The field of cantilever-based sensing emerged in the mid-1990s and is today a well-known technology for label-free sensing which holds promise as a technique for cheap, portable, sensitive and highly parallel analysis systems. The research in sensor realization as well as sensor applications has increased significantly over the past 10 years. In this review we will present the basic modes of operation in cantilever-like micromechanical sensors and discuss optical and electrical means for signal transduction. The fundamental processes for realizing miniaturized cantilevers are described with focus on silicon- and polymer-based technologies. Examples of recent sensor applications are given covering such diverse fields as drug discovery, food diagnostics, material characterizations and explosives detection.

http://iopscience.iop.org/article/10.1088/0034-4885/74/3/036101/pdf

Cantilever-like micromechanical sensors

Lab-on-a-chip devices

In this Review, the interplay between materials and microfluidics is examined, with the discussion focused on how recent advances in materials fabrication have expanded the frontiers of microfluidic platforms and how the new microfluidic capabilities are, in turn, furthering materials design.

Interplay between materials and microfluidics

We report the successful fabrication and testing of 3D printed microfluidic devices with integrated membrane-based valves. Fabrication is performed with a low-cost commercially available stereolithographic 3D printer. Horizontal microfluidic channels with designed rectangular cross sectional dimensions as small as 350 μm wide and 250 μm tall are printed with 100% yield, as are cylindrical vertical microfluidic channels with 350 μm designed (210 μm actual) diameters. Based on our previous work [Rogers et al., Anal. Chem. 83, 6418 (2011)], we use a custom resin formulation tailored for low non-specific protein adsorption. Valves are fabricated with a membrane consisting of a single build layer. The fluid pressure required to open a closed valve is the same as the control pressure holding the valve closed. 3D printed valves are successfully demonstrated for up to 800 actuations.

/pdf/3d-printed-microfluidic-devices-with-integrated-valves-3uvp3g9m5s.pdf

3D printed microfluidic devices with integrated valves

Casimir effect on the critical pull-in gap and pull-in voltage of nanoelectromechanical switches is studied. An approximate analytical expression of the critical pull-in gap with Casimir force is presented by the perturbation theory. The corresponding pull-in parameters are computed numerically, from which one can notice the nonlinear effect of Casimir force on the pull-in parameters. The detachment length has been presented, which increases with increasing thickness of the beam.

Casimir effect on the pull-in parameters of nanometer switches. Microsys Technol

To determine the maximum possible length of freestanding micromachined cantilevers, in this paper we provide a theoretical analysis of three important forces on cantilevers, namely acceleration, Casimir and Coulomb forces. The analysis provides theoretical limits to cantilever lengths separate from the well-known effects of surface adhesion and capillary collapse. This analysis offers an insight into the problem of in-use stiction in microstructures, which is a major source of functional failure in dynamic micromechanical systems. In this paper we conclude with a table that lists the maximum free standing length of microstructures that would offer reliable operation without stick–slip motion, excluding the possible effects of surface adhesion.

https://iopscience.iop.org/article/10.1088/0960-1317/12/6/317/pdf

Theoretical limits on the freestanding length of cantilevers produced by surface micromachining technology

Although PMMA can be exposed using a variety of exposure sources, deep-UV at 254 nm is of interest because it is relatively inexpensive. Additionally, deep-UV sources can be readily scaled to large area exposures. Moreover, this paper will show that depths of over 100µm can be created in commercial grade PMMA using an uncollimated source. These depths are sufficient for creating microfluidic channels. This paper will provide measurements of the dissolution depth of commercial grade PMMA as a function of the exposure dose and etch time, using an IPA:H2O developer. Additionally, experiments were run to characterize the dependence of the dissolution rate on temperature and agitation. The patterned substrates were thermally bonded to blank PMMA pieces to enclose the channels and ports were drilled into the reservoirs. The resulting fluidic systems were then tested for leakage. The work herein presents the patterning, development and system behaviour of a complete microfluidics system based on commercial grade PMMA.

http://iopscience.iop.org/article/10.1088/0960-1317/18/11/115029/pdf

Deep-UV patterning of commercial grade PMMA for low-cost, large-scale microfluidics

A novel design for hingeless out-of-plane microstructures is presented. These structures can be assembled to 90° by a single-point actuation, which can be provided by, for example, a microelectronics wirebonder or a microprober station. Both wirebonders and microprober stations are commonly available to microfabrication facilities, and therefore the assembly method described here introduces a practical and economical approach to the creation of out-of-plane structures. The microstructure designs can be used in many types of microfabrication processes, and in particular have been fabricated using both PolyMUMPs and an SU-8 technology developed at Simon Fraser University. In addition to the fabricated devices, we will present the results of finite element analysis (FEA). Also reported here are tests for positional repeatability and reliability.

https://iopscience.iop.org/article/10.1088/0960-1317/17/7/014/pdf

Automated assembly of hingeless 90° out-of-plane microstructures

Poly(methyl methacrylate) (PMMA) is a transparent thermoplastic with important applications as a positive resist for various radiation sources. When used as a photoresist, PMMA is typically used with wavelengths shorter than 240 nm, as that is the commonly accepted upper limit of effectiveness. However, the authors have shown patterning of nonamplified PMMA films at 254 nm, which is significant because 254 nm radiation can be produced using inexpensive low-pressure mercury vapor lamps. Data for the etch depth as a function of exposure dose (0–12 h), developer temperature (20–35 °C), and etch time were collected. Dissolution rates of up to many microns a minute are possible, and the dissolution rate ratio of exposed over unexposed PMMA can reach over 3000. This demonstrates the feasibility of PMMA exposure using deep-UV at 254 nm.

Deep-UV exposure of poly(methyl methacrylate) at 254 nm using low-pressure mercury vapor lamps

SU-8 is finding increased use as a structural polymer MEMS material due to its biocompatibility, mechanical properties and low cost. The goal of this work is to expand the use of SU-8 through the creation of SU-8-based surface-micromachining processes that use polydimethylglutarimide (PMGI) as a sacrificial layer. PMGI is a deep-UV positive resist, used mainly for bilayer lift-off processes. PMGI is a good sacrificial layer candidate, as it is spinable at a wide variety of thicknesses, is photopatternable and has a glass transition temperature greater than the processing temperatures required for SU-8. PMGI is shown to be useful as a sacrificial layer for SU-8 surface micromachining processes with one freestanding layer with patterned metal, single-layer devices with more than one thickness, and two layer devices. Two classes of devices were fabricated with the developed processes. The first class of devices are compliant mechanisms, including bent-beam actuators, thermal isolation platforms and out-of-plane grippers. The second class of devices fabricated are freely moving devices such as hinged plates and gears, which require the use of true kinematic joints, such as scissor hinges, staple hinges and pin joints.

https://iopscience.iop.org/article/10.1088/0960-1317/18/7/075011/pdf

R.W. Johnstone

Papers

Theoretical limits on the freestanding length of cantilevers produced by surface micromachining technology

Deep-UV patterning of commercial grade PMMA for low-cost, large-scale microfluidics

Automated assembly of hingeless 90° out-of-plane microstructures

Deep-UV exposure of poly(methyl methacrylate) at 254 nm using low-pressure mercury vapor lamps

Polydimethylglutarimide (PMGI) as a sacrificial material for SU-8 surface-micromachining