Janine K. Nunes

Scalable, shape-specific, top-down fabrication methods for the synthesis of engineered colloidal particles.

The search for a method to fabricate nonspherical colloidal particles from a variety of materials is of growing interest. As the commercialization of nanotechnology continues to expand, the ability to translate particle-fabrication methods from a laboratory to an industrial scale is of increasing significance. In this feature article, we examine several of the most readily scalable top-down methods for the fabrication of such shape-specific particles and compare their capabilities with respect to particle composition, size, shape, and complexity as well as the scalability of the method. We offer an extensive examination of particle replication in nonwetting templates (PRINT) with regard to the versatility and scalability of this technique. We also detail the specific methods used in PRINT particle fabrication, including harvesting, purification, and surface-modification techniques, with an examination of both past and current methods.

The Patterning of Sub-500 nm Inorganic Oxide Structures†

Elastomeric perfluoropolyether molds are applied to pattern arrays of sub-500 nm inorganic oxide features. This versatile soft-lithography technique can be used to pattern a wide range of materials; in this work inorganic oxides including TiO2 , SnO2 , ZnO, ITO, and BaTiO3 are patterned on a variety of substrates with different aspect ratios. An example of TiO2 posts is shown in the figure.

Multifunctional shape and size specific magneto-polymer composite particles.

Interest in uniform multifunctional magnetic particles is driven by potential applications in biomedical and materials science. Here we demonstrate the fabrication of highly tailored nanoscale and microscale magneto-polymer composite particles using a template based approach. Regiospecific surface functionalization of the particles was performed by chemical grafting and evaporative Pt deposition. Manipulation of the particles by an applied magnetic field was demonstrated in water and hydrogen peroxide.

Fabrication of multiphasic and regio-specifically functionalized PRINT ® particles of controlled size and shape

Using Particle Replication In Nonwetting Templates (PRINT ® ) technology, multiphasic and regio-specifically functionalized shape-controlled particles have been fabricated that include end-labeled particles via post- functionalization; biphasic Janus particles that integrate two compositionally different chemistries into a single particle; and more complex multiphasic shape-specific particles. Controlling the anisotropic distribution of matter within a particle creates an extra parameter in the colloidal particle design, providing opportunities to generate advanced particles with versatile and tunable compositions, properties, and thus functionalities. Owing to their robust characteristics, these multiphasic and regio-specifically functionalized PRINT particles should be promising platforms for applications in life science and materials science.

Electrically driven alignment and crystallization of unique anisotropic polymer particles.

Micrometer-sized monodisperse anisotropic polymer particles, with disk, rod, fenestrated hexagon (hexnut), and boomerang shapes, were synthesized using the particle replication in nonwetting templates (PRINT) process, and investigations were conducted on aqueous suspensions of these particles when subjected to alternating electric fields. A coplanar electrode configuration, with 1 to 2 mm electrode gaps (20-50 V ac, 0.5-5.0 kHz) was used, and the experiments were monitored with fluorescence microscopy. For all particle suspensions, the field brought about significant changes in the packing and orientation. Extensive particle chaining and packing were observed for the disk, rod, and hexnut suspensions. Because of the size and geometry of the boomerang particles, limited chaining was observed; however, the field triggered a change from random to a more ordered packing arrangement.

Fabricating Shaped Microfibers with Inertial Microfluidics

DOI: 10.1002/adma.201400268 Using microfl uidics, a few different techniques have emerged for producing fi bers with different cross-sectional shapes; however, for most of these techniques the range of shapes is limited, such as hollow, semi-circular, and ribbon cross-sectional shapes. [ 10 ] One promising microfl uidic technique that is able to realize more complex fi ber cross-sectional shapes is the hydrodynamic focusing method developed by Ligler and coworkers. [ 11 ] This method uses recessed chevron and striped structures in the channel walls to focus the precursor fi ber solution and the sheathing liquids into a desired shape, which can be predicted with computational fl uid dynamics (CFD). Ligler and coworkers demonstrated that they could use custom software, Tiny-Toolbox, [ 12 ] to design their microfl uidic components and simulate the focusing process. In this Communication, we present the synthesis of shaped polymeric fi bers using a software-enabled inertial microfl uidic technique. Using a method described by Amini et al. [ 13 ] where fl uid streams can be sculpted into desired shapes in a microchannel containing a sequence of pillars, we designate one of the sculpted streams as a template for fi ber fabrication to produce fi bers with different noncircular cross-sectional shapes. We use a computer-aided design (CAD) tool, uFlow, that has a stored library of pre-computed fl uid deformations that are produced by individual pillars in the fl ow channel. [ 14 ] In uFlow, these individual pillar-induced deformations are design elements that can be combined to create a unique sequence of pillar positions along and transverse to the fl ow direction that will result in complex sculpted fl uid fl ows of miscible fl uids. As the CFD simulation step is built into the uFlow software, the tool is quick and simple to use, and accessible to users with all computational skill levels as it circumvents the need for any additional time-consuming simulation steps. The software allows the user to immediately observe the effects of adding pillars, changing lateral pillar position, pillar diameter and fl ow rate ratio on the shape of the fl ow deformation. Consequently, uFlow is a useful predictive tool for the rapid screening and design of microchannels for shaped multi-stream fl ows. While other software, optimized for creeping fl ow, can also be used to perform design for shaped fi bers, uFlow models fl ow at a higher Reynolds number, preferred for fi ber shaping because it is associated with large fl uid deformations following fl ow past a sequence of cylinders. We use the program to design channels containing different sequences of pillars specifi cally for fi ber generation using a common poly(ethylene glycol) diacrylate (PEG-DA) photopolymerization. For the experiments presented herein, three monomer solution streams are fl owed into the channel, and the central stream, which is the only stream containing photoinitiator, becomes the template for the solid fi ber. The two outer streams, which are non-reactive because of the absence Among synthetic fi bers, the circular cross-section is most prevalent; however, it is not uncommon to manufacture fi bers with noncircular cross-sections. We will use the term ‘shaped fi bers’ to describe fi bers with any cross-sectional shape that is not circular. Depending on the application, whether the fi bers are used in fabrics and textiles, [ 1 ] as insulating materials for sound and heat, [ 2 ] for light propagation, [ 3 ] as high surface area membranes and fi ltration materials, [ 4 ] or as engineered substrates for biological applications, [ 5,6 ] it has been observed that the cross-sectional shape affects the properties of the fi ber. For example, the crosssectional shape of fi bers manufactured for textile applications is reported to have an effect on bulkiness since packing density is infl uenced by shape, coeffi cient of friction, which imparts fabric roughness and infl uences overall tactility, fl exural rigidity, which affects the softness or stiffness of fabrics, visual properties such as luster and color, and wicking properties. [ 1 ] Shaped fi bers are also being considered for applications in tissue engineering. For example, it has been shown that the higher surface area afforded by shaped fi bers is useful in fi ber scaffolds for improved cell proliferation and more rapid scaffold degradation when compared to fi bers with circular cross-sections. [ 6 ] In addition, multifaceted or ridged fi ber substrates show improved cell orientation and alignment for applications such as guided cell growth when compared to smooth fi bers. [ 5 ]

Microfluidic tailoring of the two-dimensional morphology of crimped microfibers

We synthesized uniform crimped microfibers with controlled dimensions using a microfluidic approach, whereby a liquid jet flows from a narrow channel into a wider channel. The liquid jet, sheathed by an immiscible non-reacting liquid, undergoes ultraviolet (UV)-initiated gelation upstream of the channel widening. At the channel widening, the reacting jet may buckle due to an axial compressive stress, and the transient buckled morphology is preserved in the structure of the resulting solid fiber as the gelation reaction rapidly goes to completion. We investigated the effects of different experimental conditions, such as flow rate, UV light position, concentration of photoinitiator and UV light intensity on controlling the morphology of the microfibers, and we observed that the degree of crimp in the microfiber is dependent mainly on the extent of reaction.

Control of the length of microfibers

Uniform polymeric microfibers of prescribed lengths were synthesized in microfluidic devices using two different approaches--valve actuation and pulses of ultraviolet (UV) light. The more versatile valve approach was employed to demonstrate control of the length of the microfiber as a function of the frequency of valve actuation.

Multicompartment microfibers: fabrication and selective dissolution of composite droplet-in-fiber structures

We present a microfluidic method to continuously produce multicompartment microfibers, where embedded single or double emulsion droplets are regularly spaced along the length of the fiber. Both hydrophobic and hydrophilic compounds can be encapsulated in different microcompartments of the fiber for storage, selective dissolution, and delivery applications, as well as to provide multifunctionality.

Generation of antibubbles from core-shell double emulsion templates produced by microfluidics.

We report the preparation of antibubbles by microfluidic methods. More specifically, we demonstrate a two-step approach, wherein a monodisperse water-in-oil-in-water (W/O/W) emulsion of core-shell construction is first generated via microfluidics and freeze-dried thereafter to yield, upon subsequent reconstitution, an aqueous dispersion of antibubbles. Stable antibubbles are attained by stabilization of the air-water interfaces through a combination of adsorbed particles and polymeric surfactant. The antibubbles strongly resemble the double emulsion templates from which they were formed. When triggered to release, antibubbles show complete release of their cores within about 100 ms.