Biomedical Engineering

Akihiro Miyauchi, Yuji Miyahara, Akihiro Miyauchi, Yuji Miyahara

• Publisher: Jenny Stanford Publishing
• Year: 2021
• ISBN:
  9781000333749
• Edition: 1
• Topic: Medicina
• Subtopic: Biotecnología en medicina

Several developed countries are facing serious problems in medical environments owing to the aging society, and extension of healthy lifetime has become a big challenge. Biomedical engineering, in addition to life sciences and medicine, can help tackle these problems. Innovative technologies concerning minimally invasive treatment, prognosis and early diagnosis, point-of-care testing, regenerative medicine, and personalized medicine need to be developed to realize a healthy aging society.

This book presents cutting-edge research in biomedical engineering from materials, devices, imaging, and information perspectives. The contributors are senior members of the Research Center for Biomedical Engineering, supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan. All chapters are results of collaborative research in engineering and life sciences and cover nanotechnology, materials, optical sensing technology, imaging technology, image processing technology, and biomechanics, all of which are important areas in biomedical engineering. The book will be a useful resource for researchers, students, and readers who are interested in biomedical engineering.

Chapter 1 Biosensors Using Metal Oxides as a Sensing Material

Miyuki Tabata and Yuji Miyahara

Tokyo Medical and Dental University, 2-3-10 Kanda-Surugadai, Chiyodaku, Tokyo 101-0062, Japan

[email protected]

Metal oxides, such as SiO₂, Ta₂O₅, and IrO_x, are useful materials for not only electronic devices but also biosensor elements, because they possess a high proton responsiveness. Monitoring of ion dynamics in (/ex) vivo or in (/ex) vitro is important for analyzing various biological reactions, where the proton is the main participant in such reactions. Biosensing based on pH detection provides an understanding of many biological phenomena, and such a detection method might be applied for disease diagnosis in the medical and dental fields. In this chapter, we describe the fundamental pH-sensing mechanisms of metal oxides and their biosensing applications, which include recent examples such as nucleic acid detection and dental caries detection.

1.1 Metal Oxides as a Biosensing Element

1.1.1 Metal Oxides as a pH Sensor

Various biosensors based on an optical [1, 2], thermal [3, 4], or electrical/electrochemical signal change [5, 6] have been reported and developed to detect ions [7], molecules [8], proteins [9], DNA [10–12], and viruses [13]. To detect target biomolecules, the biosensors consist of transducers whose surfaces are factionalized by immobilizing ligand specific substrates. There is a thin membrane on the transducers for specific molecular recognition, and bioassays are performed following labeling processes involving fluorescent dyes, radioactively labeled probes, or redox markers. In the case of electrical/electrochemical biosensors, the physical and chemical stimuli, which result in the molecular recognition reactions on the transducer surface, are converted into electrical signals such as potential, current, and charge change values. The most advanced electrical/ electrochemical biosensor is the device based on the working principle of a field-effect transistor (bio-transistor) (Fig. 1.1). While optical detection methods using fluorescence labeling, such as enzyme-linked immunosorbent assay (ELISA) and fluorescence-activated cell sorting (FACS), are mainstream, the label-free biosensing methods that use electrical/electrochemical biosensors are attracting attention because of the simple, cost-effective, portable, and user-friendly systems. Sensitive and specific detection of the ion species involved in a biological reaction is important for practical use, and electrical/electrochemical biosensors achieve a sensitive ion detection in a label-free manner. For example, since the first electrical/electrochemical biosensor (i.e., an ion-sensitive field-effect transistor (ISFET)) was reported by P. Bergveld in 1970 [14], various types of ISFETs and biochemical FETs have been developed, some of which have been commercialized as pH sensors for laboratory, medical, and food uses.

Figure 1.1 Conceptual structures of biosensors based on a field-effect transistor. In the case where the detection target has a negative charge, the electrical output (V_T-I_D characteristics) moves towards the right with respect to V_G after molecular recognition, which results in a decrease in I_D.

1.1.2 Electrical/Electrochemical pH Detection

Here, the electrical/electrochemical detection principle of the pH is described. Normally, a biosensor, which is able to exhibit a proton response, has a sensitive membrane that responds to protons on the top surface of the transducer, and induces an electrochemical output based on the dissociation state of the hydroxyl groups present on the top surface. As a result of an interaction between the hydroxyl groups and protons, the output signal obeys the Nernst equation, which is expressed by the standard electrode potential, temperature, electron activity, and reaction quotient of the underlying reactions and species (Eq. 1.1).

$E=E^0+\left({2.303}^{RT}{/}_{zF}\right)\log a,$ (1.1)

where E⁰ is a standard electrode potential, R is an ideal gas constant, T is a temperature in Kelvin, z is a number of moles of electrons, F is the Faraday constant, and a is an ion activity.

$E=E^0-59.16pH$ (1.2)

As shown in Eq. 1.2, when all constants are substituted in Eq. 1.1, the output signal changes by −59.2 mV against one pH unit at room temperature (25 °C).

Typical proton sensing devices used in research experiments and industry include a glass electrode, a light-addressable potentiometric sensor (LAPS), an ISFET, and a metal oxide electrode. To be used as a biosensor, not only are stability and robustness in a liquid required, but also the safety of the sensor material from the viewpoint of application to a living body. Many applications have already been reported for ISFETs and metal oxide electrodes in biosensing. The ISFET, which is the first miniaturized silicon-based chemical sensor, has three terminals: source, drain, and gate. A schematic of the pH detection using an ISFET is shown in Fig. 1.2. The gate used in pH sensing consists of a dielectric material, such as SiO₂, Si₃N₄, Ta₂O₅, and Al₂O₃, and the proton selectivity and pH sensitivity of the ISFET depend on the properties of the solid/liquid interface. The dissociation state of hydroxyl groups on the surface of the dielectric material is dominated by the donation or acceptance of protons, which determines the pH. By following this equilibrium, the drain current value is determined by the proton concentration at the gate. Metal-metal oxide pH electrodes also offer robust and accurate proton measurements, and can be used in a laboratory as an alternative for a glass electrode. In the case of such an electrode, the potentiometric output signal responds to the pH directly, owing to an equilibrium involving the metal and its oxide. The redox reaction involved in the interaction between proton and metal oxide can be described by the following equation (Eq. 1.3):

$Me{O}_x+z{e}^{-}+2H^{+}\leftrightarrow Me{O}_{x-z}{\left(OH\right)}_z,$ (1.3)

where x is the number of oxygen atoms related to the oxidization state of the metal and z is the activity of the electrons. This reaction follows the Nernst equation, as expressed in Eqs. 1.1 and 1.2. To date, various metal oxides, which include PtO₂, RuO₂, Ta₂O₅, TiO₂, SnO₂, and IrO_x, have been proposed as pH-sensing materials; in particular, it is known that IrO_x shows an excellent pH response. Metal oxides have attracted attention as electrode materials in recent decades, and their preparation methods and properties have been well studied [15]. In general, metal oxides are prepared using a thermal oxidation method or an electro-deposition method by controlling several physical experimental conditions such as treatment time, temperature, and concentration of immersion solution. It is clear that these non-glass membranes of pH-sensing materials show a low interference property of monovalent cation and are not destroyed even at sufficiently high temperature. The interference and selectivity of various metal oxide electrodes (TiO₂, RuO₂, RhO₂, SnO₂, Ta₂O₅, OsO₂, IrO₂, and PtO₂) by adding monovalent cations (Li⁺, Na₊, and K⁺) in a universal buffer (pH range of 2–12) has been reported in the literature, and the accuracy for the voltage-pH relationships follows the order Ir(IV), Ru(IV), Rh(IV), Pt(IV), and Os(IV). Thus, it has been proven that IrO_x shows a higher proton selectivity compared with the other metal oxides, which can be explained by a simple ion exchange mechanism in the surface layer and a redox equilibrium between two different solids, e.g., an oxide layer and a pure metal layer.

Figure 1.2 Schematic illustration of proton detection in an ISFET. The electrical output (V_T-I_D characteristics) changes according to hydroxyl group dissociation on a metal oxide surface.

A pH sensor using a metal oxide has been well studied and is already widely used. However, the application to biosensing is limited to certain metal oxide materials, such as SiO₂, Ta₂O₅, or IrO_x, from the viewpoints of strength, safety, and robustness. In the following, as examples of biosensors based on pH sensing technology using metal oxide materials, we introduce semiconductor DNA sequencers in commercially available products and micro pH sensors that quantitatively detect caries.

1.2 Nucleic Acids Detection or Sequencing Using Metal Oxide Devices Based on pH Sensing

1.2.1 Semiconductor-Based DNA Sequencer

Nucleic acid detection technology has been applied in various fields, such as medical treatment, administration of justice, and food science. There is no question that the detection technique of nucleic acids provides a very powerful diagnostic tool. Generally, a nucleic acid detection device requires three steps: DNA isolation, amplification, and detection. The real-time polymerase chain reaction (real-time PCR) is a robust nucleic acid detection method because of fluorescently labeled sequence-specific probes, and is already widely used in the clinical field. Recently, there has been increasing interest in electrical and electrochemical detection of nucleic acids for high-throughput and cost-effective DNA detection, as examples, differential pulse voltammetry [16, 17], square wave voltammetry [18], and chronocoulometry [10]. One of the most successful examples of commercialized products is the IonTorrent^™, which is an ISFET chip-based DNA sequencer that was launched in 2012 [11, 12]. As shown in Fig. 1.3, the commercially available ISFET-based DNA sequencer achieves massive parallel analysis with highly integrated ISFETs on a Si chip. Th...