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 masks that work as a virtual
photomask
A photomask (also simply called a mask) is an opaque plate with transparent areas that allow light to shine through in a defined pattern. Photomasks are commonly used in photolithography for the production of integrated circuits (ICs or "chips") ...
. This technique allows for rapid
photopolymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many form ...
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 graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...
light intensity, allowing control of material properties of the fabricated structure with desired spatial distribution.
Materials include
polymers
A polymer () is a substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers. Due to their broad spectrum of properties, b ...
, responsive
hydrogels
A hydrogel is a biphasic material, a mixture of porous and permeable solids and at least 10% of water or other interstitial fluid. The solid phase is a water insoluble three dimensional network of polymers, having absorbed a large amount of ...
,
shape memory polymers and
bio-materials.
Introduction
The
micro electro-mechanical systems (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
stereolithography (3D printing), which developed by
Charles Hull 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 radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of t ...
to cure the solution, which consists
initiators,
monomers
A monomer ( ; ''wikt:mono-, mono-'', "one" + ''wikt:-mer, -mer'', "part") is a molecule that can chemical reaction, react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called ...
and
absorbers, to form each layer of materials. Under the exposure of
UV light
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of t ...
, 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 system. The 2D data is used to fabricate 2D sliced planes in the liquid. Therefore, several 2D planes have to be made in the
CAD system for complicated structures. This stereolithography can be used to fabricate both polymers and metals. Metals are fabricated using the
casting process 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 employed for rituals and rites. Masks have been used since antiquity for both ceremonial and practical purposes, ...
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 employed for rituals and rites. Masks have been used since antiquity for both ceremonial and practical purposes, ...
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
A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...
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, porcela ...
items, including micro- bio reactors to support tissue growth, micromatrices for drug delivery
Drug delivery involves various methods and technologies designed to transport pharmaceutical compounds to their target sites helping therapeutic effect. It involves principles related to drug preparation, route of administration, site-specif ...
and detection and biochemical integrated circuits to simulate biological systems.[
]
Microactuator
Inspired by Mimosa pudica
''Mimosa pudica'' (also called sensitive plant, sleepy grass, sleepy plant, action plant, humble plant, touch-me-not, touch-and-die, or 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 machine element, component of a machine that produces force, torque, or Displacement (geometry), displacement, when an electrical, Pneumatics, pneumatic or Hydraulic fluid, hydraulic input is supplied to it in a system (called an ...
can swell upon the external stimulations such as solvents
A solvent (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 p ...
, temperature and light. In order to control the motion of this actuator, microfluidic
Microfluidics refers to a system that manipulates a small amount of fluids (10−9 to 10−18 liters) using small channels with sizes of ten to hundreds of micrometres. It is a multidisciplinary field that involves molecular analysis, molecular bi ...
channels are embedded inside the leaf of this actuator. With both complex external geometries and internal structures, this soft microactuator can be fabricated using Projection Micro-stereolithography, which is one of the easiest ways to obtain this complex 3D structures. The CAD 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 microactuator can mimic the motion of the human muscle
Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...
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 implicat ...
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 a multicellular organism, 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 a ...
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 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 excret ...
and oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
like the circulatory systems in living organisms. With the ability to fabricate complex 3D structures, the Projection Micro-stereolithography may provide one of the best solutions to this tissue. Like the microactuator, the mold of the artificial tissue is made by CAD. Then the CAD 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 CAD mold. The polymer used in fabricating the tissue is semi-permeable, which allows the 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 excret ...
and oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
in the capillary system go into the tissue during the transportation process. The capillary system is shown to have growth promoting function in yeast cells, which illustrate the viability of this artificial tissue.
See also
* Continuous Liquid Interface Production
References
{{reflist, 30em
3D printing processes