Microthermoforming

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Microthermoforming is the abbreviation for microscopic or microscale thermoforming, or, more precisely, for thermoforming of microproducts or microstructure products. Microstructure products means products that have structures in the micrometre range and have their technical function provided by the shape of the microstructure [1]. Thermoforming [2] in turn means shaping of heated and therefore softened semi finished products in the form of thermoplastic polymer films or plates with their edges fixed by three-dimensional stretching. Shaping is carried out mainly by forming the films or plates into female moulds (negative forming) or over male moulds (positive forming). While the other polymer microreplication processes such as micro injection moulding or (vacuum) hot embossing are primary forming processes where forming occurs already in a molten, liquid phase of the heated polymer material, microthermoforming is a secondary forming process where forming occurs in a strongly softened, but still solid phase of the heated polymer.

The microscopic scale is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens or microscope to see them clearly. In physics, the microscopic scale is sometimes regarded as the scale between the macroscopic scale and the quantum scale. Microscopic units and measurements are used to classify and describe very small objects. One common microscopic length scale unit is the micrometre, which is one millionth of a metre.

Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. The sheet, or "film" when referring to thinner gauges and certain material types, is heated in an oven to a high-enough temperature that permits it to be stretched into or onto a mold and cooled to a finished shape. Its simplified version is vacuum forming.

Microstructure small-scale structure of material

Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material can strongly influence physical properties such as strength, toughness, ductility, hardness, corrosion resistance, high/low temperature behaviour or wear resistance. These properties in turn govern the application of these materials in industrial practice. Microstructure at scales smaller than can be viewed with optical microscopes is often called nanostructure, while the structure in which individual atoms are arranged is known as crystal structure. The nanostructure of biological specimens is referred to as ultrastructure. A microstructure’s influence on the mechanical and physical properties of a material is primarily governed by the different defects present or absent of the structure. These defects can take many forms but the primary ones are the pores. Even if those pores play a very important role in the definition of the characteristics of a material, so does its composition. In fact, for many materials, different phases can exist at the same time. These phases have different properties and if managed correctly, can prevent the fracture of the material.

Contents

Moulds for polymer microreplication in general and in particular for microthermoforming can be fabricated by various methods such as mechanical micromachining, lithographic based methods in combination with electroplating (see also the so-called 'LIGA' process) and wet or dry etching. And they can be fabricated of various materials such as metal, silicon and glass.

LIGA

LIGA is a German acronym for Lithographie, Galvanoformung, Abformung that describes a fabrication technology used to create high-aspect-ratio microstructures.

State of the art

For several years now, at Karlsruhe Institute of Technology (KIT), a pressure or high pressure (thermo)forming process is used to fabricate film microchips for capillary electrophoresis (CE) [3–5] and for three-dimensional cell cultivation [6–8]. The process is derived from the macroscopic trapped sheet forming process [2]. This is a simple variation of vacuum or pressure forming without prestretching, i.e. a single stage forming, into a female mould with heating of the plastic sheet using a contact heating plate inside the forming station. The forming air is supplied via through holes in the heating plate. Still in a laboratory scale process, diverse thermoplastic films also from biodegradable polymers such as polycaprolactone (PCL) with thicknesses typically between 20 and 100 μm are thermoformed. This is performed with gas pressures up to 5 MPa into mechanically micromachined cavities of plate shaped micromoulds from brass.

Karlsruhe Institute of Technology University in Karlsruhe

The Karlsruhe Institute of Technology (KIT) is a public research university and one of the largest research and educational institutions in Germany. KIT was created in 2009 when the University of Karlsruhe, founded in 1825 as a public research university and also known as the "Fridericiana", merged with the Karlsruhe Research Center, which had originally been established in 1956 as a national nuclear research center.

Capillary electrophoresis (CE) is a family of electrokinetic separation methods performed in submillimeter diameter capillaries and in micro- and nanofluidic channels. Very often, CE refers to capillary zone electrophoresis (CZE), but other electrophoretic techniques including capillary gel electrophoresis (CGE), capillary isoelectric focusing (CIEF), capillary isotachophoresis and micellar electrokinetic chromatography (MEKC) belong also to this class of methods. In CE methods, analytes migrate through electrolyte solutions under the influence of an electric field. Analytes can be separated according to ionic mobility and/or partitioning into an alternate phase via non-covalent interactions. Additionally, analytes may be concentrated or "focused" by means of gradients in conductivity and pH.

Polycaprolactone polymer

Polycaprolactone (PCL) is a biodegradable polyester with a low melting point of around 60 °C and a glass transition temperature of about −60 °C. The most common use of polycaprolactone is in the production of speciality polyurethanes. Polycaprolactones impart good resistance to water, oil, solvent and chlorine to the polyurethane produced.

First examples of processes coming near to something that could be called 'microthermoforming' originate from the second half of the nineties. So, in 1993, dome shaped polymer microstructures for use in electrical membrane switches were fabricated [9]. This was done between a mating upper and lower metal emboss die with a concave and a convex detail, respectively, first in a hot, then in a second cold press. And in 1999, corrugated sheet like polymer microstructures for use e.g. in electrostatic actuators were fabricated [10]. This was also done between heated tools and counter tools, namely in discontinuous processes between stamps or in continuous processes between rollers. Partly, the counter tool was a soft one in the form of a thicker, unpatterned film or plate made from an easily deformable, e.g. elastomeric material which is able to assume the shape of the hard, metallic tool. In 2006, at the School of Polymer, Textile and Fiber Engineering (PTFE) of the Georgia Institute of Technology (GIT), the same technology approach was used to fabricate similar corrugated sheet like structures in a so-called 'rubber-assisted hot embossing process' [11].

Features and applications

The microthermoforming process including its products can have all the advantageous properties of the powerful macroscopic production process. Moreover, the thermoformed microparts have additional, specific properties appearing only in microscale dimensions and resulting from their unusual morphology. Thermoformed e.g. microfluidic structures have free standing microcavities such as channels and reservoirs and they are thin walled partly in the range of a few micrometers. Specific properties of thermoformed microparts are, amongst others, their high flexibility, their small volume and mass, their low thermal resistance and heat capacity, and their low light absorbance and background fluorescence. Morphology and properties of these microparts now can result in improved or even new, so far unthought of applications.

Compared to the other microreplication processes, in microthermoforming, modifications of the film to be formed remain preserved beyond the forming step due to the already mentioned material coherence during this secondary forming process. This enables surface and bulk modification and functionalisation of the three-dimensionally formed films or membranes, namely as highly resolved micro- and nanopatterns, and all side, i.e. on hardly accessible side walls and even behind undercuts. Thus, e.g. thermoformed chips for three-dimensional cell cultivation can be provided with pores, cell adhesion patterns [6–8], surface topologies and electrodes [12].

In manufacturing, an undercut is a special type of recessed surface that is inaccessible using a straight tool. In turning, it refers to a recess in a diameter generally on the inside diameter of the part. In milling, it refers to a feature which is not visible when the part is viewed from the spindle. In molding, it refers to a feature that cannot be molded using only a single pull mold. In printed circuit board construction, it refers to the portion of the copper that is etched away under the photoresist. In welding, it refers to undesired melting and removal of metal near the weld bead.

Future application fields for microthermoforming are expected to be

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