Building, powering, and operating structures that can navigate complex fluidic environments at the sub-mm scale are challenging. We discuss some of the limitations encountered when translating actuation mechanisms and design-concepts from the macro- to the micro-scale. The helical screw-propeller or drill is a particularly useful geometry at small scales and Reynolds numbers, and is one of the mechanisms employed by microorganisms to swim. The shape necessarily requires three-dimensional fabrication capabilities which become progressively more challenging for smaller sizes. Here, we report our work in building and operating these screw-propellers at different sizes. We cover the length scales from the sub 100 nm to drills that are a few hundred microns in length. We use a known physical deposition method to grow micron-sized magnetic propellers that we can transfer to solutions. We have recently succeeded in extending the fabrication scheme to grow nanohelices, and here we briefly review the technical advances that are needed to grow complex shaped nanoparticles. The microstructures can be actuated by a magnetic field and possible applications of the micro- and nanohelices are briefly discussed. We also present a system of polymeric micro-screws that can be produced by micro-injection molding and that can be wirelessly driven by an external rotating magnetic field through biological phantoms, such as agarose gels with speeds of ~200 μm/s. The molding technique faithfully reproduces features down to a few microns. These microdrills can serve as a model system to study minimally invasive surgical procedures, and they serve as an efficient propeller for wireless microrobots in complex fluids. The fabrication scheme may readily be extended to include medically approved polymers and polymeric drug carriers.

Biomedical applications of micro or nanorobots require active movement through complex biological fluids. These are generally non-Newtonian (viscoelastic) fluids that are characterized by complicated networks of macromolecules that have size-dependent rheological properties. It has been suggested that an untethered microrobot could assist in retinal surgical procedures. To do this it must navigate the vitreous humor, a hydrated double network of collagen fibrils and high molecular-weight, polyanionic hyaluronan macromolecules. Here, we examine the characteristic size that potential robots must have to traverse vitreous relatively unhindered. We have constructed magnetic tweezers that provide a large gradient of up to 320 T/m to pull sub-micron paramagnetic beads through biological fluids. A novel two-step electrical discharge machining (EDM) approach is used to construct the tips of the magnetic tweezers with a resolution of 30 μm and high aspect ratio of ~17:1 that restricts the magnetic field gradient to the plane of observation. We report measurements on porcine vitreous. In agreement with structural data and passive Brownian diffusion studies we find that the unhindered active propulsion through the eye calls for nanorobots with cross-sections of less than 500 nm.

Tissue and biological fluids are complex viscoelastic media with a nanoporous macromolecular structure. Here, we demonstrate that helical nanopropellers can be controllably steered through such a biological gel. The screw-propellers have a filament diameter of about 70 nm and are smaller than previously reported nanopropellers as well as any swimming microorganism. We show that the nanoscrews will move through high-viscosity solutions with comparable velocities to that of larger micropropellers, even though they are so small that Brownian forces suppress their actuation in pure water. When actuated in viscoelastic hyaluronan gels, the nanopropellers appear to have a significant advantage, as they are of the same size range as the gel's mesh size. Whereas larger helices will show very low or negligible propulsion in hyaluronan solutions, the nanoscrews actually display significantly enhanced propulsion velocities that exceed the highest measured speeds in Newtonian fluids. The nanopropellers are not only promising for applications in the extracellular environment but small enough to be taken up by cells.

The helix has remarkable qualities and is prevalent in many fields including mathematics, physics, chemistry, and biology. This shape, which is chiral by nature, is ubiquitous in biology with perhaps the most famous example being DNA. Other naturally occurring helices are common at the nanoscale in the form of protein secondary structures and in various macromolecules. Nanoscale helices exhibit a wide range of interesting mechanical, optical, and electrical properties which can be intentionally engineered into the structure by choosing the correct morphology and material. As technology advances, these fabrication parameters can be fine-tuned and matched to the application of interest. Herein, we focus on the fabrication and properties of nanohelices grown by a dynamic shadowing growth method combined with fast wafer-scale substrate patterning which has a number of distinct advantages. We review the fabrication methodology and provide several examples that illustrate the generality and utility of nanohelices shadow-grown on nanopatterns.

Chiral molecules play an important role in biological and chemical processes, but physical effects due to their symmetry-breaking are generally weak. Several physical chiral separation schemes which could potentially be useful, including the propeller effect, have therefore not yet been demonstrated at the molecular scale. However, it has been proposed that complex nonspherical colloidal particles could act as “colloidal molecules” in mesoscopic model systems to permit the visualization of molecular phenomena that are otherwise difficult to observe. Unfortunately, it is difficult to synthesize such colloids because surface minimization generally favors the growth of symmetric particles. Here we demonstrate the production of large numbers of complex colloids with glancing angle physical vapor deposition. We use chiral colloids to demonstrate the Baranova and Zel’dovich (BaranovaN. B.Zel’dovichB. Y.Chem. Phys. Lett.1978, 57, 435) propeller effect: the separation of a racemic mixture by application of a rotating field that couples to the dipole moment of the enantiomers and screw propels them in opposite directions. The handedness of the colloidal suspensions is monitored with circular differential light scattering. An exact solution for the colloid’s propulsion is derived, and comparisons between the colloidal system and the corresponding effect at the molecular scale are made.

Eine ideale „Schablone“ zur Herstellung von makroporösem Polystyrol sind geschäumte Öl-in-Wasser-Emulsionen aus Styrol, Wasser, Glycerin und Natriumdodecylsulfat, die unter Zusatz eines Photoinitiators mit UV-Licht polymerisiert werden. Die Schaumstruktur der Vorstufe wird direkt in das Polymer übertragen, und die erhaltenen Materialien weisen dicht gepackte Zellen mit Fenstern zwischen benachbarten Poren auf (siehe REM-Aufnahme; Skalierung 250 μm).

Angewandte Chemie International EditionVolume 51, Issue 9 p. 2213-2217 CommunicationOpen Access Synthesis of Macroporous Polystyrene by the Polymerization of Foamed Emulsions† Dr. Fabian Schüler, Dr. Fabian Schüler Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart (Germany)Search for more papers by this authorDebora Schamel, Debora Schamel Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart (Germany)Search for more papers by this authorDr. Anniina Salonen, Dr. Anniina Salonen Laboratoire de Physique des Solides, UMR 8502, Université Paris-Sud, Bâtiment 510, 91405 Orsay Cedex (France)Search for more papers by this authorDr. Wiebke Drenckhan, Dr. Wiebke Drenckhan Laboratoire de Physique des Solides, UMR 8502, Université Paris-Sud, Bâtiment 510, 91405 Orsay Cedex (France)Search for more papers by this authorProf. Dr. Michael D. Gilchrist, Prof. Dr. Michael D. Gilchrist School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4 (Ireland)Search for more papers by this authorProf. Dr. Cosima Stubenrauch, Corresponding Author Prof. Dr. Cosima Stubenrauch [email protected] Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart (Germany)Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart (Germany)Search for more papers by this author Dr. Fabian Schüler, Dr. Fabian Schüler Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart (Germany)Search for more papers by this authorDebora Schamel, Debora Schamel Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart (Germany)Search for more papers by this authorDr. Anniina Salonen, Dr. Anniina Salonen Laboratoire de Physique des Solides, UMR 8502, Université Paris-Sud, Bâtiment 510, 91405 Orsay Cedex (France)Search for more papers by this authorDr. Wiebke Drenckhan, Dr. Wiebke Drenckhan Laboratoire de Physique des Solides, UMR 8502, Université Paris-Sud, Bâtiment 510, 91405 Orsay Cedex (France)Search for more papers by this authorProf. Dr. Michael D. Gilchrist, Prof. Dr. Michael D. Gilchrist School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4 (Ireland)Search for more papers by this authorProf. Dr. Cosima Stubenrauch, Corresponding Author Prof. Dr. Cosima Stubenrauch [email protected] Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart (Germany)Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart (Germany)Search for more papers by this author First published: 20 January 2012 https://doi.org/10.1002/anie.201107806Citations: 49 † We acknowledge financial support from Science Foundation Ireland (08/RFP/ENM/1169). AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Graphical Abstract An ideal template for the production of macroporous polystyrene can be prepared from foamed oil-in-water emulsions containing styrene, water, glycerol, and sodium dodecylsulfate. After addition of a photoinitiator the mixture is polymerized with UV light and the foam structure of the precursor is transferred to the polymer. The resulting materials display densely packed cells with windows between adjacent pores (see SEM image; scale bar: 250 μm). Polymer foams are of great commercial interest and are used in diverse fields such as packaging, insulation, and impact protection.1 Depending on the application, a foam must meet specific requirements. Thus great effort has been invested in the determination and manipulation of foam properties. Material composition and cellular structure constitute crucial parameters when it comes to the tailoring of foams. As conventional manufacturing, where foams are produced from polymer melts and blowing agents, is a very complex process, it is hard to control the product’s morphology and properties. In recent years alternative methods for the synthesis of polymer foams have been proposed which make use of templates: a template is generated first and the actual polymer is subsequently synthesized. For example, emulsions have been found to be suitable templates for the synthesis of porous materials. In particular water-in-oil emulsions with a high concentration of the dispersed phase (high-internal-phase emulsions, HIPEs) have attracted a great deal of attention.2 These systems consist of a polymerizable continuous phase and a dispersed phase, which is removed after the polymerization. As most monomers relevant for polymer foam production are hydrophobic, studies usually deal with the polymerization of water-in-oil emulsions, but there are also some examples where oil-in-water emulsions have been polymerized.3 It has been shown that by varying the system parameters the foam morphology and mechanical behavior can be fine-tuned.4 New materials were obtained by subsequent processing of the porous polymers,5 and recently particle-stabilized emulsions (Pickering emulsions) were successfully applied for the synthesis of various nanocomposites.6 The discovery that particles can also attach to gas–liquid interfaces and stabilize air bubbles7 has laid the foundations for another practical route towards the controlled synthesis of porous materials from templates.8 The potential of this approach for the synthesis of different porous polymers has been shown recently.9 A similarly successful approach is to directly generate perfectly monodisperse and highly ordered polymer foams by microfluidic flow focussing techniques.10 We describe herein a novel concept for the synthesis of macroporous polystyrene by the UV-initiated photopolymerization of foamed oil-in-water emulsions. So far, there have been only a few studies on foamed emulsions,11 and, to the best of our knowledge, their use in the synthesis of polymer foams has not been reported yet. In our studies we focused on styrene-based foamed emulsions which are polymerized by UV irradiation; however, we believe that this approach can be applied to a much wider range of monomers and polymerization routes. The route we worked out consists of three principal steps. In the first step, a stable oil-in-water (here: styrene-in-water) emulsion is formulated. Next, the emulsion is foamed by bubbling N2 through the sample. Both the emulsion and the foamed emulsion are stabilized by the same surfactant; we found the anionic sodium dodecyl sulfate (SDS) to have the best performance. Finally, the resulting foamed emulsion is polymerized by exposure to UV light (Figure 1). Figure 1Open in figure viewerPowerPoint Synthesis of macroporous polymers by foaming monomer-containing emulsions and subsequent polymerization. The emulsions were prepared by consecutive addition of styrene, water, glycerol (some samples were prepared without glycerol), and SDS. We homogenized the mixtures by ultrasonication, which led to small and relatively uniform droplets 0.5–1 μm in diameter. At the outset we conducted extensive foaming experiments with mixtures stabilized by different surfactants at various styrene-to-water ratios. Emulsions stabilized by SDS turned out to foam significantly better than those prepared with a variety of non-ionic surfactants. As all SDS-containing emulsions were stable for several hours, we expected them to also be stable during foam generation. A composition of 65 vol % styrene and 35 vol % hydrophilic phase (water or water+glycerol) was found to be the best compromise regarding good foamability and sufficient foam lifetime. Note that the maximum density of a random close packing of spheres is 64 vol %. Thus at 65 vol % styrene the emulsion droplets start to jam, which renders foaming more difficult but at the same time enhances the foam lifetime once the foam has been generated.11a, 12 Figure 2 a shows an example of a single foam bubble surrounded by closely packed emulsion droplets. Figure 2Open in figure viewerPowerPoint a, b) Optical light microscopy pictures of foamed emulsions. c) Evaluation of foam lifetime for a foamed emulsion containing 5 wt % SDS and 15 vol % glycerol. Foam lifetime is defined as the time after which significant collapsing of bubbles appeared (this can be seen on the picture taken after 8 h). Pictures were taken with the optimal system (see text for details). Further optimization was achieved by adjusting the foam generation process and varying the composition of the emulsion. With regard to the first parameter, we found that whipping the emulsions with a mechanical stirrer at mixing speeds of 1600 rpm and a mixing time of 4 min leads to sufficiently stable foamed emulsions with appropriate foam densities and bubble sizes (see the Supporting Information). Figure 2 b displays a typical image of a generated foamed emulsion. The rather broad distribution of bubble sizes is due to the applied foaming technique.13 Figure 2 c illustrates the time-dependent changes in the structure of foamed emulsion. In the following we define “foam lifetime” as the time after which significant collapsing of foam bubbles appears (as seen in the picture taken after 8 h). xxAfter having optimized the foam generation process we adjusted the composition of the emulsion in order to optimize the foam lifetime. (Note that we did not optimize the process in an iterative way although changes in the composition may require an adjustment of the foam generation process and vice versa.) Gas contents were calculated comparing the volume of the nonfoamed emulsion and the volume of the foam generated from it. All foamed emulsions were examined by light microscopy, and average bubble sizes were obtained by evaluating the respective images (see Supporting Information). We first tested the influence of the SDS concentration on the stability of the foamed emulsions. The stability of a foam in the presence of oil droplets is critically governed by the stability of the so-called pseudo-emulsion film which forms between the air/water interface and an oil drop.11b If this film is stable, the oil drops do not enter the air/water interface but accumulate in the plateau borders of the foam, thus slowing down liquid drainage.11 Since the amount of added SDS must be shared between the gas/liquid and liquid/liquid interfaces to stabilize both foam and emulsion, surfactant concentrations much higher than the critical micelle concentration (cmc) were added to the mixture (the cmc for SDS in pure water at room temperature is roughly 0.26 wt %). All emulsions with total SDS concentrations equal to or exceeding 1 wt %, which corresponds to 2.9 wt % in the aqueous phase, yielded stable foams. Figure 3 shows a decrease of the bubble size and an increase of foam lifetime with increasing SDS concentration. Gas contents up to 82 % were achieved, but foams strongly degraded within 2 h, even at the highest surfactant concentrations. Average bubble diameters were between 70 and 90 μm with broad size distributions. With regard to the optimal SDS concentration, we needed to find a compromise between foam lifetime and solubility: On the one hand, the foam properties did not change significantly upon an increase of the SDS concentration from 5 to 7 wt %. On the other hand, the solubility of SDS in pure water at room temperature is approximately 15 wt %.14 Thus a total SDS concentration of 5 wt %, which corresponds to 14.3 wt % in the aqueous phase, was chosen for all further experiments in order to ensure good solubility. Figure 3Open in figure viewerPowerPoint Dependence of bubble diameter (○) and foam lifetime (□) on SDS concentration based on the total sample mass. The emulsions contained 65 vol % styrene and 35 vol % water. The next step was to vary the composition of the continuous phase by partially replacing water with glycerol. As glycerol has a much higher viscosity than water, it slows down the creaming of the emulsions and the drainage of the foamed emulsions.11c We varied the glycerol concentration (cglycerol) while maintainin the total amount of the continuous phase (35 vol %). These emulsions were then foamed and we found that the viscosity did not significantly affect the final foam density. However, foam bubble sizes and bubble size dispersities decreased with increasing cglycerol as is shown in Figure 4. This observation is due to the fact that our foaming technique is based on the application of shear forces to air/fluid interfaces, which are inversely proportional to the viscosity of the fluid.15 Most importantly, the stability of the foamed emulsion increases significantly with increasing cglycerol and thus increasing viscosity of the continuous phase: foams with cglycerol=15 vol % were completely stable for almost 7 h. There are two reasons for this tremendous increase of stability. Firstly, it can be attributed to the smaller and more uniform foam bubbles (less Ostwald ripening). Secondly, the increased viscosity of the emulsion significantly slows down the gravity-driven drainage. Because of solubility problems, the highest cglycerol tested was 15 vol %. Figure 4Open in figure viewerPowerPoint Dependence of bubble diameter (○) and foam lifetime (□) on the glycerol concentration based on the total sample mass (cglycerol=15 vol % means 15 vol % glycerol and 20 vol % water in the total system). The emulsions contained 65 vol % styrene, 35 vol % continuous phase with different amounts of glycerol and 5 wt % SDS. As a result of the prescreening tests the optimal composition of the emulsion was chosen to be 65 vol % styrene, 20 vol % water, 15 vol % glycerol, and 5 wt % SDS. Because of their higher stability during the polymerization, liquid foams with gas contents of only 60–70 % were prepared. These optimized foamed emulsions were then polymerized by exposure to UV light. For the photopolymerization we tested different photoinitiators, which were mixed with the emulsions before foaming. We tested dimethyl benzyl ketal (DBK), benzoin (BZ), diphenylacetone (DPA), and diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), each at a concentration of 2 wt % relative to the overall sample mass. In order to limit the heating of the sample, which destabilizes the foam, the lamps’ infrared radiation was extinguished by an optical filter. We found that an irradiation time of 2 h sufficed to convert the foamed emulsion into polymeric material. The samples were then dried and purified by Soxhlet extraction. Table 1 lists the results from the polymerization experiments with different photoinitiators. Table 1. Results from polymerizing foamed emulsions with different initiators at 2 wt %.[a] Initiator Effect of polymerization on foam structure Mn (GPC RI/UV) [g mol−1][b] PD (GPC RI/UV)[b] DBK macroporosity retained 21 000/20 000 1.8/1.8 BZ macroporosity retained 25 000/26 000 3.7/3.3 TPO macroporosity retained 53 000/44 000 2.2/3.1 DPA structure collapses 25 000/25 000 3.6/3.9 [a] Molecular weights Mn and polydispersities PD were obtained from GPC measurements. [b] RI=refractive index detector, UV=UV light detector. Gel permeation chromatography (GPC) measurements were performed after Soxhlet treatment of the samples, and molecular weights between 20 000 and 53 000 g mol−1 and polydispersities between 1.8 and 3.9 were obtained. It is generally known that higher molecular weights improve the mechanical performance of polymers; hence the molecular weight needs to be increased. All initiators apart from DPA gave macroporous polymers with negligible shrinkage upon solidification. On the other hand, the use of DPA led to a complete collapse of the foam. The consistency of the collapsed material was more slushy than solid, most likely because of low monomer conversion. The analysis of the purified residue indicated the formation of polystyrene with a broad polydispersity. As the highest molecular weights were obtained with the initiator TPO, the subsequent experiments and analyses were conducted with this foam. We found that sintering of the polymer material is required to obtain a smooth surface. Thus differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) measurements were performed to determine suitable conditions for the subsequent thermal treatment of the polymer. It was found that the decomposition of our best polymer foam started at 295 °C, while its glass transition was around 100 °C. Thus sintering was performed at a temperature close to Tg, namely at 110–120 °C, for 3 h. The difficulty of this step consisted in achieving a polymer with a homogeneous surface without causing structure collapse. After the thermal processing, the sample thickness was measured and compared to the initial thickness of the liquid foam. The best batches displayed shrinkages of not more than 20 %. The macroporous polymers were further analyzed by scanning electron microscopy (SEM; see Figure 5). The image shows a continuous macroporous structure with closely packed cells and some openings (so-called windows2d) between adjacent pores. Cracks and scratches in the polystyrene bulk material were eliminated by sintering (see the Supporting Information). We find that the structure of the foamed emulsion—the template—could be transferred to the solid macroporous polystyrene without significant changes. To quantify this, the liquid template and the final polymer foam are compared in Table 2. The density was calculated by weighing specimens of known volume, and the gas content was obtained from the density ratio of macroporous polystyrene and bulk polystyrene (see the Supporting Information). Figure 5Open in figure viewerPowerPoint SEM images of polystyrene foams obtained by polymerizing foamed emulsions with the UV initiator TPO. Table 2. Properties of the foamed emulsion (the template) and the final macroporous polystyrene. Gas content [%] Density [g cm−3] Average bubble/pore size [μm] foamed emulsion 64±5 0.37±0.05 46±12 polymer foam 78±4 0.24±0.04 76±30 As can be seen in Table 2, the gas content of the polymer foam is slightly higher than that of the foamed emulsion, which most likely is due to the removal of the emulsions’ hydrophilic phase. The assumption that all emulsion components except the polymerized monomer can be removed from the final product leads to a final gas content of 77 %, which is very close to our results. Moreover, as can be seen in Figure 6, the pore size distribution of the polymer foam is broader than that of the liquid template and the maximum is shifted to higher values, which is probably due to bubble coalescence during the polymerization. Figure 6Open in figure viewerPowerPoint Bubble (•) and pore size distributions (○) for liquid foamed emulsions and macroporous polystyrene. The average diameters of the windows were calculated to be (30±10) μm. Windows are commonly found in other macroporous polymers that are synthesized from HIPEs. Generally their formation is attributed to the destabilization of the emulsion films upon solidification, but the detailed mechanism of their formation is not yet fully understood.16 In our case we believe that the window-forming mechanism is already initiated in the liquid state. In some places, the small styrene droplets are expelled from the thin aqueous film separating two bubbles—as is commonly seen in foamed emulsions.11f Upon polymerization, those films do not solidify and thus create highly spherical holes. As a result, our approach allows the generation of polymer foams with a nonnegligible fraction of interconnected pores. While the high density and strong connectivity provides mechanical stability, the presence of the windows allows air, fluids, or other materials to penetrate the material. Control over this balance is searched for a wide range of applications, including solid supports, filtration materials, and bio-inspired scaffold structures.2d, 17 In summary we have presented a simple and versatile route for the creation of polystyrene foams from foamed emulsions, which is a promising alternative to other methods that make use of templates for material production. The simplicity of this approach makes any type of foaming method possible, thus offering a tunability of the bubble size and structure of the liquid foam. During polymerization and subsequent processing, the structure of the foamed emulsion was retained and the samples shrank very little. In other words, the use of a foamed emulsion as a template allows the manufacturing of specimens with controlled structural properties. Owing to its generality the strategy proposed by us can be extended to a wide range of other monomers and composites which can be polymerized from emulsions. Future goals include the increase of the molecular weight, a more detailed study of the structure of both the template and the polymer foam, and an extension of the concept to other polymers. Supporting Information Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. 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