Granite Countertops

Microstructure Synthesis by Flow Lithography and Polymerization Suitable for Wide Range of Devices Say MIT Scientists

Massachusetts Institute of Technology (Cambridge, MA) earned U.S. Patent 7,709,544 for microstructure synthesis by flow lithography and polymerization.  The morphology and chemistry of the microstructures that are synthesized can be independently controlled to produce large numbers of uniquely shaped, functionalized polymeric microstructures for applications including drug delivery, biosensing, microactuation, and fundamental studies on self-assembly and rheology, among others. The high-through-put of the synthesis processes of the invention enables the practical achievement of polymeric microstructure synthesis on a scale that is required for many of these applications. 
According to inventors Patrick S. Doyle, Daniel C. Pregibon and Dhananjay Dendukuri, the invention overcomes the limitations of conventional polymeric microstructure synthesis to provide lithographic-based microfluidic microstructure synthesis techniques that can continuously synthesize polymeric microstructures of varied complex shapes and chemistries. In one example polymeric microstructure synthesis method of the invention, a monomer stream is flowed at a selected flow rate through a fluidic channel; and at least one shaped pulse of illumination is projected to the monomer stream. This illumination projection defines in the monomer stream a shape of at least one microstructure corresponding to the illumination pulse shape while polymerizing that microstructure shape in the monomer stream by the illumination pulse.

In a further example synthesis process provided by the invention, a monomer stream is flowed at a selected flow rate, through a fluidic channel and illumination is projected to the monomer stream to polymerize at least one microstructure in the monomer stream by the illumination. At least one polymerization termination species is provided, at internal walls of the fluidic channel, which terminates at the channel walls active polymerization sites at which polymerization could occur during polymerization of the microstructure. This quenches polymerization at those sites and preserves a non-polymerized volume of the monomer stream adjacent to the channel walls.

This high-throughput technique enables superior control over microstructure geometry, shape, composition, and anisotropy. Non-spheroidal polymeric microstructures each having a plurality of distinct material regions can be synthesized by the technique, as can planar polymeric microstructures each having a plurality of distinct material regions. 

Polymeric microstructures are important for a wide range of applications, including MEMS, biomaterials, drug delivery, self-assembly, and other applications. The ability to controllably synthesize such microstructures, herein defined as structures having features in the size range of about 10 nanometers (nm)  to about 1000 microns (mu.m) is increasingly significant for enabling applications such as paints, rheological fluids, catalysis, diagnostics, and photonic materials. Monodisperse polymeric microstructures, herein defined as having a microstructure size distribution where >90% of the distribution lies within 5% of the median microstructure size, are particularly desirable as they can exhibit a constant and predictable response to external fields and can self-assemble in a predictable manner. 

Like amphiphilic molecules, microstructures possessing both hydrophilic and hydrophobic sections exhibit a tendency to orient themselves in order to minimize their surface energy. While thermal energy alone is insufficient to enable these microstructure to explore their energy landscape, external energy provided by, e.g., agitation was employed to aid the microstructures to find their energy minima and self-assemble. The wedge-shaped amphiphilic microstructures were isolated and induced, using agitation, to assemble either in a pure aqueous phase or at the interface of w/o or o/w emulsions. The results showed that the particles have a strong tendency to orient themselves in order to minimize their surface energy.

The MIT method of immiscible microstructure synthesis is of sufficient generality to enable synthesis of a wide range of non-spherical particles with such chemical anisotropy; e.g., amphiphilic particles with a rod-like hydrophobic tail and a disk-shaped hydrophilic head can be synthesized. Such a library of particles can be useful when studying the effect of geometry and chemical anisotropy on meso-scale self assembly and rheology. In addition, structures with more complicated motifs like w-o-w can also be formed quite easily.

The invention provides lithographic-based microfluidic methodology that can be employed to continuously or near-continuously synthesize polymeric microstructures of varied complex shapes, chemistries, and functionalities with an elegant dual lithography-polymerization step.

FIGS. 10A-10H are schematic views of additional example polymeric microstructures that can be synthesized by lithographically defining a microstructure shape while polymerizing that shape across or through a plurality of concurrently flowing monomer streams. In the example of FIG. 10A a rectangular mask 300 and four monomer streams are employed to produce a corresponding rectangular, four-part microstructure 305. In the example of FIG. 10B, the rectangular mask 30 is employed with three monomer streams, one of which includes, e.g., a porogen 205 and one of which includes, e.g., DNA strands 212, in the manner described above, to produce a three-part microstructure 308 having multiple distinct functionalities. 
In the example of FIG. 10C a circular mask 310 is employed with six monomer streams, three of which are identical and three of which are distinct, to produce a disc microstructure 311 that incorporates three regions 312, 314, 316 of identical composition separated by three regions 318, 320, 322 of distinct composition. In this example microstructure, each of the six resulting regions of the microstructure is characterized by a distinct width, set by the flow rates of the corresponding monomer streams.

FIG. 10D schematically shows a polymeric microstructure 330 synthesized across two monomer streams and employing a mask shape that provides a distinct feature geometry for the microstructure region corresponding to each of the monomer streams. FIG. 10E schematically shows a polymeric microstructure 334 synthesized across five monomer streams and employing an S-shaped mask 336. The differentiation in polymer species is preserved across the structure.

In accordance with the invention, the co-flowing monomer streams employed to synthesize polymeric microstructures can be miscible, chemically similar streams. In this case, the interface between different regions in a resulting microstructure is not sharp, due to molecular diffusion. Diffusion-limited mixing that is characteristic of laminar flow can be exploited to ensure that the streams flow distinctly through a microfluidic device, but molecular diffusion between adjacent polymerized regions can here occur. If co-flowing monomer streams instead are immiscible, then a sharp interface between synthesized polymeric regions can be enforced, producing microstructures having regions with sharply-segregated chemistries and differing surface energies.

FIG. 10F is an example of a polymeric microstructure 340 synthesized across two monomer streams one of which provides a hydrophilic polymer region 342 and one of which provides a hydrophobic polymer region 344. The interface 346 between the two regions in the microstructure is characteristically curved due to the disparate surface energies of the two regions. The amphiphilic nature of such a microstructure can be exploited to enable self-assembly of populations of such microstructures.

FIGS. 10G-10H are schematic views of polymeric microstructures 352, 358, synthesized by lithography and polymerization through multiple co-flowing monomer streams rather than across co-flowing monomer streams. As shown in FIG. 10G, employing a circular mask 350, a three-material microstructure is produced in which the composition of the circular cross section of the structure is changed through the thickness of the structure. Similarly, as shown in FIG. 10H, a star-shaped polymeric microstructure is synthesized across four differing monomer streams to produce a microstructure 358 having a differing composition through the thickness of the star-structure, with the composition constant in a given star-shaped plane of the structure. 

Conventionally, polymer microstructure synthesis is carried out by a batch process such as photolithography, stamping, or emulsion polymerization, or by an emulsion-based microfluidic technique such as flow-through microfluidic synthesis. Although these techniques have provided significant advances in microstructure synthesis, it is found in general that each limits microstructure composition and/or geometry. For example, photolithographic techniques generally limit the microstructure material to that which is compatible with a photolithographic process, e.g., requiring a photoresist as the structural material. Historically, the synthesis of polymeric microstructures with microfluidics has focused almost exclusively on spheroidal microstructures, in part because the minimization of microstructure interfacial energy leads to the formation of spheres or deformations of spheres such as rods, ellipsoids or discs, or cylinders.

In addition to these limitations in polymeric microstructure composition and geometry, conventional polymeric microstructure synthesis generally requires isotropic structural arrangements of materials. Further, the through-put of such processes is typically limited by a requirement for making one structure at a time or a limited photo-mask-defined field of structures at a time. These limitations in polymeric microstructure synthesis through-put, microstructure geometry, morphology, and functionality have restricted the ability to address the growing number of critical applications for which polymeric microstructures could be well suited.