Architecture of macromolecular network of soft functional materials : from structure to function

An enhanced macromolecular nanofiber network and its implications have been developed by employing the understanding of its formation with an emphasis on its topological aspect. Using agarose aqueous solution as a typical example, the macromolecular nanofiber network of soft functional materials has been clearly visualized for the first time using the developed technique of field emission scanning electronic microscopy coupled with flash-freeze-drying. Both the systematic kinetic study and the image evidence indicates that the nanofiber network in soft functional materials such as agarose turns out to form through a self-expitaxial nucleation-controlled process. This new understanding enables us to engineer ultra functions of soft materials via nanofiber network architecture, which in turn opens up a new direction in nano fabrication.

Architecture of supramolecular soft functional materials: from understanding to micro-/nanoscale engineering

This article gives an overview of the current progress of a class of supramolecular soft materials consisting of fiber networks and the trapped liquid. After discussing the up-to-date knowledge on the types of fiber networks and the correlation to the rheological properties, the gelation mechanism turns out to be one of the key subjects for this review. In this concern, the following two aspects will be focused upon: the single fiber network formation and the multi-domain fiber network formation of this type of material. Concerning the fiber network formation, taking place via nucleation, and the nucleation-mediated growth and branching mechanism, the theoretical basis of crystallographic mismatch nucleation that governs fiber branching and formation of three-dimensional fiber networks is presented. In connection to the multi-domain fiber network formation, which is governed by the primary nucleation and the subsequent formation of single fiber networks from nucleation centers, the control of the primary nucleation rate will be considered. Based on the understanding on the the gelation mechanism, the engineering strategies of soft functional materials of this type will be systematically discussed. These include the control of the nucleation and branching-controlled fiber network formation in terms of tuning the thermodynamic driving force of the gelling system and introducing suitable additives, as well as introducing ultrasound. Finally, a summary and the outlook of future research on the basis of the nucleation-growth-controlled fiber network formation are given.

Microengineering of soft functional materials by controlling the fiber network formation

The engineering of soft functional materials based on the construction of three-dimensional interconnecting self-organized nanofiber networks is reported. The system under investigation is an organogel formed by N-lauroyl-L-glutamic acid di-n-butylamide (GP-1) in propylene glycol. The engineering of soft functional materials is implemented by controlling primary nucleation kinetics of GP-1, which can be achieved by both reducing thermodynamic driving force and/or introducing a tiny amount of specific copolymers (i.e., poly(methyl methacrylate comethacrylic acid)). The primary nucleation rate of GP-1 is correlated to the number density of GP-1 spherulites, which determines the overall rheological properties of soft functional materials. The results show that the presence of a tiny amount of the polymer (0.01-0.06%) can effectively inhibit the nucleation of GP-1 spherulites, which leads to the formation of integrated fiber networks. It follows that with the additive approach, the viscoelasticity of the soft functional material is significantly enhanced (i.e., more than 1.5 times at 40 °C). A combination of the thermal and additive approach led to an improvement of 3.5 times in the viscosity of the gel.

Volume confinement induced microstructural transitions and property enhancements of supramolecular soft materials

The rheological properties of supramolecular soft functional materials are determined by the networks within the materials. This research reveals for the first time that the volume confinement during the formation of supramolecular soft functional materials will exert a significant impact on the rheological properties of the materials. A class of small molecular organogels formed by the gelation of N-lauroyl-L-glutamic acid din-butylamide (GP-1) in ethylene glycol (EG) and propylene glycol (PG) solutions were adopted as model systems for this study. It follows that within a confined space, the elasticity of the gel can be enhanced more than 15 times compared with those under un-restricted conditions. According to our optical microscopy observations and rheological measurements, this drastic enhancement is caused by the structural transition from a multi-domain network system to a single network system once the average size of the fiber network of a given material reaches the lowest dimension of the system. The understanding acquired from this work will provide a novel strategy to manipulate the network structure of soft materials, and exert a direct impact on the micro-engineering of such supramolecular materials in micro and nano scales.

Spherulitic networks : from structure to rheological property

A finite element method based on ABAQUS is employed to examine the correlation between the microstructure and the elastic response of planar Cayley treelike fiber networks. It is found that the elastic modulus of the fiber network decreases drastically with the fiber length, following the power law. The power law of elastic modulus G′ vs the correlation length ξ obtained from this simulation has an exponent of −1.71, which is close to the exponent of −1.5 for a single-domain network of agar gels. On the other hand, the experimental results from multidomain networks give rise to a power law index of −0.49. The difference between −1.5 and −0.49 can be attributed to the multidomain structure, which weakens the structure of the overall system and therefore suppresses the increase in G′. In addition, when the aspect ratio of the fiber is smaller than 20, the radius of the fiber cross-section has a great impact on the network elasticity, while, when the aspect ratio is larger than 20, it has almost no effect on the elastic property of the network. The stress distribution in the network is uniform due to the symmetrical network structure. This study provides a general understanding of the correlation between microscopic structure and the macroscopic properties of soft functional materials.

Architecture and engineering of a supramolecular functional material by manipulating the nano-structure of fiber network

Three-dimensional fiber networks were created from an organogel system consisting mainly of elongated fibrils by using a nonionic surfactant as an additive. The presence of the surfactant molecules manipulates the network structure by enhancing the mismatch nucleation on the growing fiber tips. Both the fiber network structure and the rheological properties of the material can be finely tuned by changing the surfactant concentration, which provides a robust approach to the engineering of supramolecular soft functional materials.

Critical behavior of confined supramolecular soft materials on a microscopic scale

The formation of fiber networks and the resulting rheological properties of supramolecular soft materials are dramatically influenced when the volume of the system is reduced to a threshold. Unlike un-confined systems, the formation of fiber networks under volume confinement is independent of temperature and solute concentration.

Microengineering of supramolecular soft materials by design of the crystalline fiber networks

Crystalline spherulitic fiber networks are commonly observed in polymeric and supramolecular functional materials. The elasticity of materials with this type of network is low if interactions between the individual spherulites are weak (mutually exclusive). Improving the elasticity of these materials is necessary because of their important applications in many fields. In this work, the engineering of the microstructures and rheological properties of this type of material is carried out. A small molecule organogel formed by the gelation of N-lauroyl-L-glutamic acid di-n-butylamide (GP-1) in propylene glycol (PG) is used as an example. The elasticity of this material is improved by controlling the thermodynamic driving force, the supersaturation of the gelator, and by using a selected copolymer additive to manipulate the primary nucleation of GP-1. Because of the weak interactions between the GP-1 spherulites, with the same fiber mass, the elasticity of GP-1/PG gel is less than half of those of the other two gels formed by GP-1 and 2-hydroxystearlic acid in solvent benzyl benzoate (BB), which are supported by interconnecting spherulitic fiber networks. This work develops a robust approach to the engineering of supramolecular functional materials especially those with mutually exclusive spherulite fiber networks.

Controlling nanoparticle formation via sizable cages of supramolecular soft materials

We present a new generic strategy to fabricate nanoparticles in the “cages” within the fibrous networks of supramolecular soft materials. As the cages can be acquired by a design-and-production manner, the size of nanoparticles synthesized within the cages can be tuned accordingly. To implement this idea, both selenium and silver were chosen for the detailed investigation. It follows that the sizes of selenium and silver nanoparticles can be controlled by tuning the pore size of the fiber networks in the material. When the concentration of the gelator is high enough, monodisperse nanoparticles can be prepared. More interestingly, the morphology of the nanoparticles can be altered: silver disks can be formed when the concentrations of both the gelator and silver nitrate are sufficiently low. As the fiber network serves as a physical barrier and semisolid support for the nanoparticles, the stability in the aqueous media and the ease of application of these nanoparticles can be substantially enhanced. This robust surfactant-free approach will not only allow the controlled fabrication of nanoparticles, but also can be applied to the fabrication of composite materials for robust applications.

Size invariance of fibrous networks of supramolecular soft materials during formation under critical volume confinement

The rheological properties of a hierarchically structured supramolecular soft material are mainly determined by the structure of its network. Controlling the thermodynamic driving force of physical gels (one type of such materials) during the formation has proven effective in manipulating the network structure due to the nature of nucleation and growth of the fiber network formation in such a supramolecular soft material. Nevertheless, it is shown in this study that such a property can be dramatically influenced when the volume of the system is reduced to below a threshold value. Unlike un-confined systems, the network structure of such a soft material formed under volume confinement contains a constant network size, independent of the experimental conditions, i.e. temperature and solute concentration. This implies that the size of the fiber networks in such a material is invariable and free from the influence of external factors, once the volume is reduced to a threshold. The observations of this work are significant in the control of the formation of fibrous networks in materials of this type.

Control of crystallization in supramolecular soft materials engineering

As one class of the most important supramolecular functional materials, gels formed by low molecular weight gelators (LMWGs) have many important applications. The key important parameters affecting the in-use performance of a gel are determined by the hierarchical fiber network structures. Fiber networks consisting of weakly interacting multiple domains are commonly observed in gels formed by LMWGs. The rheological properties, particularly the elasticity, of a gel with such a fiber network are weak due to the weak interactions between the individual domains. As achieving desirable rheological properties of such a gel is practically relevant, in this work, we demonstrate the engineering of gels with such a type of fiber network by controlling crystallization of the gelator. Two example gels formed by a glutamic acid derivative in a non-ionic surfactant Tween 80 and in propylene glycol were engineered by controlling the thermodynamic driving force for crystallization. For a fixed gelator concentration, the thermodynamic driving force was manipulated by controlling the temperature for fiber crystallization. It was observed that there exists an optimal temperature at which a gel with maximal elasticity can be fabricated. This will hopefully provide guidelines for producing high performance soft materials by engineering their fiber network structures.

Fitting soft center-near design multifocal lenses

The recent launch of the Air Optix Aqua Multifocal lens (Ciba Vision) means that there are three silicone hydrogel (SiHy) multifocal lenses available. The benefits of SiHy materials have been well documented (Dumbleton, 2006), but until recently they weren't available in multifocal designs. With an aging population, it's likely that a large number of contact lens wearers are approaching presbyopia, which will result in a dramatic increase in demand for multifocal lenses.

Development of a novel soft parallel robot equipped with polymeric artificial muscles

This paper presents the design, analysis and fabrication of a novel low-cost soft parallel robot for biomedical applications, including bio-micromanipulation devices. The robot consists of two active flexible polymer actuator-based links, which are connected to two rigid links by means of flexible joints. A mathematical model is established between the input voltage to the polymer actuators and the robot's end effector position. The robot has two degrees-of-freedom, making it suitable for handling planar micromanipulation tasks. Moreover, a number of robots can be configured to operate in a cooperative manner for increasing micromanipulation dexterity. Finally, the experimental results demonstrate two main motion modes of the robot.