In a world where a biological threat may take multiple forms associated with environmental, health or defense issues, the need for versatile biosensing platforms is of vital concern. The variability of biological matter is proportional to the infinite different ways in which it impacts human beings, in timescales ranging from hours (for particularly aggressive viruses such as those provoking hemorrhagic fevers) to years (for somatic evolution processes leading to cancer). Identification and quantification of one or several biological species of harmful potential are the design targets for the vast majority of the existing biosensors.
No matter what type of biological species (be they viruses, bacteria or circulating proteins in the bloodstream) are targeted by a biosensor, the bottom line of the fundamental requirements for a successful biosensing process is always the same: the best specificity, sensitivity and fastest time of analysis. More specifically, we can add portability, user-friendly exploitation interfaces, cost and a few others which are of secondary concern. In a contemporary technological context where the plethora of configurations seems to meet part or all of the previously listed requirements, it seemed of paramount importance to the authors to reassess the basics of exactly what micromechanics can do in order to overtake the biosensing area, where compatible. This book intends to shed light upon the field of microelectromechanical systems (MEMS)-based biosensors.
I.1. A brief history of biosensors
In his concise but remarkable review of the field of biosensors, Kissinger [KIS 05] looks back to the early days of biosensing (the 1960s and 1970s) to pinpoint that a “sensor seemed to always be a probe of some sort because of systematic association to pH, ion selectivity or oxygen electrodes”. Following the old literature, biosensors are found being called bioelectrodes or
enzyme electrodes, or biocatalytic membrane electrodes [ARN 88].
More generally, according to the International Union of Pure and Applied Chemistry (IUPAC) recommendations in 1999, “a biosensor is a self-contained integrated receptor-transducer device, which is capable of providing selective quantitative or semi-quantitative analytical information using a biological recognition element”.
The critical feature of the biosensor relates to the selectivity for the specific target analyte; this feature directly impacts the specificity or the process of maintaining the selectivity in the presence of other, potential interfering species. The combination of these quality criteria with miniaturization, low cost and essentially real-time measurements in various fields has generated intense commercial interest.
The last 30 years have witnessed an extraordinary growth in research on sensors in general and biosensors in particular. As underlined by Collings and Caruso in their exhaustive review on biosensor advances [COL 97], “an intensively competitive research area is the result of the combined pressure from the traditional well-springs of research and development – science push and market pull”. The growth rate of research activities on biosensors is shown in Figure I.1.
Figure I.1. Overview of the growth rate of research activities on biosensors since 1984 (Sources: World Intellectual Property Organization database (for patents) and Web of Science Thomson Reuters’ database (for publications))
In spite of all this, there is only one truly commercially successful biosensor: the blood glucose meter for people with diabetes. It is important to note that the glucose biosensor uses the technology that was developed by Clark and Lyons well over 50 years ago and only recently has the public benefited from the potential of such a biosensor. The blood glucose meter is a handheld biosens
or based on electrochemical transduction technology [ORA 03] that is produced and commercialized by many companies [TUR 99]. However, in terms of laboratory-based instrumentation, an optical detection system seems to be more commercially viable. Companies such as Affymetrix and Agilent have developed various commercial microarray optical detectors and scanners for genomic and proteomic analysis. Optical sensors that employ surface plasmon resonance (SPR) detection have also been successfully used in many laboratories and universities [RIC 03]. Hence, commercially available optical bench-size immunosensor systems such as BIAcore™ (Biacore AB, Uppsala, Sweden) and IAsys (Affinity Sensors, Cambridge, UK) have found their market in research laboratories for the detection and evaluation of biomolecular interactions.
Still, the development of disposable sensors in conjunction with handheld devices for point of care measurements has featured prominently. Microfabrication technology has played an important part in achieving miniaturized biosensors. Such technology has provided cheap, mass-producible and easy-to-use/disposable sensor strips. Similarly, electrochemical methods have played a pivotal role in detecting the changes that occur during a biorecognition event, and the merging of microfabrication with electrochemical detection has led to the development of various handheld biosensor devices. In fact, i-STAT has developed the world’s first handheld device for point-of-care clinical assaying of blood (Figure I.2), noting that this biosensor array employs several electrochemical-based transduction methods (i.e. potentiometric, amperometric and conductometric) [PEJ 06]. However, this is the only example demonstrating the power of microfabrication technologies for the development of biosensors with high integration and multiplex analysis capabilities.
Will markets harvest the fruits of the next generation of biosensors? In fact, it is sugge
sted that a major part of research and development (R&D) activity in this area rarely results in a commercial product [FUJ 04]. However, the observed growth in biosensor research increases the probability of witnessing another success story in the next couple of decades. The future R&D outlook for biosensors looks positive despite very little market growth/progress over the past few years.
Figure I.2. The i-STAT multisensor for monitoring various blood electrolytes, gases and metabolites (www.abottpointofcare.com)
I.2. What is biosensing?
To introduce the field of MEMS biosensors, the concepts and terminology that will be discussed in the next sections and chapters of this book are first to be clarified and described.
Biosensing: this term is used when a “search and quantify” cycle of operations for one or more biological species (proteins, viruses, bacteria, etc.) is conducted, starting from a sample (either in a gaseous, liquid or solid state) and making use of analytical means of variable complexity.
Biosensor: this is a biosensing device or system made up of two fundamental components: a functionalized solid surface and a transducer which, in turn, transforms a biological reaction (or biological recognition event) taking place on the functionalized surface into a measurable physical signal (Figure I.3).
Figure I.3. Principle and main components of a biosensor
MEMS biosensor: this is a biosensor using a microelectromechanical system as a transducer.
Multiplexed biosensing: this consists of detecting/dosing several different kinds of biological species present in the same sample, at the same time, in the same fluidic chamber by means of an array of biosensors that are deterministically functionalized prior to the contact w
ith the sample.
Functionalization: this is a succession of chemical and biological reactions on a solid surface, aiming to provide specific reactivity with biological species that are the final targets of a biosensing operation. Functionalization can be either deterministic (i.e. precisely localized) or arbitrary (i.e. the whole functionalized surface will bear the same chemical and/or biological functionalities).
Fluidics: this is a set of techniques enabling fluid sample circulation toward one or more biosensors. Except for specific requirements, the fluidics operated in most practical cases is “basic fluidics”, meaning a reduced volume (several milli-/microliters) closed chamber (or reservoir) made up of biocompatible material (glass, plastic or biologically friendly metals) and bearing access ways (inlets and outlets) which ensure the contact with the outside world by means of flexible capillary tubes.
Sample preparation: this consists of a sequence of steps carried out before sample analysis and aims to render a raw biological sample in a solid, liquid or gaseous phase (e.g. food, blood and air sample) appropriate for biosensing. A clean sample is often required when using sensing techniques that are not responsive to the analyte in its in situ form or when the measurement results are distorted by interfering species. Sample preparation includes filtering and separation of unwanted entities (particles, biological entities or chemical species), and dissolution or preconcentration and isolation of the target analyte within an appropriate diluent using various techniques.
I.2.2. Important numbers and characteristics
For a biosensor to be a “meet-all-expectations” device, it must fulfill a set of requirements (also known as specifications) that are specific to the biosensing domain. The most important of them are defined as...