Two techniques using artificially developed materials, THz metamaterial and aptamer-based techniques for liver cancer diagnosis are explained in this article. Both methods hold great potential for ultrasensitive detection at the incipient stage of cancer development.
by Vinod Kumar Khanna
Liver cancer is a leading cause of death around the world. Primary liver cancer originating in the liver itself represents a significant fraction of cancer mortalities in the undeveloped world. The underlying cause is the widespread rampage of hepatitis which increases the patient’s predisposition to liver cancer.
Liver cancer manifests itself in diverse types. Among these different types, hepatocellular carcinoma (HCC) derived from hepatocytes is the more common malignant tumour. The current gold standard for HCC diagnosis with certain limitations, is the detection of alpha fetoprotein (AFP) biomarker in conjunction with ultrasound and/or CT scan. Various sensing platforms have been described for biomarker detection, notably microcantilever and magnetic tunnel junction (Khanna 2018). In this article, two approaches that exploit artificially developed materials offering several advantages in comparison to traditional techniques are described. The first approach is based on THz metamaterials spectroscopy and the second on aptamers using electrochemical impedance spectroscopy.
THz metamaterial-based biosensor
The THz or submillimeter radiation, (0.3 to 3)×1012 Hz occupies the space between microwaves and infrared radiation called the terahertz gap. It is an intrinsically safe radiation, non-ionising and non-destructive. Its absorption in biomolecular systems is dominated by intermolecular and intramolecular vibrations. In this context, metamaterials play a vital role because their resonance frequency can be tuned according to the vibrational frequency of the cancer biomarker molecules.
Metamaterials are electromagnetic materials that are artificially structured as a periodic array of unit cells of metallic elements called meta-atoms. The dimensions of the unit cell are much smaller than the wavelength of the stimulating electromagnetic waves. The unit cell geometry is tailored to produce materials with controlled refractive index, permittivity and permeability. Exotic properties such as negative refraction is achievable. More importantly, these properties are dependent on the geometry of the atomic arrangement instead of composition of the material.
Metamaterial devices serve as valuable tools for manoeuvring electromagnetic waves at sub-wavelength scales. A THz metamaterial is functionalised with a suitable coating. Then the concentration of analyte binding to the coating influences the resonance frequency of the metamaterial. The change in resonance frequency provides a quantitative estimation of the analyte.
The presence of water in the specimens is an impediment to detection at THz frequencies. Therefore, Geng., et al. (2017) have integrated a microfluidic chip with the biosensor which allows only a small quantity of liquid to be used. The main components of their sensor are split-ring resonators (SRRs) in the form of loops of 200 nm thick RF magnetron sputtered gold film on Si/SiO2 substrates. These loops have one or two square gaps. The microchannel is made in PDMS. When they injected 1 μg/mL of AFP antibody and then 0.02524 μg/mL of blood serum antigen, the transmission spectrum of biosensor with single gap (2μm) shifted in frequency by 8.6 GHz. Similar experiment with another liver cancer biomarker GGT (gamma-glutamyl transferase)-II antibody (1 μg/mL) and blood serum antigen (5mu/mL) yield a frequency shift of transmission spectra by 18.7 GHz. Figure 1 is an illustration of the experimental protocol of Geng., et al. (2017) for correlating the change in resonance frequency of THz material with concentration of AFP antigen.
Aptamers, often termed artificial chemical antibodies, are short single-stranded DNA or RNA molecules, usually 20-60 nucleotides long. They possess high affinity for specific bonding to molecular targets, ranging from simple inorganic molecules to protein complexes, through three-dimensional structures. Easy and less expensive generation of aptamers than antibodies along with their non-immunogenicity and non-toxicity have made them appealing for diagnostics.
Kashefi-Kheyrabadi., et al. (2014) captured HepG2, hepatocellular carcinoma cells in a sandwich architecture between a primary TLS11a recognition aptamer covalently bonded to a gold disk electrode and a secondary TLS11a aptamer. On the lower side, as few cells as 2 cells/mL were detectable. Using cyclic voltammetry, a linear relationship between electron transfer resistance and cell concentration was obtained in the concentration range of 100 cells/mL to 106 cells/mL. Figure 2 depicts the scheme of Kashefi-Kheyrabadi., et al. (2014) for HepG2 cell concentration measurements.
Sensitive hepatocellar carcinoma detection is a serious challenge to biomedical engineers. Metamaterials engineering enables access to properties of materials and behavior not seen in natural forms of materials. Proper utilisation of these properties can provide immense opportunities for construction of high sensitivity devices for specific applications. Aptamers have significant attractive features over antibodies. Recently, researchers have completed a phase II study comparing a set of abnormal DNA markers to alpha fetoprotein for liver cancer diagnosis (Mayo Clinic, 2018). Hopefully, early stage detection of this malady will soon be a reality.
- Geng Z., Zhang X., Fan Z., Lv X. and Chen H. (2017) A route to terahertz metamaterial biosensor integrated with microfluidics for liver cancer biomarker testing in early stage, Scientific Reports, 7: 16378, 11pp.
- Kashefi-Kheyrabadi L., Mehrgardi M. A., Wiechec E., Turner A. P. F., and Tiwari A. (2014) Ultrasensitive detection of human liver hepatocellular carcinoma cells using a label-free aptasensor, Analytical Chemistry, 86, 4956−4960.
- Khanna V. K. (2018) Medical Sensors and Lab-on-a-Chip Devices: Mechanisms, Biofunctionalization and Measurement Techniques, World Scientific Publishing Co. Pte. Ltd,. Singapore, pp. 387-406.
- Mayo Clinic. (2018) A step closer to developing a DNA test for liver cancer. ScienceDaily, Available at <www.sciencedaily.com/releases/2018/06/180605172516.htm> Accessed: 5 June 2018.
Vinod Kumar Khanna is former Emeritus Scientist & Chief Scientist, CSIR-CEERI, Pilani, India. He has worked on power semiconductor devices, microelectronics, MEMS and microsensors. He has published over 190 research papers in journals/conference proceedings and 12 books, and holds 2 US and 2 Indian patents.