From MEMS to Bio-MEMS and Bio-NEMS : Manufacturing Techniques and Applications

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However, silicon and SU-8 have shown increased biofouling. In vitro cytotoxicity of MEMS materials [ 7 ]. The interaction between blood proteins and the material is regarded as an important source of thrombogenesis. The adsorption of proteins is explained, from the thermodynamic point of view, in terms of the systems free energy or surface energy. However, adsorption itself does not induce thrombosis.

Theories regarding correlations between thrombogenicity of a material and its surface charge or its binding properties proved not to be useful. Thrombus formation on implant materials is one of the first reactions after deployment and may lead to acute failure due to occlusion and serve as a trigger for neointimal formation. Next to the direct activation by the intrinsic or extrinsic coagulation cascade, thrombus formation can also be initiated directly by an electron transfer process while fibrinogen is close to the surface.

The electronic nature of a molecule can be defined as semiconductor or insulator. Contact activation is possible in the case of a metal since electrons in the fibrinogen molecule are able to occupy empty electronic states with the same energy. Therefore, the obvious way to avoid this transfer is to use a material with a significantly reduced density of empty electronic states within the range of the valence band of the fibrinogen.

This is the case for the used silicon carbide coating. Hemocompatibility leads to the following physical requirements: 1 to prevent the electron transfer, the solid must have no empty electronic states at the transfer level, i.

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This requirements met by a semiconductor with a sufficiently large band gap its valence band edge must be deeper than 1. A material that meets these electronic requirements is silicon carbide in an amorphous, heavily n-doped, hydrogen-rich modification a-SiC:H. The amorphous structure is required in order to avoid any point of increased density of electronic states, especially at grain boundaries. At present, a-SiC:H is known for its high thromboresistance induced by the optimal barrier that this material presents for protein adhesion.

These properties may translate into less protein biofouling and better compatibility for intravascular applications rather than Si. SiC has a relatively low level of fibrinogen and fibrin deposition when contacting blood. These proteins promote local clot formation; thus, the tendency not to adsorb them will resist blood clotting. It is now well established that SiC coatings are resistant to platelet adhesion and clotting both in vitro and in vivo [ 5 ].

In the Bolz et al. The technique provides the most suitable coating process due to its high inherent hydrogen concentration which satisfies the electronically active defects in the amorphous layers.

Bio Micro and Nano Electro-Mechanical Systems (BioMEMS and BioNEMS) - ANU

They used fibrinogen as an example model for thrombogenesis in implants, although most haemoproteins are organic semiconductors. These results support the electrochemical model for thrombogenesis at artificial surfaces and prove that a proper tailoring of the electronic properties leads to a material with superior hemocompatibility. The in vitro test showed that the morphology of the cells was regular.

The a-SiC:H samples showed the same behaviour as the control samples. Blood and membrane proteins have similar band-gaps, because the electronic properties depend mainly on the periodicity of the amino acids, and the proteins differ only in the acid sequence, not in their structural periodicity. Apparently, similar reactions inducing a modification of proteins are responsible for the cell culture results.

A-SiC: H has superior hemocompatibility; its clotting time is percent longer than to that of titanium and pyrolytic carbon. Furthermore, it has been shown that small variations in the preparation conditions cause a significant change in hemocompatibility.

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Therefore, it is of paramount importance to know the exact physical properties of the material in use. This property makes amorphous silicon carbide a suitable coating material for all hybrid designs of biomedical devices. The substrate material can be fitted to the mechanical needs, disregarding its hemocompatibility, whereas the coating ensures the hemocompatibility of the device.

Possible applications are catheters or sensors in blood contact and implants, especially artificial heart valves. They showed decreased thrombogenicity of an amorphous layer of SiC compared to titanium. Several other studies showed that a hydrogen-rich amorphous SiC coating on coronary artery stents is anti-thrombogenic. Three studies showed a benefit that was attributed to the SiC-coated stent. In a direct comparison of the blood compatibility silicon wafers and SiC-coated PECVD silicon wafers, both appeared to provoke clot formation to a greater extent than diamond-like coated silicon wafers; silicon was worse than SiC-coated silicon.

In conclusion, the hemocompatibility of SiC was demonstrated [ 5 ]. MEMS or microelectromechanical systems, is the integration of mechanical elements, sensors, actuators, and electronics on a substrate, in which micro-fabrication technologies are used. The first step when designing a process flow for fabrication of a micromachined device is to choose the structural and other materials which are to be used in the process flow.

Some of the properties of materials that are commonly used in MEMS fabrication are listed in Table 2. Traditionally, silicon and polysilicon have been used most often as the structural materials for MEMS. This was initially due to the wealth of existing knowledge on processing of silicon samples from microelectronic fabrication.

Luckily for the micromachining engineers, silicon has several favourable mechanical properties in addition to its superb electrical specifications that have made it the material of choice for microelectronics. Nevertheless, there are numerous cases where other materials offer significant advantages over silicon. Examples include applications where a piezoelectric material is needed or when the devices are designed to work in harsh environments. On the other hand, the same advantages of SiC over silicon bring up challenges in deposition and etching of SiC films.

The fabrication technologies differ depending on what needs to be accomplished. The aim is to incorporate electronics; they are normally fabricated using standard integrated circuit processes or sequences, such as Complementary Metal-Oxide Semiconductor CMOS technology or bipolar technology. On the mechanical side, the micromechanical components are fabricated using compatible micromachining processes that in essence selectively etch away parts of the silicon wafer or add new structural layers to form a mechanical device, or an electromechanical device if it has additional integrated electronics.

There are several types of methods used for micromechanical fabrications.

From MEMS to Bio-MEMS and Bio-NEMS: Manufacturing Techniques and Applications

Through MEMS, it is possible to incorporate micro-scale types of devices such as motors, pumps, fluidic channels, sample preparation including mixing or vaporization chambers, and various types of sensors including optical sensors that will perform assorted tasks, such as monitoring. Monitoring can be performed in both the physical and chemical sense.

Therefore, MEMS can be thought of as an enabling technology that allows for the development of what we commonly call smart or intelligent systems, which operate without the need for external computing resources. Because MEMS devices are manufactured using batch fabrication techniques similar to those used over multiple decades in the integrated circuit industry, we see unprecedented levels of functionality, reliability, and sophistication being placed on small silicon chips at a relatively low cost.

Therefore, there is significant potential for MEMS technologies. At present, 6H-SiC and 4H-SiC are the only polytypes that are commercially available in large-area, integrated circuit IC -grade wafer form suitable for epitaxial growth of single crystalline films. In contrast, 3C-SiC is not widely available as bulk substrates, but single crystalline films can be epitaxially grown directly on Si wafers despite a significant mismatch in both lattice constant and thermal coefficient of expansion.

Single crystalline 3C-SiC piezoresistive pressure sensors have been fabricated using bulk micromachining for high temperature gas turbine applications. Bare silicon exhibits inadequate tribological performance. SiC films exhibit good tribological performance.

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Surface micromachining involves the monolithic fabrication of suspended microscale structures by selective removal of underlying thin film sacrificial layers. Surface micromachining is inherently an additive process utilizing thin film deposition techniques to produce both structural and sacrificial layers. As such, the primary function of the substrate is to provide mechanical support for the resulting device.

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Patterning of the thin film layers involves wet and dry etching techniques that are sensitive only to the chemical properties of the materials, and not their microstructure or crystallinity, thereby enabling a high degree of flexibility with respect to planar designs. In concept, there is no restriction on the structural and sacrificial materials to be used in the fabrication of a particular device as long as the materials are compatible with each other during the fabrication process.

As such, surface micromachining is not constrained by the properties of the substrate and, thus, can accommodate an extremely wide range of materials, including SiC.

Lecture - 32 MEMS for Biomedical Applications (Bio-MEMS)

The silicon carbide analog to polysilicon is polycrystalline 3C-SiC, hereafter referred to simply as poly-SiC. Poly-SiC is actually more versatile than polysilicon in that it can be deposited directly on SiO 2 and polysilicon. In essence, the process of fabricating MEMS structures in poly-SiC by surface micromachining mirrors that of polysilicon. The main differences are process used to deposit the SiC films, the selection of sacrificial layer material, and the etch recipes used to pattern the structural films. A significant breakthrough in the advancement of SiC surface micromachining was the development of reactive ion etching techniques that are highly selective to SiC, which when combined with MEMS-friendly SiC deposition techniques, allow SiC surface micromachining to follow directly from polysilicon micromachining.

A wide range of micromachined structures, such as lateral resonators, flow sensors, capacitive pressure sensors, micromotors, and microbridge resonators can be fabricated using the deposition, patterning, etching, and sacrificial release techniques commonly used in polysilicon surface micromachining [ 11 ].

Several groups have demonstrated surface micromachining using a-SiC films as structural layers. Examples include RF switches and accelerometers [ 12 ]. Amorphous-SiC films deposited by PECVD generally exhibit a very wide range of residual stress typically compressive in as-deposited films that exhibit a strong dependence on deposition conditions [ 13 ]. Bulk micromachining can generally be defined as a process to fabricate suspended structures by selective bulk removal of the supporting substrate.

Bulk micromachined structures can be comprised of the substrate material itself or thin films that are deposited directly onto the substrate. Unlike surface micromachining, the substrate in bulk micromachined devices is not merely a solid mechanical support, but rather forms a key component of the device structure. To create a SiC-on-insulator substrate, the Si wafer that was originally used as the substrate for SiC growth is removed by etching. The principal factor affecting yield is wafer bowing due to high tensile residual stress in the 3C-SiC films. The techniques developed for Si bulk micromachining, including photoelectrochemical etching, DRIE, and laser micromachining, have been successfully adapted for SiC albeit typically with much lower etch rates [ 13 ].

Among the first SiC MEMS structures to be routinely fabricated were diaphragms, cantilever beams, and related structures fabricated out of single crystalline 3C-SiC films by silicon anisotropic etching.