Jason J. Benkoski

Dipolar organization and magnetic actuation of flagella-like nanoparticle assemblies

Modeled after the design of eukaryotic protozoa, we fabricated artificial microscopic swimmers through the dipolar assembly of a bidisperse mixture of 250 nm superparamagnetic magnetite colloids and 24 nm ferromagnetic cobalt nanoparticles. The cobalt nanoparticles self-assemble into long, 1-D chains measuring approximately 24 nm × 5 µm. These chains then co-assemble with the magnetite beads to form “head” + “tail” structures. These types of asymmetric “flagella-like” colloidal assemblies were formed and maintained solely through dipolar interactions and is the first demonstration using randomly mixed dispersions of disparate magnetic colloids. When actuated by a pair of orthogonal static and sinusoidal magnetic fields, they undergo an asymmetric undulation that is the essential condition for locomotion at low Reynolds numbers. Based upon their shape, size, and articulation, these assemblies are potentially among the smallest structures capable of overcoming Brownian motion to perform useful locomotion. In addition to the head and tail structure, a variety of irregular structures formed that were incapable of swimming. A design of experiments (DOE) study was therefore implemented to optimize the production of artificial swimmers within a large parameter space that included concentration, the amount of sonication, and magnetic field strength. The artificial swimmers were most prevalent for intermediate concentrations of Co and magnetite particles. Statistical analysis suggested that the permanent dipole of the Co nanoparticles stimulated the assembly of the bidisperse mixture into complex, heterogeneous structures. Demonstration of in situ imaging of the magnetic actuation of these dipolar NP assemblies was conducted by optical microscopy.

Dipolar assembly of ferromagnetic nanoparticles into magnetically driven artificial cilia

Taking inspiration from eukaryotic cilia, we report a method for growing dense arrays of magnetically actuated microscopic filaments. Fabricated from the bottom-up assembly of polymer-coated cobalt nanoparticles, each segmented filament measures approximately 5–15 µm in length and 23.5 nm in diameter, which was commensurate with the width of a single nanoparticle. A custom microscope stage actuates the filaments through orthogonal permanent and alternating magnetic fields. We implemented design of experiments (DOE) to efficiently screen the effects of cobalt nanoparticle concentration, crosslinker concentration, and surface chemistry. The results indicated that the formation of dense, cilia-mimetic arrays could be explained by physical, non-covalent interactions (i.e. dipolar association forces) rather than chemistry. The experiments also determined an optimal Co nanoparticle concentration of approximately 500 µg ml−1 for forming dense arrays near the ends of the permanent magnets, and a critical concentration of approximately 0.3 µg ml−1, below which particle assembly into chains was not observed.

Liquid-Filled Metal Microcapsules

A moisture-sensitive diisocyanate liquid is microencapsulated within a metal shell measuring less than 2 μm thick and 50 μm in diameter. This mild synthesis takes place through a series aqueous processing steps that occur at or near room temperature. Through a combination of emulsification, interfacial polymerization, and electroless plating, one can microencapsulate moisture- or air-sensitive chemicals within a metal seal. The liquid-filled metal microcapsules promise a number of advantages compared to conventional polymeric microencapsulation, including improved mechanical properties and improved barrier properties to gases and organic molecules.

Robust Composite-Shell Microcapsules via Pickering Emulsification

Microencapsulation technology has been increasingly applied toward the development of self-healing paints. Added to paint as a dry powder prior to spraying, the microcapsules store a liquid that can repair the protective barrier layer if released into a scratch. However, self-healing will not occur unless the microcapsules can withstand spray-painting, aggressive solvents in the paint, and long-term exposure to the elements. We have therefore developed a one-pot synthesis for the production of Pickering microcapsules with outstanding strength, solvent resistance, and barrier properties. Octadecyltrimethoxysilane-filled (OTS) microcapsules form via standard interfacial polycondensation, except that silica nanopowder (10-20 nm diameter) replaces the conventional surfactant or hydrocolloid emulsifier. Isophorone diisocyanate (IPDI) in the OTS core reacts with diethylenetriamine, polyethylenimine, and water to form a hard polymer shell along the interface. Compared to pure polyurea, the silica-polyurea composite improves the shelf life of the OTS by 10 times. The addition of SiO2 prevents leaching of OTS into xylenes and hexanes for up to 80 days, and the resulting microcapsules survive nebulization through a spray gun at 620 kPa in a 500 cSt fluid.

Synergy between Galvanic Protection and Self-Healing Paints.

Painting is a cost-effective technique to delay the onset of corrosion in metals. However, the protection is only temporary, as corrosion begins once the coating becomes scratched. Thus, an increasingly common practice is to add microencapsulated chemical agents to paint in order to confer self-healing capabilities. The additives ability to protect the exposed surface from corrosion depends upon (i) how long the chemical agent takes to spread across the exposed metal; (ii) how long the agent takes to form an effective barrier layer; and (iii) what happens to the metal surface before the first two steps are complete. To understand this process, we first synthesized 23 ± 10 μm polyurea microcapsules filled with octadecyltrimethoxysilane (OTS), a liquid self-healing agent, and added them to a primer rich in zinc, a cathodic protection agent. In response to coating damage, the microcapsules release OTS into the scratch and initiate the self-healing process. By combining electrochemical impedance spectroscopy, chronoamperometry, and linear polarization techniques, we monitored the progress of self-healing. The results demonstrate how on-demand chemical passivation works synergistically with the cathodic protection: zinc preserves the surface long enough for self-healing by OTS to reach completion, and OTS prolongs the lifetime of cathodic protection.

Effects of Engineered Wettability on the Efficiency of Dew Collection

Surface wettability plays an important role in dew collection. Nucleation is faster on hydrophilic surfaces, while droplets slide more readily on hydrophobic surfaces. Plants and animals in coastal desert environments appear to overcome this trade-off through biphilic surfaces with patterned wettability. In this study, we investigate the effects of millimeter-scale wettability patterns, mimicking those of the Stenocara beetle, on the rate of water collection from humid air. The rate of water collection per unit area is measured as a function of subcooling (ΔT = 1, 7, and 27 °C) and angle of inclination (from 10° to 90°). It is then compared for superbiphilic, hydrophilic, hydrophobic, and surperhydrophobic surfaces. For large subcooling, neither wettability nor tilt angle has a significant effect because the rate of condensation is so great. For 1 °C subcooling and large angles, hydrophilic surfaces perform best because condensation is the rate-limiting step. For low angles of inclination, superhydrophobic samples are best because droplet sliding is the rate-limiting step. Superbiphilic surfaces, in contrast to their superior fog collecting capabilities, generally collected dew at the slowest rate due to their inherent contact angle hysteresis. Theoretical considerations suggest that this finding may apply more generally to surfaces with patterned wettability.

Solid Lipid Nanoparticles (SLNs) for Intracellular Targeting Applications.

Nanoparticle-based delivery vehicles have shown great promise for intracellular targeting applications, providing a mechanism to specifically alter cellular signaling and gene expression. In a previous investigation, the synthesis of ultra-small solid lipid nanoparticles (SLNs) for topical drug delivery and biomarker detection applications was demonstrated. SLNs are a well-studied example of a nanoparticle delivery system that has emerged as a promising drug delivery vehicle. In this study, SLNs were loaded with a fluorescent dye and used as a model to investigate particle-cell interactions. The phase inversion temperature (PIT) method was used for the synthesis of ultra-small populations of biocompatible nanoparticles. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenylphenyltetrazolium bromide (MTT) assay was utilized in order to establish appropriate dosing levels prior to the nanoparticle-cell interaction studies. Furthermore, primary human dermal fibroblasts and mouse dendritic cells were exposed to dye-loaded SLN over time and the interactions with respect to toxicity and particle uptake were characterized using fluorescence microscopy and flow cytometry. This study demonstrated that ultra-small SLNs, as a nanoparticle delivery system, are suitable for intracellular targeting of different cell types.

Surfactant sculpting of biologically inspired hierarchical surfaces

We describe a method for fabricating biologically inspired hierarchical surfaces in a single step through surfactant self-assembly at an oil/water interface. The key to this system is the use of polydimethylsiloxane-diacrylate for the oil phase, which makes it possible to solidify these delicate structures with UV photocuring. Scanning electron microscopy (SEM) and 3-D optical profilometry reveals morphologies that capture the randomness, fractal geometry, and hierarchical organization of natural materials. The morphology is controlled by surfactant type, surfactant concentration, viscosity, film thickness, and time. The experimental evidence is consistent with a spontaneous increase in surface area driven by a transiently negative surface tension. Spontaneous emulsification generates distinct morphologies for a given surfactant and surfactant concentration in a manner reminiscent of phase behavior in a ternary phase diagram. When emulsification cannot keep pace with the increase in surface area, buckles form. These perturbations are then amplified at increasing length scales by dewetting and the Rayleigh–Taylor instability.

Imaging magnetic flux lines with iron oxide nanoparticles using a “fossilized liquid assembly”

Directed self-assembly of nanomaterials via external fields is an attractive processing tool for industry as it is inherently inexpensive and flexible. Direct observations of this process are however challenging due to the nano and meso length scales involved. The self-assembly of magnetic nanoparticles in particular has gained much recent interest for applications ranging from biomedical imaging and targeted cancer therapy to ferrofluid mechanical damping devices, that rely on the state of aggregation and alignment of the nanoparticles. We utilize an oil–water platform to directly observe directed self-assembly of magnetic nanoparticles that are field ordered into two-dimensional mesostructures through the fossilized liquid assembly (FLA) method. Our system consisted of polymer-coated iron-oxide nanoparticles (25 nm) which were assembled in the vicinity of the interface between a crosslinkable hydrophobic monomer (UV-polymerizable) oil, and water through the use of external magnetic fields, and then cured with UV light. This flash curing system effectively provides a snapshot of the assembly process and allows for direct visualization of assemblies through the use of both atomic force and optical microscopy. In this study, entire magnetic flux field lines in various geometrical configurations were successfully modelled and mapped out by the magnetic nanoparticles, both in-plane and in perpendicular orientations utilizing FLA. The assemblies showed strong directional selectivity and alignment with the flux field lines and provided evidence of strong dipole interactions which partially caused aggregate sedimentation.

Chemiluminescent solid lipid nanoparticles (SLN) and interations with intact skin

We report the synthesis and characterization of a novel nanoparticle formulation designed for skin penetration for the purpose of skin imaging. Solid lipid nanoparticles (SLNs), a drug delivery vehicle, were used as the matrix for targeted delivery of peroxide-sensitive chemiluminescent compounds to the epidermis. Luminol and oxalate were chosen as the chemiluminescent test systems, and a formulation was determined based upon non-toxic components, lotion-like properties, and longevity/visibility of a chemiluminescent signal. The luminescence lifetime was extended in the lipid formulation in comparison to the chemiluminescent system in solution. When applied to porcine skin, our formulation remained detectable relative to negative and positive controls. Initial MTT toxicity testing using HepG2 cells have indicated that this formulation is relatively non-toxic. This formulation could be used to image native peroxides present in tissue that may be indicative of skin disease.