# Novel Microfluidics Device for Rapid Antibiotics Susceptibility Screening
## 1 Introduction  
Since the appearance of the first commercially available antibiotic in the late 1930s and their massive introduction into modern medicine in the 1940s, they have become an essential part of civilization as we know it. Indeed, their high efficiency and affordability made them a popular cure among individuals from all around the world. In the last few decades, however, the extensive use of antibiotics led to the emergence of microorganisms that are resistant to previously known efficient suppressing agents [1]. Consequently, the treatment of some infections became somewhat difficult, raising general morbidity and mortality and, as a result, increasing economic burden on healthcare systems. For example, between 2005 and 2015, almost one in every five infections in the EU was due to antibiotic-resistant bacteria. The resistance to second-line and third-line antibiotics increased by 20% and 35%, respectively, in 2010 compared to 2005 and is also expected to grow by more than 72% by 2030. Each year, antimicrobial resistance (AMR) is accountable for about 33,000 deaths and about 1.1 billion Euros in cost to the healthcare systems of European countries [2]. Similar tendencies were reported for the USA [1] and the Asia Pacific region [3]. In order to tackle the issue of AMR, it is mandatory to develop fast, cheap and reliable experimental methods for antibiotics susceptibility screening of microbial cells. There are several techniques typically utilized for the monitoring of antibiotic resistance. Most of the methods (broth dilution [4], disk diffusion [5], polymerase chain reaction (PCR)-based resistant gene detection [6]) are based on evaluation of bacterial growth on agar plates or molecular detection of resistant genes. The main drawbacks of the aforementionedtechniques are connected to long incubation times and poor quantification capabilities [7]. To address these issues, novel AMR methods have been designed and assessed, such as monitoring of bacterial biofilm evolution through optical and electrical measurements [8], matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) [9], Raman spectroscopy [10], digital time-lapse microscopy [11] and microarrays [12]. Even though some of those techniques provide accurate antimicrobial resistance (AST) determinations in an ultra-rapid manner (2-3 h reported in [13] versus 8-10 h with automated conventional AST systems [14,15]), their complexity, high accompanying costs and the need for trained staff make them rather unaffordable tools for most laboratories worldwide. Furthermore, test result dissemination speeds remain limited. As indeed described in [16], less than 5% of European laboratories could validate and transmit the results of an AST to clinicians 24 h/day without delay. The even lower laboratory capacities for testing and reporting of results, combined with the inadequate access to basic health services, make it almost impossible for people living in developing countries to have access to such novel ultra-rapid technologies [17]. Additionally, the current health crisis provoked by the COVID-19 pandemic has demonstrated that supply chains are fragile and clinician specialists are insufficient even in developed countries. Therefore, it seems essential to focus research efforts as much as possible towards low-cost, stand-alone and specialist-independent technologies if we are to provide adequate care to as many people as possible without discrimination. In recent years, microfluidics emerged as a dominant approach to solve a number of questions arising from the limited observability of biological processes at the single-cell level [18]. Concisely, microfluidic devices consist of interconnected fluidic channels and chambers having at least one of their dimensions smaller than a millimeter, i.e., micron-scale, hence \"microfluidics\". The most important aspect of the physics of those devices is the extremely low Reynolds number (fluid inertia vs. viscosity ratio), resulting in strictly laminar flow.