Microfluidics for point of care diagnostic devices: a review

Written by Audrey Nsamela
Published July 29th 2020
Contact: partnership@elvesys.com, Elvesys SAS, 172 Rue de Charonne 75011 Paris

Introduction to Microfluidics for point of care diagnostic devices

It has now been a few months that the new coronavirus, named Covid-19, has been raging throughout mainland China. The World Health Organization (WHO) has declared it an international emergency. They fear what may happen if the virus spread to a country with less effective or absent health care settings. In those countries and among the poorest populations, not only is it a problem to treat the diseases, but it’s also an issue to diagnose them. Covid-19 is only one example of a deadly virus that presents a challenge for healthcare services and communities. 

Many different types of diseases can threaten human lives and we can split them into two main categories: transmittable diseases (influenza, coronavirus, Hepatitis, HIV, …) and non-transmittable diseases (cancer, diabetes, cardiovascular diseases…) With a growing and ageing population, researchers worldwide are struggling to find effective ways to treat these pathologies. The transmittable diseases are usually caused by external pathogens. The first step in the process is then to rapidly and accurately identify the pathogens, whether they are foodborne, bacterial or viral. 

Download the free PDF of this presentation

Such identification in remote locations or places with no access to medical laboratories is made possible with the recent development of point of care (POC) diagnostic devices. WHO has described criteria for choosing a POC diagnostic device, saying it should be “affordable, sensitive, user-friendly, rapid and robust, equipment-free and deliverable to end users” or ASSURED [1]. Most of the conventional diagnostic methods involve culturing cells for harvesting sufficient number of pathogens needed for detection. As POC diagnostics should be fast, molecular diagnostic techniques with no cell culture involved are preferred. However, currently used techniques such as PCR, ELISA and other immunoassays involve many sample preparation steps, and are time consuming. These techniques need to be miniaturized and integrated into a single chip that can perform all the sample preparation steps, assay, detection and analysis. This very challenging task could be achieved with the help of recent advances in microfluidic technology. Microfluidic-based POC devices, often also called lab-on-a-chip POC devices, overcome many challenges as they are small, cost-effective, highly sensitive, accurate and allow the detection of multiple analytes in one sample [2-3].

Scheme of a microfluidic-based POC diagnostic system integrated into a hand-held device. [4]

This short review does not intend to be an extensive description of all microfluidic systems designed for POC diagnostics. It rather will be an overview of the most common materials for the microfluidic chips, the most relevant detection methods and the application of those techniques to detect different kind of pathogens. Finally, there will be a discussion on the remaining challenges and the future prospects of commercialization of these products.

Microfluidics for point-of-care diagnostic devices: materials for POC lab-on-a-chip

The following table summarizes the commonly used materials for microfluidic-based POC chips with their fabrication methods, advantages and disadvantages [2]. 

For more extensive information about materials used in microfluidic chips, a short review has been published by Elveflow.

Graphical description of some common fabrication techniques for microfluidic chips. Reproduced from [2, 5-6].

Microfluidics for point of care diagnostic devices: detection methods for infectious diseases

Microfluidic based SPR biosensor that can be used for bacteria, DNA or protein detection. Reproduced with permission of Nature Publishing [8].

The choice of a particular detection method for microfluidic based POC diagnostic devices depends on the types of analyte we are looking for. Is it proteins? Cells? Nucleic acids?

Different techniques exist for detecting different pathogens. Most of the reported systems focus on viruses and foodborne pathogens rather than bacteria. This is partly due to the fact that few antibodies exist for bacteria detection and the microfluidic POC diagnostic devices often rely on these antibodies for fast and efficient bacteria identification. Other techniques that don’t require the use of antibodies will be discussed further.

Microfluidic-based POC systems were develop for the detection of foodborne pathogens such as Salmonella, Listeria, Cholera and E. Coli using a combination of functionalized nanoparticles with microfluidics. This combination is largely reported in the literature and is often used for optical detection. Whether it is fluorescence, colorimetry, SERS or Raman, the type of signal collected during the optical detection depends on the material properties of the probe. For example, fluorescent nanoparticles, like quantum dots, can be functionalized with antibodies specific to a certain bacteria or virus and then form a “sandwich-like” complex with the targeted analytes. The number of pathogens can be deduced by the fluorescence recording [7]. In other settings, Surface Plasmon Resonance (SPR) detection method is employed with gold coated microfluidic channels and immobilized antibodies for selective bacteria detection [3,8].

In a similar way, optical detection can be achieved by the naked eye with a simple colorimetric assay. Colorimetric assay is possible when a chemical reaction following the capture of the analyte result in a visible color change. It can be achieved with plasmonic nanoparticles due to a change in the peak of absorbance wavelength when the particles are getting aggregated. However, nanoparticles are not the only way of producing a visible color change, it can also be created by the oxidizing action of an enzyme called Horse Radish Peroxidase (HRP). This enzyme is often used in immunoassay and lateral-flow assay (LFA) and allows for colorimetric as well as electrochemical detection. LFA are methods employed in many microfluidic POC diagnostic devices, especially paper-based microfluidics.

Scheme of a typical optical detection based on antibody-antigen bonding and HRP enzymatic activity. Copyright 2019 American Scientific Publishers [3].

The advantage of mixing nanoparticles with microfluidic systems is the reduction in the necessary volume of sample and reagents as well as a better sensitivity and faster process [10]. Elveflow developed a nanoparticle synthesis sheath flow Beta Pack and a herringbone micromixer lipid nanoparticle synthesis pack. You can also read our protocol on active micromixing.

The biggest challenge with combining molecular diagnosis techniques and microfluidics is to integrate the sample preparation and the sorting of the analytes on-chip [3-4].

Microfluidics for point of care diagnostic devices: smartphone-based diagnostics

Most of the microfluidic integrated biosensing techniques require more elaborate imaging systems or data analysis than naked eye evaluation. As the ideal POC testing device would be portable and lightweight, it is important to think of miniaturizing the detection and analysis scheme. There is no wonder why many researchers have taken seriously into consideration the use of smartphones in combination with microfluidic-based POC biosensors. According to statistical data, around 50% of the world’s population owns a smartphone.

There was a 40% increase in the number of people owning a smartphone in the past 4 years and the trend is growing. Although less people in developing countries are included in that number, it is still a very promising and cost-effective way to deploy and commercialize POC diagnostics [11-12]. Bright field or fluorescence microscopy adapter can be attached on a smartphone having a good CMOS camera. Such attachments are usually 3D-printed, thus easy to fabricate and cost-effective. For bright field imaging, a set of LED’s, the smartphone’s flashlight or ambient light are used as illumination source. Fluorescence imaging prototypes include excitation/emission filters as well. Aside from optical detection, smartphones can also be integrated with amperometric biosensors for electrochemical detection and serve as analyzers/processors. For more information about smartphone based POC diagnostic devices combined with microfluidic chips, the reader can consult the excellent review of Xu and al. (2018) [12].

Improvements and future perspectives

Microfluidics for POC biosensors and diagnostics has proven to be an effective way of reducing the size, weight, cost, and assay time of infectious pathogens detection techniques. Combined with nanoparticles, those devices show an enhanced sensitivity compared with their corresponding bench top assays. Most of the POC diagnostics devices using microfluidic chips, with or without a smartphone attachment, are at the “proof of concept” stage. Mass production and commercialization of lab-on-a-chip POC devices are very big challenges that still need to be overcome.

References