Dispersion stabilities and rheological properties of fumed silica suspensions

Porous Microneedles Through Direct Ink Drawing with Nanocomposite Inks for Transdermal Collection of Interstitial Fluid

Interstitial fluid (ISF) is an attractive alternative to regular blood sampling for health checks and disease diagnosis. Porous microneedles (MNs) are well suited for collecting ISF in a minimally invasive manner. However, traditional methods of molding MNs from microfabricated templates involve prohibitive fabrication costs and fixed designs. To overcome these limitations, this study presents a facile and economical additive manufacturing approach to create porous MNs. Compared to traditional layerwise build sequences, direct ink drawing with nanocomposite inks can define sharp MNs with tailored shapes and achieve vastly improved fabrication efficiency. The key to this fabrication strategy is the yield-stress fluid ink that is easily formulated by dispersing silica nanoparticles into the cellulose acetate polymer solution. As-printed MNs are solidified into interconnected porous microstructure inside a coagulation bath of deionized water. The resulting MNs exhibit high mechanical strength and high porosity. This approach also allows porous MNs to be easily integrated on various substrates. In particular, MNs on filter paper substrates are highly flexible to rapidly collect ISF on non-flat skin sites. The extracted ISF is used for quantitative analysis of biomarkers, including glucose, = calcium ions, and calcium ions. Overall, the developments allow facile fabrication of porous MNs for transdermal diagnosis and therapy.

Multifunctional Nanocomposite Yield-Stress Fluids for Printable and Stretchable Electronics

Compliant materials are crucial for stretchable electronics. Stretchable solids and gels have limitations in deformability and durability, whereas active liquids struggle to create complex devices. This study presents multifunctional yield-stress fluids as printable ink materials to construct stretchable electronic devices. Ionic nanocomposites comprise silica nanoparticles and ion liquids, while electrical nanocomposites use the natural oxidation of liquid metals to produce gallium oxide nanoflake additives. These nanocomposite inks can be printed on an elastomer substrate and stay in a solid state for easy encapsulation. However, their transition into a liquid state during stretching allows ultrahigh deformability up to the fracture strain of the elastomer. The ionic inks produce strain sensors with high stretchability and temperature sensors with high sensitivity of 7% °C-1. Smart gloves are further created by integrating these sensors with printed electrical interconnects, demonstrating bimodal detection of temperatures and hand gestures. The nanocomposite yield-stress fluids combine the desirable qualities of solids and liquids for stretchable devices and systems.

Multifunctional Microneedle Patches via Direct Ink Drawing of Nanocomposite Inks for Personalized Transdermal Drug Delivery

Additive manufacturing, commonly known as 3D printing, allows decentralized drug fabrication of orally administered tablets. Microneedles are comparatively favorable for self-administered transdermal drug delivery with improved absorption and bioavailability. Due to the cross-scale geometric characteristics, 3D-printed microneedles face a significant trade-off between the feature resolution and production speed in conventional layer-wise deposition sequences. In this study, we introduce an economical and scalable direct ink drawing strategy to create drug-loaded microneedles. A freestanding microneedle is efficiently generated upon each pneumatic extrusion and controlled drawing process. Sharp tips of ∼5 μm are formed with submillimeter nozzles, representing 2 orders of magnitude improved resolution. As the key enabler of this fabrication strategy, the yield-stress fluid inks are formulated by simply filling silica nanoparticles into regular polymer solutions. The approach is compatible with various microneedles based on dissolvable, biodegradable, and nondegradable polymers. Various matrices are readily adopted to adjust the release behaviors of the drug-loaded microneedles. Successful fabrication of multifunctional patches with heterogeneously integrated microneedles allows the treatment of melanoma via synergistic photothermal therapy and combination chemotherapy. The personalized patches are designed for cancer severity to achieve high therapeutic efficacy with minimal side effects. The direct ink drawing reported here provides a facile and low-cost fabrication strategy for multifunctional microneedle patches for self-administering transdermal drug delivery.

Simultaneous rheology and cure kinetics dictate thermal post-curing of thermoset composite resins for material extrusion

Thermoset composites are excellent candidates for material extrusion because they shear thin during extrusion but retain their shape once deposited via a yield stress. However, thermal post-curing is often required to solidify these materials, which can destabilize printed parts. Elevated temperatures can decrease the rheological properties responsible for stabilizing the printed structure before crosslinking solidifies the material. These properties, namely the storage modulus and yield stress, must therefore be characterized as a function of temperature and extent of reaction for various filler loadings. This work utilizes rheo-Raman spectroscopy to measure the storage modulus and dynamic yield stress as a function of temperature and conversion in epoxy-amine resins with fumed silica mass fractions up to 10 %. Both rheological properties are sensitive to conversion and particle loading, but only the dynamic yield stress is reduced by elevated temperatures early in the cure. Notably, the dynamic yield stress increases with conversion well before the chemical gel point. These findings motivate a two-step cure protocol that starts at a low temperature to mitigate the drop in dynamic yield stress, then ramps up to a high temperature when the dynamic yield stress is no longer at risk of decreasing to rapidly drive conversion to near completion. The results suggests that structural stability can be improved without resorting to increasing filler content, which limits control over the final properties, laying the groundwork for future studies to evaluate the stability improvements provided by the multi-step curing schedules.

Poly(lactic acid)/Plasticizer/Nano-Silica Ternary Systems: Properties Evolution and Effects on Degradation Rate

Plasticized nanocomposites based on poly(lactic acid) have been prepared by melt mixing following a two-step approach, adding two different oligomeric esters of lactic acid (OLAs) as plasticizers and fumed silica nanoparticles. The nanocomposites maintained a remarkable elongation at break in the presence of the nanoparticles, with no strong effects on modulus and strength. Measuring thermo-mechanical properties as a function of aging time revealed a progressive deterioration of properties, with the buildup of phase separation, related to the nature of the plasticizer. Materials containing hydroxyl-terminated OLA showed a higher stability of properties upon aging. On the contrary, a synergistic effect of the acid-terminated plasticizer and silica nanoparticles was pointed out, inducing an accelerated hydrolytic degradation of PLA: materials at high silica content exhibited a marked brittleness and a dramatic decrease of molecular weight after 16 weeks of aging.

A Printable and Conductive Yield-Stress Fluid as an Ultrastretchable Transparent Conductor

In contrast to ionically conductive liquids and gels, a new type of yield-stress fluid featuring reversible transitions between solid and liquid states is introduced in this study as a printable, ultrastretchable, and transparent conductor. The fluid is formulated by dispersing silica nanoparticles into the concentrated aqueous electrolyte. The as-printed features show solid-state appearances to allow facile encapsulation with elastomers. The transition into liquid-like behavior upon tensile deformations is the enabler for ultrahigh stretchability up to the fracture strain of the elastomer. Successful integrations of yield-stress fluid electrodes in highly stretchable strain sensors and light-emitting devices illustrate the practical suitability. The yield-stress fluid represents an attractive building block for stretchable electronic devices and systems in terms of giant deformability, high ionic conductivity, excellent optical transmittance, and compatibility with various elastomers.

Selective Laser Sintering-Based 4D Printing of Magnetism-Responsive Grippers

Components fabricated by four-dimensional (4D) printing hold the potential for applications in soft robotics because of their characteristics of responding to external stimuli. Grippers, being the common structures used in robotics, were fabricated by the selective laser sintering (SLS)-based 4D printing of magnetism-responsive materials and tested for remote-controllable deformation in an external magnetic field. A composite material consisting of magnetic Nd2Fe14B powder and thermoplastic polyurethane powder was selected as the raw material for the SLS; the magnetic particle acquired permanent magnetism by magnetization after the SLS process. Microscopic characterization showed the homogeneous dispersion of magnetic particles inside the polymer matrix. The magnetic induction intensity distribution was systematically investigated by both experiments and numerical simulations. The reliability of the numerical model proposed was justified by the excellent consistency between them. The deformation of the grippers could be regulated by tuning the magnetic particle content and the distance from the external magnet; the deformation mechanism is investigated numerically. The magnetic driving force and the corresponding horizontal displacement are calculated, thus having high accuracy compared with the existing research that obtained the deformation amount by only visual inspection. Mechanical properties of the SLS-fabricated magnetic polymer composite specimens were also studied because of their close relationship with the deformation behaviors. These findings provide guidance for future research on controllable deformation and driving force calculation for 4D printing.

Dispersion stability and rheological properties of silica suspensions in aqueous solutions

This historical perspective overviews the dispersion stability and rheological properties of fumed and colloidal silica suspensions in aqueous solutions as a function of the volume fraction of silica (ϕ) (where ϕ is ≤0.1). The silica suspensions exist in a gel state at lower ϕ in acidic conditions than at alkaline pH. The steady-state shear viscosities of silica suspensions at acid conditions exhibit shear thinning behavior at lower ϕ than in alkaline conditions; the magnitudes of the dynamic moduli of the silica suspensions at acidic pH are larger than those at alkaline pH. Changes in the dispersion stability and rheological behavior of the silica suspensions may be attributable to the addition of salt, which decreases electrostatic repulsion. Furthermore, the effects of polymer adsorption on the dispersion stability and rheological behavior of hydrophilic or hydrophobic silica suspensions are discussed.

Aggregation of Silica Nanoparticles in Sol-Gel Processes to Create Optical Coatings with Controllable Ultralow Refractive Indices

Optical coatings with controllable ultralow refractive indices are of profound significance in optical areas. However, it remains a challenge to fabricate such coatings using a simple method. Here we develop an effective and simple approach to create ultra-low-index coatings. This approach was based on a modified sol-gel process, with a key process that involved the aggregation of silica nanoparticles via the addition of a polymer surfactant (e.g., polyvinylpyrrolydone) in sols before coating. The approach involves three steps: the synthesis of silica sols under ammonia catalysis in ethanol (Stöber method), the addition of polyvinylpyrrolydone in the silica sols to induce the aggregation of the silica nanoparticles, and the formation of ultra-low-index coatings by depositing the aggregated silica sols on substrates. Through varying the aggregation extent, this approach produced coatings with controllable refractive indices ranging from 1.17 to 1.07. To the best of our knowledge, the minimum index value of 1.07 from our coating is among the lowest refractive indices ever reported. The ultra-low-index coatings demonstrated excellent optical properties, with which perfect quarter-wavelength antireflection coatings (maximum transmittance ∼100%) and broadband antireflection coatings (transmittance >98% from 400 to 1100 nm) can be prepared. One advantage of the antireflection coatings is that their transmission is less dependent on the refractive index and the thickness of the stacking layer, which make it promising in large-scale production. Moreover, the coatings can be made hydrophobic (water contact angle 136°) by exposing the coatings to a hexamethyldisilazane atmosphere, exhibiting high environmental stability in a humid environment. The aggregation of silica nanoparticles in sol-gel processes provides a scalable alternative to the current approaches for creating ultra-low-index coatings.

Stability and rheological properties of silica suspensions in water- immiscible liquids

This paper overviews silica suspensions in water-immiscible liquids, with an emphasis on their dispersion stability and rheological properties as a function of the surface characteristics of silica powders at lower silica volume fractions, ϕ, than 0.1. In addition, a critical review is presented the manufacturing process of silica powder by considering their microstructures. Hydrophilic fumed silica powders are in a gel state at lower ϕ than hydrophobic fumed silica powders in water-immiscible liquids. The interaction between the surface silanol groups is dominant in the former; whereas in the latter, the mutual interaction between the surface hydrophobic moieties and the dispersion media is favored. Moreover, the dynamic moduli of the hydrophobic fumed silica suspensions strongly depend on the mutual interaction between the hydrophobic moieties and the dispersion media. Their magnitudes become larger as mutual interactions increase. In addition, the effects of the adsorption of polymers and non-adsorbing polymers on the dispersion stability and rheological behavior of hydrophilic or hydrophobic fumed silica suspensions are discussed, by considering their small-angle neutron scattering (SANS) curves. The precipitated silica suspensions are more compact and form smaller microstructures than the fumed silica suspensions and their gels correspond to the weak-link gel.

Application of Fumed Silica as a Support during Oxidative Desulfurization

Here, a hydrophilic fumed silica (F-SiO₂) was used as a support, and we place phosphotungstic acid (HPW) onto the F-SiO₂ via a simple impregnation method normally used to prepare a HPW/F-SiO₂ catalyst, which is used in oxidative desulfurization processes. A number of characterization analyses were used, such as X-ray diffraction, Fourier transform infrared, and Transmission electron microscopy, to prove that the HPW catalyst was homogeneously distributed on the F-SiO₂. The structural parameters of the catalyst and the support were tested with Brunauer-Emmett-Teller, and it was confirmed that the catalyst is a mesoporous material. Energy-dispersive spectrometry was used to characterize the distribution of the active component distribution. Catalytic performance was investigated using the catalytic oxidative desulfurization process. During optimal conditions, the removal effect of dibenzothiophene (DBT) in simulated oil can reach 100%. After 13 cycles, catalytic activity is still high, and the DBT conversion can still attain 95.362%.

Understanding the role of hydrogen bonding in the aggregation of fumed silica particles in triglyceride solvents

HYPOTHESIS

Fumed silica particles are thought to thicken organic solvents into gels by aggregating to form networks. Hydrogen bonding between silanol groups on different particle surfaces causes the aggregation. The gel structure and hence flow behaviour is altered by varying the proportion of silanol groups on the particle surfaces. However, characterising the gel using rheology measurements alone is not sufficient to optimise the aggregation. We have used confocal microscopy to characterise the changes in the network microstructure caused by altering the particle surface chemistry.

EXPERIMENTS

Organogels were formed by dispersing fumed silica nanoparticles in a triglyceride solvent. The particle surface chemistry was systematically varied from oleophobic to oleophilic by functionalisation with hydrocarbons. We directly visualised the particle networks using confocal scanning laser microscopy and investigated the correlations between the network structure and the shear response of the organogels.

FINDINGS

Our key finding is that the sizes of the pore spaces in the networks depend on the fraction of silanol groups available to form hydrogen bonds. The reduction in the network elasticity of gels formed by methylated particles can be accounted for by the increasing pore size and tenuous nature of the networks. This is the first report that characterises the changes in the microstructure of fumed silica particle networks in non-polar solvents caused by manipulating the particle surface chemistry.