Christophe A. Serra

A Continuous Flow Synthesis of Micrometer‐Sized Actuators from Liquid Crystalline Elastomers

Adv. Mater. 2009, 21, 4859–4862 2009 WILEY-VCH Verlag G T IO N Liquid crystalline elastomers (LCEs) are weakly crosslinked polymers that contain shape-anisotropic moieties (mesogens) that self organize into liquid crystalline phases. It was proposed by de Gennes in 1975 that these materials would undergo a shape change during the phase transition from the liquid crystalline to the isotropic state, if all mesogens were ordered into the same direction, forming a monodomain. The ability to change shape on application of a certain external stimulus led to the fabrication of actuators based on these ‘‘intelligent’’ materials. These are mainly macroscopic actuators, having sizes of millimeters or centimeters. However, in recent years there has been a growing interest in the preparation of micrometer sized actuators, as these are interesting for novel fields of science such as micromechanics and robotics. Here we present the use of a microfluidic setup to prepare monodisperse, monodomainic, and micrometer-sized liquid-crystalline elastomer beads that show a strong and rapid shape change of about 70% in length during the phase transition. We show that the particle size as well as the quality of the monodomain can be controlled by the operating parameters of the microfluidic setup. The key step in preparing LCE-based actuators is the overall orientation of the liquid crystalline director (ideally the formation of a monodomain) before the material is crosslinked. Methods used in the previous cited works are mainly the stretching of pre-crosslinked films, the drawing of fibers from a polymer melt and the use of electric or magnetic fields. All these methods have in common that they are complex multistep processes that are difficult to automate and not suitable for the continuous preparation of a large number of actuators. In addition, they are problematic for preparing samples in the micrometer size region. Using microfluidics on the other hand allows the continuous preparation of a large number of monodisperse micro-particles with a minimum of time and effort. In addition we argue, that the polymerization of the droplets while they are flowing through a tube increases the tendency of the mesogens to adopt a monodomainic director field configuration, thus giving the particles actuation properties. In our approach a liquid crystallinemonomer wasmixed with a crosslinker and a photoinitiator, melted into the isotropic phase and injected through a thin needle into a co-flowing stream of immiscible silicone oil. The resulting droplets were cooled into the liquid-crystalline phase and polymerized by irradiation with UV-light while flowing through a piece of thin tubing. To achieve this, we constructed a novel microfluidic setup, which is based on earlier works of Serra and Zhang. In this approach the mixing of monomer and continuous phase is carried out via a fused silica capillary in a T-junction. The main challenge for the new setup was temperature control. As all known LC-monomers are solid at room temperature, we placed the T-junction as well as the tube containing the monomer mixture in a heat bath, which was set above the monomer clearing temperature. Two syringe pumps were used, one providing the flow of the continuous phase, the other one for pushing a low viscous oil into the monomer tube, while also providing flow. An uplight microscope was used to observe droplet formation at the end of the needle. The tube containing the monomer droplets continued out of the heat bath and was rolled onto a hot plate with its temperature set to that of the liquid crystalline phase of the monomer. UV-light was shone on the tube, initiating radical polymerization and crosslinking inside the droplets in the LC-phase. Several aspects were considered for choosing an appropriate LC material for this project. Polymeric materials were excluded because their viscosities are too high to be pumped through thin capillaries while in melt. In order to induce an orientation of the mesogens, the material has to be processed (crosslinked) in the liquid crystalline phase, thus making the use of solvents to reduce viscosity impossible. Therefore we needed a monomer that was already liquid crystalline and could easily and rapidly be polymerized and crosslinked in the flow setup. In addition, a strong coupling between the mesogen and the resulting polymer is important, as this is a prerequisite for a strong shape change of the actuator. Finally, the transition temperature for the material must occur in a temperature range to which the whole reactor setup can be heated. The choice fell on a three-core mesogen with a polymerizable acrylate group attached laterally over a flexible spacer (see Fig. 1 for chemical structure), which was described earlier by Patrick Keller. It has a nematic phase between 72 and 98 8C and has been used for actuator applications before. For the preparation of crosslinked polymer particles this LC-monomer was mixed with 10mol % of hexanedioldiacrylate (chemical structure also in Fig. 1) and 2wt % of the photoinitiator Lucirin TPO. The mixture was melted and injected to the monomer tube of the setup. In a microfluidic setup the particle size is controlled mainly by two parameters: the viscosity of

Mass transfer improvement by secondary flows: Dean vortices in coiled tubular membranes

A helically wound hollow-fibre (or tubular) membrane module was studied in an oxygenation operation with water flowing in the laminar regime inside the tube. Data are compared with a conventional module where straight hollow-fibre membranes are in parallel alignment. In the former design, the results are analyzed taking into account the formation of secondary flow (Dean vortices) and a new mass transfer correlation is presented. In the latter design, results are consistent with the Leveque equation. It is shown that the presence of vortices gives better performance in terms of oxygen transfer. Improvement factors were in the range of 2 to 4.

Use of air sparging to improve backwash efficiency in hollow-fiber modules

The use of air in backwash of hollow-fiber modules was investigated experimentally from bench to full scale. Modules operated in a dead-end and outside-in mode: they were fouled by either a bentonite suspension or a raw river water and then backwashed in presence of air. The air was injected into the retentate compartment either in combination with a reversed permeate flux or together with feed water after a brief permeate back flow. Results indicate that the cake layer is instantaneously lifted off by the reversed permeate flux and is concentrated in the free volume of the module. To remove it from the module and recover the feed concentration, this volume has to be rinsed with a volume at least three times as big. The air, by its piston-like action, improves material removal and reduces the volume of concentrated foulant to be flushed. So the backwash time is reduced and its efficiency is improved. An optimum air flow rate can be found that is independent of the water flow rate used to flush the module free-volume.

Microfluidics: a focus on improved cancer targeted drug delivery systems.

Pharmaceutical science aims to localize the pharmacological activity of the drug at the site of action. Targeted drug delivery systems can directly deliver the payload to the desired site of action without undesired interaction with normal cells. This is especially important for anticancer drugs to avoid side effects and improve therapeutic response and patient compliance. Number of targeted drug delivery systems for anticancer drugs are in market and many more are in research phase. Most of the methods so far used suffer from poor drug loading, variation in composition, attachment of targeting ligands to carriers, and in vivo and in vitro cellular uptake in cancer cell. Recently microfluidic techniques are gaining attention from researchers and formulation scientists due to the ability of having a better control over the above said parameters not to mention saving cost, material, time and the possibility offered to synthesize different system morphologies from nano to microscale. This article reviews the recent advances in the design of various targeted systems obtained through microfluidics and to some extent addresses challenges and hurdles faced during cancer cell treatment.

Dead-end ultrafiltration in hollow fiber modules: Module design and process simulation

Abstract Ultrafiltration in a hollow-fiber module operating with outside-in and dead-end flow at a constant flow rate was simulated using a model that takes into account the longitudinal pressure drops inside the fibers and within the fiber bundle. The model considers both the filtration phase during which the membrane is fouled by the formation of a filter cake and the backwash phase in which it is cleaned, so as to predict the net rate of production of the module during an operating cycle. The results show that there is a combination of packing density and fiber diameter that gives a maximum net flow rate. Furthermore, this model allows the influence of operating conditions and feed properties on the module performance to be estimated. This can be used to determine how operating parameters must be modified when there is a change in the feed properties.

Co-axial capillaries microfluidic device for synthesizing size- and morphology-controlled polymer core-polymer shell particles

An easy assembling-disassembling co-axial capillaries microfluidic device was built up for the production of double droplets. Uniform polymer core-polymer shell particles were synthesized by polymerizing the two immiscible monomer phases composing the double droplet. Thus poly(acrylamide) core-poly(tri(propylene glycol) diacrylate) shell particles with controlled core diameter and shell thickness were simply obtained by adjusting operating parameters. An empirical law was extracted from experiments to predict core and shell sizes. Additionally uniform and predictable non-spherical polymer objects were also prepared without adding shape-formation procedures in the experimental device. An empirical equation for describing the lengths of rod-like polymer particles is also presented.

Flux improvement by Dean vortices: ultrafiltration of colloidal suspensions and macromolecular solutions

Coiled and straight hollow-fibre modules have been built and tested; the permeate flux obtained in ultrafiltration with these two geometries is compared for two feeds: a colloidal bentonite suspension and a dextran solution. In the case of colloidal suspensions, the secondary flows induced by the coiled geometry allow fouling to be reduced and the permeate flux is multiplied by a factor of up to 2. An empirical relationship is proposed to express the limiting flux of permeate as a function of both the velocity and some geometrical parameters of the coiled modules. Analogous results are obtained during the ultrafiltration of dextran. It is also shown that under certain conditions almost no deposit was formed; the permeate flux under these conditions is three times higher for coiled modules than for straight ones. For a given energy expenditure and ultrafiltration process, the gain in permeate flux can reach a factor of 1.8.

Polyacrylamid/silver composite particles produced via microfluidic photopolymerization for single particle-based SERS microsensorics.

A micro-continuous-flow process was applied for the preparation of swellable polyacrylamide particles incorporating silver nanoparticles. These sensor particles are formed from a mixture of a colloidal solution of silver nanoparticles and monomer by a droplet-based procedure with in situ photoinitiation of polymerization and a subsequent silver enforcement in batch. The obtained polymer composite particles show a strong SERS effect. Characteristic Raman signals of aqueous solutions of adenine could be detected down to 0.1 μM by the use of single sensor particles. The chosen example demonstrates that the composite particles are suitable for quantitative microanalytical procedures with a high dynamic range (3 orders of magnitude for adenine).

Numerical simulation of polymerization in interdigital multilamination micromixers

Free radical polymerization in microfluidic devices modeled with the help of numerical simulations is discussed. The simulation method used allows the simultaneous solvation of partial differential equations resulting from the hydrodynamics, thermal and mass transfer (convection, diffusion and chemical reaction). Three microfluidic devices are modeled, two interdigital multilamination micromixers respectively with a large and short focusing section, and a simple T-junction followed by a microtube reactor together considered as a bilamination micromixer with a large focusing section. The simulations show that in spite of the heat released by the polymerization reaction, the thermal transfer in such microfluidic devices is high enough to ensure isothermal conditions. Moreover, for low radial Peclet number, microfluidic devices with a large focusing section can achieve better control over the polymerization than a laboratory scale reactor as the polydispersity index obtained is very close to the theoretical limiting value. As the characteristic dimension of the microfluidic device increases, i.e. for high radial Peclet number, the reactive medium cannot be fully homogenized by the diffusion transport before leaving the system resulting in a high polydispersity index and a loss in the control of the polymerization.

Microfluidic conceived drug loaded Janus particles in side-by-side capillaries device

A side-by-side capillaries microfluidic device was developed to fabricate drug loaded poly(acrylamide)/poly(methyl acrylate) Janus particles in the range of 59-240 μm by UV-assisted free radical polymerization. This system was characterized in terms of continuous and dispersed phases flow rates (Qc/Qd), monomer composition of the two compartments, surfactant nature and concentration, outlet tube diameter and UV intensity. These factors were adequately controlled to get different particle shapes ranging from core-shell to bi-compartmental particles. For the latter, a low surfactant concentration (0.75 wt.%) was necessary when the two dispersed phases were pumped at equal flow rate, while at high surfactant concentration, dispersed phases flow rates have to be changed. FTIR analysis suggested complete polymerization of monomers and cytotoxicity test showed these particles were biocompatible having LD 50 of 9 mg/mL. Both ketoprofen and sodium fluorescein were released in sustained release manner at pH 6.8 by following a diffusion type release mechanism. Drug release was faster for bigger particles and found to result from the irregular distribution of the two phases and indentation on bigger particles as revealed by SEM analysis. In comparison, sodium fluorescein release was slower which was attributed to low encapsulation but could be modified by decreasing crosslinker concentration.