Projection micro-stereolithography (PµSL) adapts

3D printing 3D printing or additive manufacturing is the construction of a three-dimensional object from a CAD model or a digital 3D model. It can be done in a variety of processes in which material is deposited, joined or solidified under computer ...

technology for micro-fabrication. Digital micro display technology provides dynamic

stereolithography Stereolithography (SLA or SL; also known as vat photopolymerisation, optical fabrication, photo-solidification, or resin printing) is a form of 3D printing technology used for creating models, prototypes, patterns, and production parts in a la ...

masks that work as a virtual

photomask A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography and the production of integrated circuits (ICs or "chips") in particular. Masks are used ...

. This technique allows for rapid

photopolymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...

of an entire layer with a flash of UV illumination at micro-scale resolution. The mask can control individual

pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the s ...

light intensity, allowing control of material properties of the fabricated structure with desired spatial distribution. Materials include

polymers A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic an ...

, responsive

hydrogels A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still dif ...

, shape memory polymers and bio-materials.

Introduction

The

micro electro-mechanical systems Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...

(MEMS) is developing quickly in the past 30 years. Relying on the integration of sensors and actuators, MEMS always demand cheaper, easier and more precise method to fabricate micro size 3-D structures using different materials such as polymers, ceramics and semiconductor materials. The appearance of the Projection Micro-stereolithography improves the development of MEMS by achieving most of the requirements above. This invention is based on the

(3D printing), which developed by

Charles Hull Charles Hull VC (24 July 1890 – 13 February 1953) was an English recipient of the Victoria Cross, the highest and most prestigious award for gallantry in the face of the enemy that can be awarded to British and Commonwealth forces during ...

in 1984. This machine is primarily used to fabricate soft materials such as hydro gels and polymers. The basic theory behind this invention is using

UV light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

to cure the solution, which consists initiators,

monomers In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...

and absorbers, to form each layer of materials. Under the exposure of

, the initiators are transferred into the radicals. Radicals connect monomers together to begin the polymerization process. The absorbers are mixed with monomers to control the depth of UV light penetration. This chemical process allows the areas under UV exposure to become solid state polymers.

History

At first, all micro size stereolithography method utilized the same method as the macro size stereolithography that they direct write materials on the base. The first micro size stereolithography that use the UV light to cure the liquid resin surface is developed by professor Ikuta and Hirowatari in 1993. This fabrication approach is the prototype of today's projection micro-stereolithography. Compared with previous direct writing fabrication methods, this approach has the advantage that it can fabricate each layer simultaneously which increases the yield rate for large production. At that time, 2D shape data was obtained in a

CAD Computer-aided design (CAD) is the use of computers (or ) to aid in the creation, modification, analysis, or optimization of a design. This software is used to increase the productivity of the designer, improve the quality of design, improve c ...

system. The 2D data is used to fabricate 2D sliced planes in the liquid. Therefore, several 2D planes have to be made in the

system for complicated structures. This stereolithography can be used to fabricate both polymers and metals. Metals are fabricated using the

casting process Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a ''casting'', which is ejected ...

after a polymer mold is made. Although improves the yield rate, this method requires a

mask A mask is an object normally worn on the face, typically for protection, disguise, performance, or entertainment and often they have been employed for rituals and rights. Masks have been used since antiquity for both ceremonial and pra ...

for each layer of the final product, which increases process time and cost. Therefore, the fabricate technology is developed again that the

masks A mask is an object normally worn on the face, typically for protection, disguise, performance, or entertainment and often they have been employed for rituals and rights. Masks have been used since antiquity for both ceremonial and practic ...

are replaced by the micromirror display device, which is similar to the projector in our daily life. The micromirror display provides a dynamic mask that can change the patterns electronically. Since multiple masks are displaced by one mask, the processing time and fabrication cost are greatly decrease.

Process

The dynamic mask defines the beam. The beam is focused on the surface of a UV-curable polymer

resin In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on n ...

through a projection lens that reduces the image to the desired size. Once a layer is polymerized, the stage drops the substrate by a predefined layer thickness, and the dynamic mask displays the image for the next layer on top of the preceding one. This proceeds iteratively until complete. The process can create layer thickness on the order of 400 nm. Sub 2 µm horizontal and sub-1 µm vertical resolutions have been achieved, with sub-1 µm feature sizes. Process can work at ambient temperature and atmosphere, although increased nitrogen improves polymerization Production rates of 4 cu mm/hr have been achieved, depending on resin viscosity. Materials can be easily switched during fabrication, enabling integration of multiple material elements in a single process.

Applications

Applications include fabricating microactuators, creating molds,

electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the redox, reduction of cations of that metal by means of a direct current, direct electric cur ...

or (with resin additives)

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelai ...

items, including micro- bio reactors to support tissue growth, micromatrices for

drug delivery Drug delivery refers to approaches, formulations, manufacturing techniques, storage systems, and technologies involved in transporting a pharmaceutical compound to its target site to achieve a desired therapeutic effect. Principles related to dr ...

and detection and biochemical integrated circuits to simulate biological systems.

Microactuator A microactuator is a microscopic servomechanism that supplies and transmits a measured amount of energy for the operation of another mechanism or system. As a general actuator, following standards have to be met: * Large travel * High precision * Fa ...

Inspired by

Mimosa pudica ''Mimosa pudica'' (from la, pudica "shy, bashful or shrinking"; also called sensitive plant, sleepy plant, action plant, touch-me-not, shameplant) is a creeping annual or perennial flowering plant of the pea/legume family Fabaceae. It is often ...

, the leaf of this

actuator An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover". An actuator requires a control device (controlled by control signal) a ...

can swell upon the external stimulations such as

solvents A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

, temperature and light. In order to control the motion of this actuator,

microfluidic Microfluidics refers to the behavior, precise control, and manipulation of fluids that are geometrically constrained to a small scale (typically sub-millimeter) at which surface forces dominate volumetric forces. It is a multidisciplinary field tha ...

channels are embedded inside the leaf of this actuator. With both complex external geometries and internal structures, this soft

microactuator A microactuator is a microscopic servomechanism that supplies and transmits a measured amount of energy for the operation of another mechanism or system. As a general actuator, following standards have to be met: * Large travel * High precision * Fa ...

can be fabricated using Projection Micro-stereolithography, which is one of the easiest ways to obtain this complex 3D structures. The

mold of this actuator is generated in a computer. The sliced 2D images are obtained next. Each 2D image is then projected by the micromirror display and go through the lens to a desired size to the surface of the polymer resin. Since Projection Micro-stereolithography is time-saving, the same experiment can be done on different liquid soft materials in order to learn the swelling effect of them. Based on this contraction and extension of materials caused by a small drop of solvent or a small change in environmental conditions, this

can mimic the motion of the human

muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other ...

and can be used on many soft robotic applications.

Artificial Tissue

Many

reconstructive surgery Reconstructive surgery is surgery performed to restore normal appearance and function to body parts malformed by a disease or medical condition. Description Reconstructive surgery is a term with training, clinical, and reimbursement implica ...

procedures require new tissues when the original tissues are removed because of illnesses. One way to generate this new tissue is to take one part of tissue from another part of the human body and transfer it to the new site. However, this method causes damage to other

organs In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a f ...

while generating new tissues. Therefore, fabricating artificial tissues is a preferred approach to solve this problem. The major limitation of this artificial tissue is the absence of the

capillary system A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...

to transport

nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excr ...

and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

like the circulatory systems in

living organisms In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fungi; ...

. With the ability to fabricate complex 3D structures, the Projection Micro-stereolithography may provide one of the best solutions to this tissue. Like the

, the mold of the artificial tissue is made by

. Then the

mold is transferred to 2D images and projected to the surface of the polymer resin through a lens. The capillary system is embedded in the tissue during the mold designing process in the

mold. The polymer used in fabricating the tissue is semi-permeable, which allows the

and

in the

go into the tissue during the transportation process. The

is shown to have growth promoting function in yeast cells, which illustrate the viability of this artificial tissue.

References

{{reflist, 30em 3D printing processes