Mems for Biomedical Applications

Shekhar Bhansali, Abhay Vasudev

• Publisher: Woodhead Publishing
• Year: 2012
• Print ISBN: 9780857091291
• eBook ISBN: 9780857096272
• Topic: Technology & Engineering
• Subtopic: Biomedical Science
• Index: Technology & Engineering

The application of Micro Electro Mechanical Systems (MEMS) in the biomedical field is leading to a new generation of medical devices. MEMS for biomedical applications reviews the wealth of recent research on fabrication technologies and applications of this exciting technology.The book is divided into four parts: Part one introduces the fundamentals of MEMS for biomedical applications, exploring the microfabrication of polymers and reviewing sensor and actuator mechanisms. Part two describes applications of MEMS for biomedical sensing and diagnostic applications. MEMS for in vivo sensing and electrical impedance spectroscopy are investigated, along with ultrasonic transducers, and lab-on-chip devices. MEMS for tissue engineering and clinical applications are the focus of part three, which considers cell culture and tissue scaffolding devices, BioMEMS for drug delivery and minimally invasive medical procedures. Finally, part four reviews emerging biomedical applications of MEMS, from implantable neuroprobes and ocular implants to cellular microinjection and hybrid MEMS.With its distinguished editors and international team of expert contributors, MEMS for biomedical applications provides an authoritative review for scientists and manufacturers involved in the design and development of medical devices as well as clinicians using this important technology.- Reviews the wealth of recent research on fabrication technologies and applications of Micro Electro Mechanical Systems (MEMS) in the biomedical field- Introduces the fundamentals of MEMS for biomedical applications, exploring the microfabrication of polymers and reviewing sensor and actuator mechanisms- Considers MEMS for biomedical sensing and diagnostic applications, along with MEMS for in vivo sensing and electrical impedance spectroscopy

1

Microfabrication of polymers for bioMEMS

P. Rezai, W-I. Wu and P.R. Selvaganapathy, McMaster University, Canada

Abstract:

Use of microfabrication methods derived from the semiconductor industry have been adapted to new materials in the recent past to produce electromechanical and fluidic systems in the microscale. Polymers are one such class of new materials as they are considered more suited for biomedical applications due to low cost, abundance, and availability of a wide range of functionality in addition to properties such as low protein adsorption, chemical resistance, and low electrical and thermal conductivities. This chapter describes in detail the properties, microfabrication methods and applications associated with most of the widely used polymers such as polydimethylsiloxane, parylene, SU-8, hydrogels, biodegradable materials and thermoplastics.

Key words

polymers

microfabrication

polydimethylsiloxane (PDMS)

parylene

SU-8

hydrogels

porous monoliths

biodegradable polymers

paraffin

thermoplastic polymers

1.1 Introduction

BioMEMS and lab-on-chip-based automation and miniaturization of analytical assays have significantly improved their performance, throughput, and the cost associated with them in areas as diverse as medical diagnostics, drug delivery, drug discovery, analytical chemistry, and molecular diagnosis (Dittrich and Manz, 2006). Use of microfabrication methods to produce lots of precisely replicable devices has led to repeatable and reliable performances. Automation eliminates the human interfering factors and increases the confidence in the analysis (Selvaganapathy et al., 2003).

Polymers have been widely used in bioMEMS devices primarily due their low cost, chemical inertness, low electrical and thermal conductivities, ease of surface modification, and their biocompatibility. Since polymers cost less, they are ideally suited for disposable bioMEMS devices where cross contamination is an issue. The low cost of polymeric materials and their processing technique is one of the biggest advantages that provide impetus for development of novel processing technologies for microfabrication of polymeric MEMS/microfluidic systems. This chapter describes some of the widely used polymers in MEMS, their properties, and fabrication methods.

1.1.1 Polymers and their classification

Polymers have high molecular masses (> 1000 Da, having more than 100 repeat units). They are macromolecules polymerized from smaller molecules called monomers through a series of chemical reactions. Depending on the position of the reacting groups in the monomer and the cross-linker, these chemical reactions can produce linear or cross-linked (large three-dimensional (3D) network) polymers. The process of polymerization is statistically dependent resulting in the development of a range of polymer chain lengths, causing a nondefined melting temperature point, rather softening over a temperature range called the melt interval. Polymers are classified according to their structure and behavior (Nicholson, 1997).

Polymers are mostly classified according to their response to thermal treatment. One of the most important characteristic properties in this classification is the glass transition temperature (T_g) above which the polymers melt and hence can be molded into specific shapes. After cooling below T_g, polymers can regain their solidity while taking the shape of the mold insert. Linear or branched polymers such as thermoplastics (e.g., polyethylene (PE) and polystyrene (PS)) are not polymerized by cross-linking and hence have a reversible thermal behavior (they undergo the same phase transition without hysterisis). They melt into plastic forms upon heating above their glass transition temperature and solidify upon cooling. This property is ideal in plastic molding applications. Elastomers (such as polydimethylsiloxane, PDMS) are weakly cross-linked polymers that have small elastic modulus with high ranges of deformability. Due to their cross-linked nature, they decompose by excessive heating rather than melting. Finally, thermosetting polymers (e.g., bakelite and vulcanized rubber) are heavily cross-linked polymers, mostly in a rigid and brittle nature with a low range of elasticity and a high resistance against heat.

1.2 Microfabrication

Lithography-based microfabrication was originally developed for integrated circuit (IC) fabrication in the semiconductor industry. It involves thin-film deposition and etching techniques combined with photolithography to define specific patterns to produce micro/nanoscale structures in the order of 0.1–5 μm thickness on planar substrates. These techniques were adapted in the 1970s to create high aspect ratio structures (20–200 μm) that could be used for construction of micromechanical components. Known as surface and bulk micromachining, these techniques allowed the traditional materials used in microfabrication such as silicon and glass to be structured microscopically. Surface micromachining was developed in the late 1980s to create micro-and nanostructures for MEMS and microfluidic devices. In this process, alternative layers of structural (that will be retained in the end) and sacrificial (that will be removed in the end) materials are deposited, lithographically defined, and then etched to create a 3D structure. This allowed extending the range of materials that can be used to include oxides and nitrides of various elements as well as some polymeric materials.

With the advent of chemical and biological sensing and processing applications of MEMS (Manz et al., 1990), bulk and surface micromachining were initially adapted to produce fluidic devices in silicon and glass (Harrison et al., 1993; Liang et al., 1996). Nevertheless, these lithography-based microfabrication processes have certain disadvantages. The range of materials that could be used was restricted. Functional materials such as hydrogels, porous materials, and polymers with specific properties could not be incorporated into these devices. The cost of lithographic fabrication was substantially higher compared to other methods and became an important consideration since many of the biological and chemical sensing devices were designed to be disposable. Polymers as functional materials are considered more suited for chemical and biomedical applications as they are abundantly available at low cost and can be produced with a wide range of functionality while providing properties such as low protein adsorption, chemical resistance, and low electrical and thermal conductivities. Many standard laboratory tools (cell culture plates, catheter, feeding tubes, pipette tips, etc.) have been made of various polymers and the protocols developed have included the surface chemistry associated with these polymers in the biochemical reaction. Most importantly, fabrication of polymer macrostructures is a well-established and low-cost process, which is ideally suited for disposable devices. These factors provided a significant impetus to the adoption of polymeric materials as substrate and functional materials in fluidic MEMS devices as well as spurred the development of alternate microfabrication processes in the 1990s for polymeric materials adapted for their large-scale manufacturing processes.

Several manufacturing techniques such as hot embossing, injection molding, and casting allow polymeric materials to be microstructured by replication from a master mold. The replication can be performed in ambient while still retaining similar resolution to photolithography. Several reviews of these polymer-based microfabrication techniques have been recently published (Xia and Whitesides, 1998; Hec...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Woodhead Publishing Series in Biomaterials
• Introduction
• Chapter 1: Microfabrication of polymers for bioMEMS
• Chapter 2: Review of sensor and actuator mechanisms for bioMEMS
• Chapter 3: MEMS for in vivo sensing
• Chapter 4: MEMS and electrical impedance spectroscopy (EIS) for non-invasive measurement of cells
• Chapter 5: MEMS ultrasonic transducers for biomedical applications
• Chapter 6: Lab-on-chip (LOC) devices and microfluidics for biomedical applications
• Chapter 7: Fabrication of cell culture microdevices for tissue engineering applications
• Chapter 8: MEMS manufacturing techniques for tissue scaffolding devices
• Chapter 9: BioMEMs for drug delivery applications
• Chapter 10: Applications of MEMS technologies for minimally invasive medical procedures
• Chapter 11: Smart microgrippers for bioMEMS applications
• Chapter 12: Microfluidic techniques for the detection, manipulation and isolation of rare cells
• Chapter 13: MEMS as implantable neuroprobes
• Chapter 14: MEMS as ocular implants
• Chapter 15: Cellular microinjection for therapeutic and research applications
• Chapter 16: Hybrid MEMS: Integrating inorganic structures into live organisms
• Index