such amphiphiles in quantitative yields. Amygdalin amphiphiles showed unique gelation behavior in a broad range of solvents such as nonpolar hexanes to polar aqueous solutions. Importantly, an enzyme triggered drug-delivery model for hydrophobic drugs was demonstrated by using these supramolecularly assembled hydrogels. Following a similar biocatalytic approach, vitamin C amphiphiles were synthesized with different hydrocarbon chain lengths, and their ability to self-assemble into molecular gels and liquid crystals has been studied in detail. Such biobased soft materials were successfully used to develop novel organic-inorganic hybrid materials by in situ synthesis of metal nanoparticles. The self-assembled soft materials were characterized by several spectroscopic techniques, UV-visible, infrared, and fluorescence spectrophotometers, as well as microscopic methods including polarized optical, confocal, scanning, and transmission electron microscopes, and thermal analysis. The molecular packing of the hierarchically assembled bilayer membranes was fully elucidated by X-ray analysis. We envision that the results summarized in this Account will encourage interdisciplinary collaboration between scientists in the fields of organic synthesis, soft materials research, and green chemistry to develop functional materials from underutilized crop-based renewable feedstock, with innovation driven both by material needs and environmentally benign design principles.     

Material extrusion-based additive manufacturing of polypropylene: A review on how to improve dimensional inaccuracy and warpage

Material extrusion-based additive manufacturing (ME-AM) is an emerging processing technique that is characterized by the selective deposition of thermoplastic filaments in a layer-by-layer manner based on digital part models. Recently, it has attracted considerable attention, as this technique offers manifold benefits over conventional manufacturing technologies. However, to meet the challenges of complex industrial applications, certain shortcomings of ME-AM still need to be overcome. A case in point is the limited amount of semicrystalline thermoplastics, which are still not established as reliable, commercial filament materials. Particularly, polypropylene (PP) offers attractive properties that are unique among the ME-AM material portfolio. This review describes the current approaches of fabricating PP components by ME-AM. Both commercial and scientific strategies to make PP 3D-printable are elaborated and compared. As dimensional issues are especially problematic for PP, a comprehensive section of this review focuses on the strategies developed for mitigating warpage for PP parts fabricated by ME-AM. © 2019 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48545.     

Development of Bigels Based on Stearic Acid–Rice Bran Oil Oleogels and Tamarind Gum Hydrogels for Controlled Delivery Applications

In the last few decades, different types of gels have been widely studied as potential drug delivery carriers. In this paper, we propose the synthesis of an oleogel, a tamarind gum hydrogel, and bigels for applications as drug delivery matrices. The oleogel was prepared by mixing stearic acid and rice bran oil, whereas the hydrogel was prepared by mixing tamarind gum with a hydroethanolic solution. Hydrogel‐in‐oleogel and oleogel‐in‐hydrogel bigels were prepared by mixing the hydrogel and the oleogel. The suitability of the formulations for controlled drug release applications was thoroughly examined using microscopy, Fourier transform infrared (FTIR) spectroscopy, as well as mechanical, electrical, thermal, drug release, and antimicrobial studies. An alteration in the microarchitecture of the bigels is observed when the oleogel and the hydrogel are mixed in varying proportions. The associative interactions within the formulations increase with the increase in the hydrogel content. The bigels exhibit the presence of stearic acid melting endotherm (associated with the oleogel) and water evaporation endotherm (associated with the hydrogel). This study suggests that the hydrogel has lowest bulk resistance compared to the other formulations. The structural breakdown of the bigels is composition‐dependent, and the bulk electrical resistance is mainly governed by the oleogel phase. An increase in the diffusion of the moxifloxacin HCl from the formulations is observed with the increase of the hydrogel proportion, which in turn increases the rate of release of the drug. The proposed formulations also exhibit good antimicrobial efficacy. The analysis of these properties suggests that specific formulations can be tailored by need‐based applications of the drug release rate.     

PASP/PAA互穿网络水凝胶的制备及其作为保水剂的应用

采用水相法制备了聚天冬氨酸-聚丙烯酸互穿网络(PASP/PAA IPN)水凝胶,通过正交实验优化其制备工艺。利用FTIR、SEM、TGA和DSC对PASP/PAAIPN水凝胶的结构及性能进行表征,考察了PASP/PAAIPN水凝胶在不同温度及pH条件下的吸水率,最后作为保水剂进行芹菜盆栽实验。结果表明,互穿网络组成对水凝胶的稳定性和吸水性具有明显的影响,最终工艺条件为聚琥珀酰亚胺3 g、丙烯酸2 mL、交联剂乙二醇二缩水甘油醚1 g、交联剂N, N′-亚甲基双丙烯酰胺16 mg,在该条件下得到PASP/PAA IPN水凝胶吸液倍率为266 g/g,吸盐水倍率为63g/g,具有良好的热性能、pH敏感性和温度敏感性。盆栽实验证明,PASP/PAAIPN水凝胶无论正常浇水还是干旱胁迫下,芹菜生长情况、光合速率及叶绿素含量均优于市售PAA保水剂,对应的光合速率较PAA保水剂分别高7.82%和53.40%;叶绿素含量较PAA保水剂分别高58.33%和249.70%。     

Measurement of low yield-stress materials

A special method has been devised to measure yield stress of biosolids material or cake. A material with yield stress is placed in a container initially resting on its base. By slowly rotating the container 90° incrementally under “quasi-equilibrium”, the profile of the material is reformed in which the height is deeper on one end and shallower on the opposite end. To measure material with low yield stress, an immiscible lighter liquid is introduced with the material fully immersed in the pool of the lighter liquid and the above procedure is repeated. Also the dimension of the side of the container should be much greater that of the base. Despite the yield stress of the material is small the interface profile between the lighter liquid and the material can be established under reduced weight due to buoyancy force of the lighter liquid. The measured material profile is compared with the theoretical prediction based on a model assuming the material establishes a hydrostatic equilibrium in its final position with the container resting on its side. The effective density used in the test is the density difference between the lighter liquid and the material, which can be made small, catered for measuring material with very low yield stress.The new method was used to measure xanthan gum with low yield stress with magnitude less than 1 Pa. The cross sectional shape of the container affects the measurement and the subsequent interpretation. Both rectangular and cylindrical geometries have been investigated and both give comparable yield stress for the xanthan gum being tested. The proposed method is attractive for providing reasonable and accurate measurement despite its simplicity.     

Scalable Trehalose‐Functionalized Hydrogel Synthesis for High‐Temperature Protection of Enzymes

Herein, a scalable, two‐step synthesis of a trehalose hydrogel for the thermostabilization of enzymes is reported. A reaction between vinylbenzyl chloride and trehalose in base, followed by a redox‐initiated radical polymerization of the resulting mixture, produces the gel in 88% yield. The reaction scale can be increased 100‐fold while maintaining a 76% yield. Additionally, various solvents are investigated for purification, and more sustainable manufacturing solvents are selected. When the three major enzymes utilized in animal feed, phytase, β‐glucanase, and xylanase, are heated to 90 °C in the hydrogel, greater than 98% activity is retained. Lastly, quantitative release of enzyme from the gel within 4 h is demonstrated. The scalable synthesis of the trehalose hydrogel, combined with its ability to stabilize and release a variety of animal feed enzymes, makes this technology promising for use with enzymes important in animal food production.     

Gas marbles: ultra-long-lasting and ultra-robust bubbles formed by particle stabilization

Bubbles and foams are ubiquitous in daily life and industrial processes. Studying their dynamic behaviors is of key importance for foam manufacturing processes in food packaging, cosmetics and pharmaceuticals. Bare bubbles are inherently fragile and transient; enhancing their robustness and shelf lives is an ongoing challenge. Their rupture can be attributed to liquid evaporation, thin film drainage and the nuclei of environmental dust. Inspired by particle-stabilized interfaces in Pickering emulsions, armored bubbles and liquid marble, bubbles are protected by an enclosed particle-entrapping liquid thin film, and the resultant soft object is termed gas marble. The gas marble exhibits mechanical strength orders of magnitude higher than that of soap bubbles when subjected to overpressure and underpressure, owing to the compact particle monolayer straddling the surface liquid film. By using a water-absorbent glycerol solution, the resulting gas marble can persist for 465 d in normal atmospheric settings. This particle-stabilizing approach not only has practical implications for foam manufacturing processes but also can inspire the new design and fabrication of functional biomaterials and biomedicines.     

Heavy-duty laundry detergents

Current powder and liquid household laundry detergent formulations are reviewed. Methods for quantifying the important performance and physical properties of these formulations are described. Optimization of ingredients and manufacturing methods are included.     

Highly Mechanical and Fatigue‐Resistant Double Network Hydrogels by Dual Physically Hydrophobic Association and Ionic Crosslinking

Double network (DN) hydrogels with high strength and toughness are considered as promising soft materials. Herein, a dual physically cross‐linked hydrophobic association polyacrylamide (HPAAm)/alginate‐Ca²⁺ DN hydrogel is reported, consisting of a HPAAm network and a Ca²⁺ cross‐linked alginate network. The HPAAm/alginate‐Ca²⁺ DN hydrogel exhibits excellent mechanical properties with the fracture stress of 1.16 MPa (3.0 and 1.7 times higher than that of HPAAm hydrogel and HPAAm/alginate hydrogel, respectively), fracture strain of 2604%, elastic modulus of 71.79 kPa, and toughness of 14.20 MJ m^?3. HPAAm/alginate‐Ca²⁺ DN hydrogels also demonstrate self‐recovery, notch‐insensitivity, and fatigue resistance properties without any external stimuli at room temperature through reversible physical bonds consisting of hydrophobic association and ionic crosslinking. As a result, the dual physical crosslinking would offer an avenue to design DN hydrogels with desirable properties for broadening current applications of soft materials.     

面向柔性超级电容器的功能水凝胶材料的研究进展

功能水凝胶作为一种三维高分子网络结构的软湿材料,具有可灵活调控的功能特性,为设计和构建高性能柔性超级电容器提供了理想的材料。本文综述了近年来面向柔性超级电容器领域的功能水凝胶材料的研究进展,重点分类介绍了面向电化学双层电容器和赝电容器的功能水凝胶材料的设计构建和性能强化。探讨了通过水凝胶电解质及电极材料的组成结构设计和性能调控来提升超级电容器的电化学性能和力学性能的策略。同时,探讨了水凝胶电解质及电极材料的组成结构设计和性能调控在实现其自愈合、高耐寒等多样化功能特性方面的重要作用。最后,对功能水凝胶材料柔性超级电容器在高储能、高柔性、高保水、自愈合、高耐寒、绿色可降解等方面的未来发展进行了展望。     

Enhancing the performance of flexible vinyl by using the formulary approach

Each vinyl plastisol processor has different requirements. The features of the finished goods determine the specifications and physical properties that are critical in the manufacturing process. Processors may also have varying equipment limitations that require a specific working range of each plastisol property, such as viscosity, for acceptable processing. Although the benzoate esters have been utilized for many years by plastisol formulators and processors to their advantage, usage of these esters has been limited because of their tendency to yield high‐viscosity plastisols. By utilizing a formulary approach, it is possible to fine‐tune the properties of the plastisol, thus allowing greater latitude. Design of experiment (DOE) was employed to accomplish this objective. For this work, generic resilient‐flooring formulations were selected as the model. The formulary approach was employed to adjust the formulation and processing parameters, thereby allowing plastisol formulators and processors to develop cost‐efficient formulations. J. VINYL ADDIT. TECHNOL., 13:201–205, 2007. © 2007 Society of Plastics Engineers     

Computer aided applications to injection molding: Transfinite/finite element thermal/stress response formulations

The paper describes two different computational approaches for simulating the effects due to temperature variations and therein the associated thermally induced stresses and warpage with emphasis on computer aided engineering applications to ‘plastic’ components. One approach is the conventional finite element approach, and the other is termed as the transfinite element approach. The development of unified thermal/stress formulations for each of the aforementioned approaches is described for applications to injection molded plastic parts. Results of both computational formulations are in excellent agreement for predicting the thermal response and unified thermal/stress response and agree qualitatively with previous studies. For this study, it is found that the transfinite element formulations are computationally more efficient although the conventional formulations can still be effectively used. Both formulations are capable of serving as powerful computational tools to help part designers to predict the thermal/stress responses accurately and effectively. The unified concepts and approaches offer potential for extension to more complex geometries and for combined heat transfer and stress problems.     

Fabrication of dense SiSiC ceramics by a hybrid additive manufacturing process

In this work, we report the fabrication of Silicon infiltrated Silicon Carbide (SiSiC) components by a hybrid additive manufacturing process. Selective laser sintering of polyamide powders was used to 3D print a polymeric preform with controlled relative density, which allows manufacturing geometrically complex parts with small features. Preceramic polymer infiltration with a silicon carbide precursor followed by pyrolysis (PIP) was used to convert the preform into an amorphous SiC ceramic, and five PIP cycles were performed to increase the relative density of the part. The final densification was achieved via liquid silicon infiltration (LSI) at 1500°C, obtaining a SiSiC ceramic component without change of size and shape distortion. The crystallization of the previously generated SiC phase, with associated volume change, allowed to fully infiltrate the part leading to an almost fully dense material consisting of β-SiC and Si in the volume fraction of 45% and 55% respectively. The advantage of this approach is the possibility of manufacturing SiSiC ceramics directly from the preceramic precursor, without the need of adding ceramic powder to the infiltrating solution. This can be seen as an alternative AM approach to Binder jetting and direct ink writing for the production of templates to be further processed by silicon infiltration.     

Influence of shear thinning and material flow on robotic dispensing of poly(ethylene glycol) diacrylate/poloxamer 407 hydrogels

Robotic dispensing of hydrogels offers a direct way for generating complex hydrogel shapes. For this, there is a general need for hydrogel formulations with suitable rheological properties. In this contribution, hydrogel formulations containing poly(ethylene glycol) diacrylate (PEG‐DA) and Poloxamer 407 are characterized regarding their flow behavior during robotic dispensing. Formulations contain between 15% and 20% PEG‐DA and 22.5% and 25% Poloxamer 407. All formulations show shear thinning which can be described using a power law with a power law index between 0.10 and 0.11 and calculated shear rates at the wall of the dispensing needle of 379 s^?1 with a dispensing speed of 8 mm s^?1 and a dispensing needle inner diameter of 0.51 mm. Thus, facilitating the generation of smooth hydrogel strands, three‐dimensional hydrogel objects can be prepared without flow after robotic dispensing and can be cured afterward to elastic hydrogels, retaining the shape of the dispensed object. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45083.     

Flaw Sensitivity and Tensile Fatigue of Poly(Vinyl Alcohol) Hydrogels

Tensile fatigue behavior is commonly overlooked as researchers pursue the toughest hydrogels. This work describes a poly(vinyl alcohol) (PVA) hydrogel prepared through freezing–thawing (FT) processing to achieve varied monotonic strength and toughness. The monotonic tensile responses of relatively strong and weak versions of the hydrogel are studied with cylindrical hole and crack-like flaws of different sizes to develop an understanding of monotonic strength in the presence of two different, extreme defect types. The monotonic strength of the samples with cylindrical defects is reasonably predicted using nominal stress which accounts for a loss of load-bearing area, while linear-elastic fracture mechanics gives a first-order approximation of the impact of crack-like flaw size on monotonic strength. A subset of key defected samples are further subjected to cyclic loading and fatigue failure at varying stress amplitude. The cylindrical defect samples outperformed cracked samples in fatigue, and the utilization of four FT cycles instead of two improved both monotonic toughness and fatigue properties. This work represents the first tensile fatigue analysis on defected hydrogel materials, sheds light on the behavior of hydrogels in cyclic loading environments, and evaluates both the monotonic toughness and fatigue behavior of soft materials with and without defects.     

Preparation and physical properties of a macroscopically aligned lyotropic hexagonal phase templated hydrogel

Lyotropic liquid crystal (LLC) materials, such as lyotropic liquid single crystal hydrogels (LLSCHs) and LLC templated hydrogels, have the potential for a wide range of applications from nanomaterials to drug delivery. Most of the applications are dependent on transport processes through the gels. While powder distribution LLC hydrogels and elastomers have been shown to be alignable by uniaxial stress/strain, LLSCH that are macroscopically aligned can be made by photopolymerisation after alignment of suitable polymerisable surfactants. Cross-linked polyethylene glycol diacrylate (PEG-DA) hydrogels, have previously been templated using powder distribution LLC phases of non-polymerisable surfactants. In this work, a macroscopically aligned LLC hexagonal phase templated PEG-DA hydrogel was made. The alignment was monitored with ²H NMR quadrupole splitting which showed that the hydrogel retained the macroscopic alignment after photopolymerisation. The effects of compression/deswelling were recorded using optical and polarised optical microscopy (POM). However, the focus of this work was on studying the self-diffusion of water in the hydrogel, which is pertinent not only for the typical applications of these materials but for potentially new applications. Pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) provides a way of investigating the self-diffusion easily and non-invasively. Interestingly the measured self-diffusion of water, at least for the sample in this study, was relatively independent of the diffusion time used (i.e., 70 ms to 3 s).     

Reactive extraction of acylglycerides using <Emphasis Type="Italic">Aspergillus flavus</Emphasis> resting cells

A one-pot TAG extraction and FAME formation using fungal resting cells and oilseeds at moderate temperature are described. The final yield of methyl esters is increased by the sequential addition of water and methanol. The process can be carried out either with solvents or in a solvent-free system. When a solvent-free medium is used, the final yield will increase if soft oilseeds are used.     

辐射合成水凝胶材料的动力学进程及影响因素

本文主要介绍以电子加速器的高剂量率电子射线合成水凝胶材料的研究,主要结果如下: 1.在动力学进程中,当转化率约20％时才产生凝胶。完成动力学进程所需辐照剂量只需0.5—2.0×10~0rad,辐照时间为1—4分钟; 2.聚合初速率(V_0)与剂最率(Ⅰ)的关系如下:HEMA+EGDMA体系:V_0∞I~(0.53); HEMA+IOMA+EGDMA体系:V_0∞I~(0.48); HEMA+IOAM+EGDMA+RH体系:V_0∞I~(0.50); 3.在聚合物链中引入疏水性单体,可以大大改善水凝胶材料的强度,但含水量相应降低。 4.本材料已用于超薄型软接触镜的生产。