Advances in passively driven microfluidics and lab-on-chip devices: a comprehensive literature review and patent analysis

The development of passively driven microfluidic labs on chips has been increasing over the years. In the passive approach, the microfluids are usually driven and operated without any external actuators, fields, or power sources. Passive microfluidic techniques adopt osmosis, capillary action, surface tension, pressure, gravity-driven flow, hydrostatic flow, and vacuums to achieve fluid flow. There is a great need to explore labs on chips that are rapid, compact, portable, and easy to use. The evolution of these techniques is essential to meet current needs. Researchers have highlighted the vast potential in the field that needs to be explored to develop rapid passive labs on chips to suit market/researcher demands. A comprehensive review, along with patent analysis, is presented here, listing the latest advances in passive microfluidic techniques, along with the related mechanisms and applications.


Introduction
Handling small volumes of uids is very important in highthroughput screening, diagnosis, and research applications. 1 Microuidics is one way to handle small volumes of uids between microlitres (10 À6 ) and picolitres (10 À12 ). [2][3][4] Hundreds of simultaneous biochemical reactions can be performed in a collection of microarrays arranged on a solid substrate which acts as laboratories, embedded in which are chips known as biochips. 5,6 There are three main types of biochips: lab on chips (LOCs), DNA chips, and protein chips. LOCs employ a combination of one or more laboratory functions within a single integrated chip. 7 Some elds utilizing LOCs, such as sub-micrometer and nano-sized channels, DNA labyrinths, single-cell detection and analysis, 8 and nano-sensors, might become feasible, allowing new ways to interact with biological species and large molecules. In addition, a large number of biochemical analyses can be screened at a faster rate in disease diagnosis and the detection of bioterrorism agents. 9 Several reports have been published on the various aspects of these devices, including uid transport, system properties, sensing techniques, 10 and bioanalytical applications. Advantages 11,12 include lower fabrication costs, allowing cost-effective disposable chips and mass production. Simple tests that could be performed by the bedside are known as point-of-care (POC) testing. 13 The ultimate aim of this technique is to obtain results in a concise period at or near the location of the patient, so that the treatment plan can be adjusted. 14 Microuidics can be used for various lab experiments, such as drug testing and discovery, [15][16][17][18][19][20][21][22][23] ltration and separation of particles, 24 cell sorting and counting, [25][26][27][28][29][30][31] cell culture, 32-40 pointof-care diagnosis, 41,42 3D printing, stoichiometry, and ow synthesis. 43,44 Due to their simplicity with high throughput and very low reagent consumption, 45 microuidic chips are vital components in research, for the delivery of accurate results. 46 Microuidic chips are mostly made up of PDMS (polydimethylsiloxane). PDMS is commonly used because it is a transparent elastic polymer, permeable to oxygen and carbon dioxide. [47][48][49][50][51] Additionally, PDMS is now becoming a standard material as it can be easily fabricated for microuidic devices (MFDs), and its high gas solubility, which obeys Henry's law, is a signicant advantage of using PDMS material. 52 Microuidic operation techniques involves the movement or transport of biological samples and analysis of those samples through an external power source/ eld [57][58][59] or actuators, 60 such as peristaltic pumps, 61 electrokinetics, 62,63 electro-wetting, 64 electro-osmotic pumps, 65 electrostatics, 66,67 centrifugal and magnetic pumps 68 and some other large power sources to power the pumps and actuators. [69][70][71] Thus the complexity of structure and size is increased, which requires additional human resources. Hence, the probability of integrating active microuidics with LOCs and (POC) applications has dropped off. To counter these drawbacks, [72][73][74] the movement of the test sample is achieved either using uid properties or passive mechanisms without any external supporting power sources. Hence, passive microuidics 75 has been adopted and used to a large extent in modern-day research projects to avoid the use of external supporting power devices. This method is simple, easy to manufacture, and does not need any actuators or external power supplies, as it employs basic laboratory instruments, like micropipettes, and medical devices, such as syringe pumps.

Systematic literature search
A systematic literature search has been performed using Google Scholar with the following keywords: "passive pumping platform, passively driven microuidics, pressure head driven microuidics, and ow-driven microuidics" throughout 2000-2018. Fig. 1 presents a pie chart revealing the aggregate number of articles on passive microuidics published from 2000 to 2018. This review article will be useful for researchers who plan further investigations in this eld.
From Fig. 1, it is evident that pressure-driven, capillary, hydrostatic, surface-tension and vacuum-based methods are leading trends. From the search, articles were selected and some were hand picked based on their relevancy with an exclusive focus on the passively driven approach and they are presented according to their timeline. In 2002, David Beebe et al. published a short report on the fabrication of MFDs and the physics applied to passive valves, mixers, and pumps which facilitate uid ow. 53 Due to the extensive usage of PDMS in MFDs, Sia et al. reviewed the advantages of using PDMS in miniaturized biological assay devices, such as efficiency and spectral insights into cell biology, where the uidic ow was obtained by applying a positive or negative pressure at the inlet or outlet, respectively. 76 Subsequently, Bayraktar et al. reviewed the available knowledge based on the areas that required additional investigation in pressure-driven and electro-osmotic ows in microchannels. 77 Later, Fiorini et al. in 2005 reported a review of different modes of on-chip operations, such as the pumping and valving of uid ow; and the separation and detection of different chemical species that have been implemented in a microuidic format. 54 Haeberle et al. reported platforms for LOC applications. 78 Eventually, a microuidic cell culture was developed, which implied the use of surface tension, where a differential pressure was generated due to the different volumes in the inlet and outlet port to assist the uid ow and which was based on concepts related to physical and microenvironments on passive pumping. 79 Additionally, Ahn et al. reported a short review in 2010 based on various methods of passive pumping and their applications to LOC biochemical analysis. 80 Furthermore, Gervais et al. focused on various techniques that have been adapted for passive pumping. 74 Later, Su et al. reviewed the latest advances in microuidic platforms for POC testing in the context of infectious diseases, along with the integration of multiple functions into a single unit with full automation and analysed the challenges involved. 81 Subsequently, Byun et al. summarized recent advances in pumping techniques for microuidic cell culture in an effort to support current and potential users of microuidic-based devices for advanced in vitro cellular studies. 57 The relevant biophysical laws, along with their experimental details and the designs of various passive separation techniques, were explained by Tripathi et al. in 2015. 82 These separation techniques advanced the development of single-cell capture. Eventually, Narayanamurthy et al. focussed on the development of single-cell trapping using hydrodynamic effects for the purpose of developments in cell separation and rapid viral detection and explained its benets. 83 In this article, we wish to discuss the current proposals, developments, updates, and future of passive microuidic and LOC devices. The different methods adopted in every passively driven type of microuidics are briey covered, so that one can obtain a better and clearer knowledge of previous techniques. [84][85][86] This also throws light on the advantages, ow rates and applications of all the techniques. The limitations of each method are also explained.

Passive microuidic techniques
Passive microuidics is an emerging technique with a tremendous scope that is being adopted more commonly in LOC and POC diagnosis due to the following characteristics: Easy fabrication: Fabrication of passive microuidics is simple and easy, as it avoids complicated fabrication resulting in complex structures.
Less expertise: Passive microuidics operation does not usually require training or experience, as it is straightforward and easy to operate.
No external power source required: Passive microuidics does not involve any external power source for its working.
Low cost: Passive microuidics is substantially low in price due to less complicated procedures and no auxiliaries being involved.
Compact and portable: Passive microuidics is very compact and portable in size; hence, it can be driven almost anywhere.
Different techniques employed in the eld of microuidics and LOC devices to achieve passive operations are surface tension, pressure-driven, osmosis, capillary action, gravityinduced ow, vacuum suction, and hydrostatic pressure, as shown in Fig. 3. Each technique has its pros and cons. In the sections below, each technique is described in detail, and the articles are listed in year order.

Surface tension
In general, surface tension is dened as a property of the surface of a liquid that allows it to resist an external force due to the cohesive nature of water molecules. This cohesive forces between the liquid molecules are responsible for the phenomenon known as surface tension. 88 In microuidics, the generation of strong passive ows can be achieved by selecting a suitable surface and liquid combinations that can create the required solid-liquid surfacetension gradients. 89 Mathematical models are required to nalize the design of the channel, so Makhijani et al. developed a numerical model to simulate the liquid lling due to the presence of surface tension in the liquid-air interface and demonstrated the application of disposable biochips for clinical diagnosis. It was mainly used for analysis and optimization to achieve the desired ow. 90 Subsequently, Walker et al. proposed a simple, semi-autonomous method of pumping uids where devices producing small droplets, such as a pipettes, were required. 91 Meanwhile, evaporation occurred during the surface-tension-based ow, which could be used to increase the concentration of the sample in the channel. 92 Due to the requirement of microuid mixing, for the purpose of drug delivery and research, Chen et al. demonstrated a micromixing device without the use of any active devices; hence, the surface tension of the uid provided transport, merging, mixing, and stopping in the microchannel by varying the channel geometry. 93 Ward et al. analysed droplet formation using ow- focusing geometry and microuidic technology and compared the two methods of supplying uids using a syringe-pump methodology that dictated the volume ow rates and a method which controlled the inlet uid pressure. 94 Later, Tan et al. controlled the droplets by the formation of a bifurcated junction in an MFD, and thereby controlled the chemical concentration in the droplet by the use of droplet ssion. 95 Furthermore, a passive pumping technique was employed in a direct methanol fuel cell (DMFC), through passive fuel delivery, designed based on a surface-tension driving mechanism and integrated with a laboratory-made prototype to achieve xed consumption depending on fuel concentration and power-free fuel delivery. Due to different surface-tension properties, water was separated from methanol through a Teon membrane, and forward ow occurred in the capillary. This was believed by Yang et al. to be more applicable to future smallscale DMFCs in portable electronics. 96 Thanks to their analysis of channel pressure and ow, Berthier et al. reported that greater pressure created a large drop in output at the channel exit when a small droplet was placed on the entrance of the microuidic channel. 97 Eventually, in 2008 Ivar et al. published an article that implied the use of surface tension to induce a pressure difference in the uid that causes pumping, routing, and compartmentalization without the use of any additional micro-components. Moreover, they demonstrated the applications of patterning multiple monolayer cell colonies and threedimensional cell compartments and co-cultures. 98 Backow is a serious issue faced in surface-tension-driven pumps, so Ju et al. explained the optimum conditions required to avoid backow in surface-tension-driven passive pumping by determining the ratio of inlet and outlet pressure during the occurrence of backow. 99 Du et al. reported that the concentration gradient in the channel induced forward ow, and it was further enhanced by evaporation-induced backward ow. Thus, the gradient concentration generated was controlled by convection and molecular diffusion. This approach was particularly used for chemical and biological processes in portable MFDs, where long-range gradients are required. 100 Jane et al. analysed the ow rate with the Hagen-Poiseuille equation, thermodynamics, and the Young-Laplace equation where the ow rates could be controlled by channel geometry, dimension of inlet and outlet wells and hydrophobicity. Later, Chen et al. provided a clear idea about the surface-tension-induced ow rates, theoretical relationships among sample volume, induced ow rate, and surface tension of the drops at the inlet and outlet ports, and resistance to ow. 101 As soon as the droplet was placed at the inlet port, it began to collapse due to varying velocity in the channel and it transformed from a smaller drop to a larger drop. Theoretically, the maximum ow rate was obtained at a contact angle of 90 degrees, but practically it was maintained between 30 and 60 degrees due to relling. Resto et al. provided a clear idea of both theoretical and practical aspects of the pumping and the contact angle to achieve the maximum ow rate. 102 The surface tension caused a pressure difference in the channel to induce uid ow from the inlet to the outlet. Equilibrium of pressure in the channel led to the immediate arrest of the inow. This sudden stop allowed its use in a wide range of biological This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 11652-11680 | 11655 applications from reagent delivery to drug-cell studies. 103 Jane et al. explained that the surface tension in the uid induced the ow through the microchannels due to the change in volumes of uid. Passive microuidics with electrochemical sensors inside the microchannel was considered for LOC ow injection. Various factors affecting the ow in the microuidic channel were also discussed. This was the rst report on a passive pump based on ow injection analysis (FIA). 104 In 2010, Amy et al. integrated particle counting with a passive pumping mechanism by placing a 0.5 microliter drop of saline and sample uid on the focussing inlet and the sample inlet, respectively. The surface tension in the uid experienced a ow due to the change in volume. These ows to the reservoir traversed a pore that caused a change in resistivity, and the pulse was counted. Thus, particle counting was achieved with the help of surface-tension pumping. 105 Puccinelli et al. explained that the pumping process of a small drop placed at the inlet was due to the surface tension in the inlet uid. They validated the performance of a complete, reliable, and repeatable cell-based biological assay. The robustness of each technique was also discussed. 106 Chung et al. described the involvement of surface tension, as well as evaporation, leading to the generation of passive pumping in a forward and backward direction with a concentration gradient a few centimeters long in the channel. Recent developments in microuidic gradient generators were also described in their work. 107 Besides research, Lin et al. demonstrated the uses and applications of microuidics in the elds of food, environment, and physiological health monitoring. 108 Berthier et al. suggested that the surface tension of the uid enabled a short-term laminar ow patterning in multiple uids when the sample was loaded in any sequence. Numerical simulations and practical experiments were conducted to study the laminar behaviour. This method was well suited to a cell-based assay and reduced the complexities of laminar ow patterning (LPF). 109 In 2012, Resto et al. developed inertia-enhanced passive pumping that reduced uid exchange and inertia-activated ow, which initiates the ow in an empty channel where uid ow took place due to surface tension. They also analysed the transfer of momentum to the incoming uid and the effect which induced the pumping mechanism. 110 Microdevices are capable of targeted focal delivery of chemicals for axonal growth studies. Hence, Kuo et al. varied the drop volume to passively drive the ow into the microchannel. With this manipulation technique, the bio-chemicals delivered were combined with neuronal cells, and the required ow rate was achieved. 111 Groot et al. enabled the dynamic culture and analysis of tissues in a hanging drop example, which employed surface tension as the driving force with two droplets, namely the culture droplet and the userinterface droplet. 112 Computer simulations were used to give detailed information on ow patterns and physical phenomena under different conditions. The pumping process was divided into three planes with deceleration followed by acceleration and deceleration that relied on the physical properties of the operating uid, and geometrical characteristics of the channel. 113 Fig. 4 shows an MFD utilizing surface tension for passive operation. The latest reports on passive MFD using surface tension techniques are summarised in Table 1.
Limitations. As the uid droplet at the inlet reduces, the surface tension decreases causing a decrease in uid ow. So relling is necessary for continuous ow. 57 Additionally, the presence of reected light and mirage effects in millimetre-wide spherical caps reduce the accuracy of goniometer measurement. 97

Pressure-driven
Fluid pressure is a measurement of the force per unit area. Pressure in liquids is equally divided in all directions; therefore, if a force is applied to one point of the liquid, it will be transmitted to all other points within the liquid. 114 MFD and LOC devices employing pressure for their passive operations are shown in Fig. 5. In a passive pressure-driven technique, the pressure created in the reservoirs to drive the sample is achieved either by pipetting or by a nger-force pressure. 115 Liu et al. reported a twisted microchannel with chaotic advection that possessed high potential mixing even for uids with a low  Reynolds number. 116 Ahn et al. suggested the uid control technique and veried that the low-pressure drop in the uid tended to maintain the ow without any complicated pumps. 117 Also, Jeon et al. described the design and fabrication of passive valves and pumps, which used the pressure-driven mechanism instead of electro-osmotic pumps. The fabrication included aligning, stacking, and bonding of a patterned membrane. 118 Rush et al. established a 2D serpentine channel with the ow of low-Reynolds-number Brownian solute particles. 119 Later, Moorthy et al. developed an on-chip porous lter that separates minimal volumes of biological uids in real-time applications and analysis. 120 Also, Hu et al. investigated the relationship between the pressure drop in the rough channel and smooth channels and also the pressure drop due to a change in height. It was found that in a rough channel the pressure drop increased and when the channel height increased the pressure drop decreased. 121 Chen et al. explained that a wavy-wall section incorporated within the microchannel developed a centimetre-long high concentration gradient by increasing the interfacial leads. 122 Jiang et al. explained the push or pull in the sample uid due to a negative or positive pressure that a syringe pump would create. 123 Hattori et al. conrmed that uid ow commenced only when the pressure was applied to the uid by using a syringe pump or micropipette through an air vent lter. 124 In addition, when a nger-powered pressure of about 3-4 kPa was applied at the inlet, sample movement occurred. 114 In their investigations, Davey et al. recommended a system where the inlet and the outlet were a hydrophobic and a hydrophilic needle, respectively, and the channel was made of a hydrophilic region where the uid from the inlet traversed the channel and reached the outlet. 125 Tice et al. reported an electrostatic microvalve with passive components embedded in them, which was used to regulate the pressure in hydraulic control lines and actuate pressure-driven components. 126 Finger-power integrated pumping systems had been utilized to eradicate the limitations produced by the use of external pumps. Pressure generated inside the pressure chamber played a vital role in determining the efficiency of nger-powered pumps. 127 Thus the requirement for pre-evacuation of PDMS devices in a vacuum chamber was eliminated. Thereby, the design of a simple POC pumping method using a single-layer structure where the dead-end microuidic channel was partly surrounded by an embedded microchamber, with a thin PDMS wall separating the dead-end channel and the embedded microchamber, was reported by Xu et al. 128 But the working liquid created a reduced pressure in the analytical channel and induced sequential sample ow into the microuidic circuits. Kokalj et al. reported simplied activation by ngertip pressure with no external power or control for a wide range of applications in POC diagnostic settings. 129 Jeong et al. developed an onchip microow control technology with the ability to mimic in vivo conditions at an in vitro microscale for long-term tissue culture with a continuous ow rate. The passive ow was initiated using a siphon effect, and a yarn ow resistor to regulate the ow rate in the microchannel. 130 Thanks to implantable drug delivery systems, a micropump for controlled, automated inner-ear drug delivery with the ultimate aim of producing a long-term implantable/wearable delivery system was investigated by Tandon et al. 131 Currently accessible pressure-driven approaches in passively driven microuidics are summarised in Table 2.
Satoh et al. in 2016 reported multiple medium-circulation units on a single MFD that were driven only by two pneumatic pressure lines with three independent culture units, in which the cells were cultured under medium circulation ow. The authors stated that pneumatic pressure could be easily distributed to multiple wells in a reservoir with a common gas-phase space without any changes in tube connections. 134 Zhang et al. developed a microuidic passive ow regulator with an in-built ve-layer structure valve for high-throughput owrate control in microuidic environments by constructing a gas-driven ow system and analyzed the ow regulation. 135 Moon et al. investigated a passive microuidic ow-focusing method to generate water-in-water aqueous two-phase system (ATPS) droplets without the involvement of any external components. This was the rst microuidic technique that formed low interfacial tension ATPS droplets without applying external perturbations. 136 Ryu et al. examined a closed-loop operation of inertial microuidics, which could dissociate clinical airway secretions and isolate/enrich immune-related (iv) A self-powered imbibing microfluidic pump by liquid encapsulation (SIMPLE): (a) sequential pump operation and (b) an experimental presentation. (c) Initially, the chip is prefilled with the working liquid (blue) through the inlet denoted by 25 thick blue arrows and encapsulated by impermeable protective foil patches (green circles). Before activation, the foil is removed, the sample (red) is deposited over the inlet hole, and temporary finger force can activate the pump. When the working liquid touches the paper, the finger can be removed, and the pump is activated. The pump works until the working liquid saturates the paper or until all of the fluid has been sucked into the paper (figure (iv) has been adapted from ref. 129 with permission from the Royal Society of Chemistry, copyright: 2014).
This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 11652-11680 | 11659 cells for in vitro downstream assays. They found their applications in clinical in vitro cell-based biological assays of various pulmonary diseases like acute respiratory distress syndrome, pneumonia, cystic brosis and bronchiectasis. 137 Lee et al. also developed a negative pressure-driven uid ow generated by a simple nger-triggered operation where the PDMS suction cup was placed at the outlet of the device and dispensed the uid at the inlet. 132 A new infusion-based simple method (ISIMPLE) for drug delivery into the skin was developed by Dosso et al. with a self-contained skin patch without any external driving source, where the expelled air increased the pressure due to the extreme exibility of the design and manufacture. The ISIMPLE concept offered enormous opportunities for entirely autonomous, portable, and cost-effective LOC devices. 139 Later, Liu et al. developed a PDMS pump that utilized the air released from the aerated PDMS to create a positive pressure in the MFD. 140 Limitations. The application of external pressure using a nger might affect the accuracy and repeatability of the assays due to pressure difference, and multi-step diagnostic assays may not succeed via hand-powered mechanisms since several reagents/buffers need to be driven in a controlled manner. 141 Osmosis Osmosis is the spontaneous net movement of solvent molecules to a region of higher solute concentration in the direction that tends to equalize the solute concentrations on both sides through a selectively permeable membrane. Permeability depends on solubility, charge, or chemistry, and solute size. Water molecules diffuse through the solute from the solvent layer. 142 Bruhn et al. demonstrated the development of devices that are capable of pressure generation based on the osmosis principle, which could be made from available low-cost materials. 143 Some MFD and LOC devices using osmosis as a passive approach for their operation are shown in Fig. 6.
Later, Good et al. integrated and analyzed a polymeric microuidic device in a portable mechanical micro-pumping system that used uid-responsive polymer particles as an actuator, without external power. 144 In 2009, Xu et al. reported that, when a semi-permeable membrane was sandwiched between the inner osmotic reagent chamber and the outer water chamber, the water ow to the osmotic reagent chamber via the semi-permeable membrane facilitated the ow of uid in the channel by the process of osmosis. 145 Furthermore, stable concentration gradients are required for cell analysis and culture. The development of a microuidic platform provided stable concentration gradients for cells of various signaling molecules for more than a week with only the least amount of handling and no external power source. Later, Park et al. optimized the osmotic pumping performance by balancing the capillary action and hydraulic pressure in the inlet reagent reservoirs. 146 Updates on osmotic-driven passive pumping techniques in microuidics are summarised in Table 3. Limitations. An osmotically driven mechanism needs a complex setup compared to surface-tension, capillary, and gravity-driven setups. 57 In osmotic drug delivery systems relling the reservoirs was mostly impossible or complicated with the constant drug delivery rate. 144 The development of an additional reservoir for the solvents increased the device complexity for operation in any environment. 150 Capillary Capillary action is dened as the movement of a uid within the spaces of a porous material due to the intermolecular forces between the liquid and the surrounding solid surfaces that enables the liquid to ow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. 87 Juncker et al. reported a capillary pump assisted capillary ow microuidic system, where the ow was activated from the open and closed channels of the device. 151 MFD and LOC devices incorporating the capillary technique are shown in Fig. 7. Simultaneously, a microuidic capillary system was presented for the continuous transport of uid using capillary force, when the uid was placed in the service port. 151 Mixing of uids is necessary for drug delivery and research. So, Hosokawa et al. reported an MFD which used capillary force for pumping. 152 To avoid clogging by uids, Kim et al. presented a capillary passive retarding microvalve at the junction of the microuidic channel where the propagation occurred only aer the merging of two uids. 153 An exact discussion of interface motion driven by capillary action in a microchannel was reported by Ichikawa et al. where a dimensionless variable of the driving force was used to predict the interface motion. 154 In order to maintain and control the ow properties of capillary systems (CSs), Zimmermann et al. developed a design for capillary pumps. These capillary pumps were designed to have a small ow resistance and were preceded by a constricted microchannel, which caused ow resistance. 155   capillary ow with a passive method where the ow could be retarded with appropriate hydrophobic patterns in hydrophilic channel surfaces. The microuidic system was designed in such a way that it contained two planar parallel surfaces, separated by spacers. 156 Flow rates may vary in different substrates, so Zhu et al. reported a study of capillary ow rate in several substances, including glass, polycarbonate (PC) and polydimethylsiloxane (PDMS) to measure the contact angles and to examine the longevity of capillary ow in PDMS and PC chips to give better clarity in measuring the ow rates. 157 Lynn et al. realized a pressure difference arising from the small curved meniscus at the bottom of the outlet reservoir that drove the uid with a constant uid ow for more than an hour. 158 Later, Gervais et al. described the uid ow from high capillary pressure to low capillary pressure and increased the channel width at each level to reduce the friction, thereby leading to a high ow rate. 159 Mukhopadhyay et al. proposed a microchannel bend in polymethylmethacrylate (PMMA) of different widths. The effects of channel aspect ratio and different separation angles were studied for uid ow. 160 Eventually, Alphonsus et al. explained in a short review on microuidic immunoassay that the capillary pressure in the channel pushed the sample, causing the uid to ow within the device. 161 Kim et al. realized that the surfactant-added PDMS could be used to increase the hydrophobicity of PDMS to increase the lling rate of blood by capillary action. 162 Souza et al. fabricated a toner-based micro-uidic device in which the uid owed towards the outlet as soon as the serum was placed in the sample inlet. 163 Later, Horiuchi et al. also developed an immunoassay chip which consisted of vertically integrated capillary tubes to create negative pressure and pump the uid towards it. 164 Once the uid had been placed in the inlet well, Kim et al. veried the ow timing control and direction of multiple solutions in the channel. 165 Kistrup et al. reported aspects of using rapid prototyping instead of pilot (mass) production. They included the fabrication of the microuidic system that employed injection moulding and ultrasonic welding in which the uid ow was assisted by the capillary pressure at the interface nozzle. 166 Berthier et al. proposed the onset of the suspended capillary ows (SCF) and the viscous friction at the walls using the force balance between the capillary forces that drove the ow. 167 Plasma separation is possible in passive pumping microuidic technology. Madadi et al. demonstrated a self-driven blood plasma separation microuidic chip, which was capable of extracting more than 0.1 ml of plasma from a single droplet of undiluted fresh human blood (5 ml) with high purity without any external pumps. 168 Nie et al. reported a exible microuidic device that lled the channel through capillary force to provide continuous uid pumping through an evaporation micropump. The hexagonal arrangement at the pore array drove the uid ow and automatically absorbed liquid through a lter paper interface. 169 Mukhopadhyay et al. described a leakage-free PMMA fabricated MFD with microchannel bends with the effect of surface wettability on surface-driven capillary ow. This type of microuidic system was utilized in blood cell  172 Wu et al. generated ow-focusing droplets through the capillary effect to reduce the requirements of subsequent systems in addition to high exibility. 173 Zhai et al. developed a microuidic device, which was insensitive to backow due to the integration of a syringe pump to balance the capillary pressure within the channel. 174 A study on the microuidic self-owing chip to understand the inuence of micro-scale topographies was reported by Xie et al. According to this work, the ow rate increased with increasing gradient on the surface and the ow speed was 40 times greater with high efficiency. 175 Vasilakis et al. reviewed a simple highspeed lling passive capillary pump integrated with lab-onprinted circuit board technology (Lo-PCB), with induced capillary pressure to produce stable ow rates. 176 Akyazi et al. developed a new concept for uid ow manipulation in micro paper-based analytical devices (PADs), where the ionogel (considered to be a negative passive pump) could drive uids by the swelling effect, which controlled the ow direction and volume to the outlet. 177    Mei et al. developed a capillary-based open microuidic device (COMD) for monodispersed droplet generation, from gas bubbles to highly viscous polymer solutions to provide high throughput in industrial emulsication. Capillary action was used as a portable sidewall of another microchannel with controllable size. 179 Kim et al. described a liquid additive, which passively controlled the velocity of cells within a detectable range during capillary sample loading, thereby eliminating the need for bulky and expensive pumping equipment. It also required the adoption of an immune bead assay, which was quantied with a portable uorescence cell counter based on a blue-light-emitting diode. 180 Moonen et al. investigated capillary-based passive pumping for optimized neuronal cell trapping across a microsieve with gentle velocity proles and This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 11652-11680 | 11667 high survival rates. 181 Reports on capillary-based passive pumping in microuidics are summarised in Table 4.
Limitations. Though the capillary circuit and capillary elements contribute many characteristics to microuidics, they are unable to deliver self-regulated ows for performing advanced functions. 184 The capillary action depends on the surface tension and adhesion of the analyte to the channel surfaces. 151 The concentration of uid in the outlet reservoir limits the overall ow duration; however, to alleviate this, multiple outlet reservoirs are used to reduce the rate of concentration in each reservoir to maintain longer ow duration. 158

Gravity-driven ow
This is a technique in which the uid ow is driven or assisted by the earth's gravity, depending on the viscosity of the uid and the height of the inlet from the surface. A comfortable and straightforward analysis of the ow rate in the MFD can be determined with the help of the gravity-driven ow principles which accelerate the passive pumping system was reported by Mäki et al. in 2014. 185 The microuidics and LOC devices utilizing the gravity for their passive operations are shown in Fig. 8.
In 2003, Cho et al. described a simple disposable polymeric microscale integrated sperm sorter (MISS) to isolate sperm from semen, which was unattainable by traditional sperm-sorting methods. 186 Similarly, Suh et al. explained the purpose of obtaining the motile sperm through gravity-driven ow with increased morphology from normal semen. 187 Not only sperm but blood cells could be counted using a gravity-driven MFD that was fabricated using UV laser ablation and resin lm lamination by Yamada et al. In 2005. 188 Huh et al. developed a unique combination of a simple, self-contained microfabricated device using eld-driven separation with microhydrodynamics-assisted separation, where the earth's gravity was used to drive the uids across the channel to assist in separation. 189 For cell culture, Lee et al. designed a compatible passive gravity-driven MFD with long-term continuous cell culture perfusion, which was adapted from the standard 96-well plate format to remove tubing and connectors. 190 In 2010, Zhang et al. presented a gravity-actuated technique where an oil phase was loaded into the infusion set and then the water phase was pumped into the PTFE (polytetrauoroethylene) tubing by the gravity of the oil phase. 191 Thereby, Sung et al. developed a gravity-induced-ow MFD to provide longterm ow by eliminating the bubble formation based on the mathematical PK-PD (pharmacokinetic/pharmacodynamic) model. 192 Abaci et al. in 2014 developed a pumpless recirculating gravity-driven human skin equivalent (HSE)-on-a-chip platform that was simple to fabricate, handle and operate when placed on a rocking platform. 22 A way of inducing uid ow, by elevating the inlet of the channel against gravity by placing it on a rocking platform was demonstrated by Esch et al. in 2015. 193 A compactly congured differential ow resistance microuidic single-cell trapping device with a shorter ow path was introduced and demonstrated by Jin et al. to increase the speed and throughput (in both mathematical and numerical simulations). 194 James et al. developed an MFD with efficient trapping of single cells through hydrodynamic ow by positioning the microwells along the ow path, which acted as a mechanical barrier. The hydrodynamic ow inside the channel was analyzed with Comsol Multiphysics with different boundary conditions for varying particle sizes. 195 Simultaneously, Narayanamurthy et al. described the rate of single-cell trapping based on the shape of the channel in a passive biochip and concluded that hexagonally positioned microwells possessed high single-cell capture (SCC) percentages. The SCC potential of microuidic biochips was found to be an improvement over straight channels, branched channels, or serpentine channels. Multiple cell capture (MCC) began to decrease from the straight channel, branched channel, or serpentine channels. 196 Kim et al. presented a design for gravity-driven microuidic systems that could generate self-switching pulsatile ows to mimic physiological blood ow pulsing. 197 Comprehensive developments in gravity-based passive pumping in microuidics are summarised in Table 5.
Limitations. For specic cell culture applications, dynamic or pulsatile ow was necessary, while gravity-driven micro-uidic systems could only generate continuous ow. 57 However, with the insertion of a periodic rocking device, an active pump would be suitable to produce a dynamic ow. 192 Upon tilting, this method for establishing an air-epidermal interface within a gravity-driven ow system is highly complex. Hence, the reservoir volume was subjected to minor adjustments and properly levelled surface for tilting. 22

Hydrostatic pressure
Hydrostatic pressure is the force exerted by a uid at equilibrium at a given point acting with equal magnitude in all directions, due to the force of gravity. Hydrostatic pressure increases in proportion to the depth measured from the surface because of the increasing weight of the uid, exerting a downward force from above. 201 Weigl et al. presented hydrostatic pressure-driven micro-uidic elements, including mixers, valves, and detectors that were employed in ultra-low-cost disposable qualitative and semi-quantitative medical and environmental assays for the home, office, and eld use, and for sample or reagent preparation tools to provide processed liquids for downstream analysis. 202 Subsequently, a microuidic cartridge was planned for the extraction of small molecules by the hydrostatic pressure from the mixture of small and large molecules. 203 For the culture medium, Marimuthu et al. developed a pumpless perfusion microuidic chip that could deliver a constant ow rate with reduced pressure due to the intravenous (IV) setup used at the inlet, over the siphon-based gravity-driven micro-uidics. 204 Later, Seo et al. employed hydrostatic pressure to sort motile sperms of three species, namely bull, mouse, and human, with an average sorting rate. 198 The culturing of mouse testis tissue and spermatogenesis in a hydrostatic pressure environment were developed by Komeya et al. In addition, researchers used a resistance circuit to induce slow and longlasting medium ow in the channel. 205 MFD and LOC devices employing hydrostatic pressure for their passive operations are shown in Fig. 9. Comprehensive reports on the hydrostatic pressure-driven passive pumping technique in microuidics are summarised in Table 6.
Limitations. In a few cell cultures and trapping experiments, a more accurate ow rate was not obtained due to the reservoir

Vacuum driven
A vacuum refers to any space in which the pressure is lower than atmospheric pressure (negative pressure). Vacuum-driven devices use the ability of an MFD to suck the sample through negative pressure without any extra on/off-chip microuidic units. Low vapour pressure and degassing become essential when the vacuum pressure falls below this vapour pressure.
Degassing is the process of removal of any gas in the channel through permeability or solubility within the membrane to generate a vacuum. 206 Song et al. worked to produce a better demonstration of vacuum degassed ow in POC applications using a PDMS-based material, coated with Parylene C. 207 Later, Monahan et al. developed a channel outgassing technique, where the channel was evacuated, and the negative pressure generated inside the channel assisted the ow with 90% efficiency in eliminating bubble formation. 208 A new powerfree pumping method for PDMS MFD was developed by  Hosokawa et al. to overcome a signicant issue in the detection of gold nanoparticle DNA analysis. 75 Subsequently, they presented an on-chip heterogeneous immunoassay with a simple structure and operational procedure. Redissolution through the microchannel walls developed the capacity to drive the solution movement and prevent air bubble trapping. 209 Dimov et al. reported a microuidic blood analysis system where the plasma was separated by trapping the RBC and WBC in a trench, and the inlet uid was pumped due to the suction of the chamber, as a result of pre-evacuating the channel. 210 Liang et al. developed a degassed channel with negative pressure to pump the uid irrespective of the surface tension of the uid. Before sample loading, channel geometry, surface area, PDMS thickness, exposure area, vacuum degassing time, and post-vacuum idle time when the device was exposed to atmospheric conditions were inspected. 211 Eventually, Li et al. designed a PDMS modular pump in a vacuum desiccator with high exibility and reduced fabrication complexity to create negative pressure in the channel for sample movement. 212 Furthermore, Li et al. developed a self-powered one-touch nger-press-activated blood extraction system based on the negative pressure-driven force developed in the pre-vacuum actuator. 213 Xu et al. designed a vacuum-driven power-free MFD that depended on the gas solubility or permeability of PDMS that restored the air inside it and encouraged the transfer of air into a vacuum. 214 Also, Li et al. worked on the development of a self-powered PDMS-based microuidic droplet generator with mono-dispersed droplet generation and multi-sample introduction in a controlled way. 215 A simplied and inexpensive passive microuidic channel with excellent analytical performance to carry out microow injection analysis (mFIA) was investigated by Agustini et al. 216 Subsequently, Liu et al. developed a paraffin wax and glass MFD through higher driving pressure to maintain longer working times where the inlet was exposed to air with a slower decline in ow rate. 217 Li et al. proposed a high-pressure MFD to drive the uid using gas permeation and suppressing the generation of bubbles under high temperatures. 218 Then, a power-free and self-contained uid reactor array that performed parallel uid loading, metering and mixing through pre-degassed PDMS and the change in capillary force due to sudden narrowing of the channel cross-section was developed by Liu et al. 219 Diffusion. This is the movement of solute particles from a region of higher concentration to a region of lower concentration due to the random motion of atoms or molecules with or without a semi-permeable membrane. Warrick et al. compared the theoretical and experimental data of microchannels and standard wells on the metrics of sample washing and experimental error in treatment concentrations. 220 Permeation. Verneuil et al. described the process of water permeation in microstructures through silicone which has a strong inuence on uid displacement. 221 Randall et al. conrmed the use of lubrication approximation to solve gapaveraged velocity while transferring water from a single rectangular channel into a PDMS using permeation-driven ows. 222 Eddings et al. conrmed that the uid ow in a PDMS-based MFD was generated through high gas permeability at the membrane. 223 The evaporation of water in the polymer walls through permeation resulted in uid ow due to the excess pressure stored in the compartment. 224 A hybrid PDMS utilizing permeability for uid ow is shown in Fig. 10. MFD and LOC devices using a vacuum for their passive operation are shown in Fig. 11. Comprehensive developments in vacuum/permeationdriven passive pumping in microuidics are summarised in Table 7.
Limitations. Due to the limited air diffusion and hydrophobicity of the PDMS material, the ow rates acquired were reduced to nanoliters per second. This could be enhanced by increasing the surface area of the material used. 223

Critical discussions and limitations
The evolution of new microuidic tools for genomics, proteomics, and metabolomics is progressing swily in research laboratories and will provide the motive for large-scale production. Passive ow methodologies, such as surface-tension-driven ow, capillary-based ow, gravity-driven ow, hydrostatic-pressuredriven ow, and osmosis-driven ow techniques are found to be suitable in assisting the ow without any external sources. Despite the benecial properties of PDMS that permitted its fast enactment in applied elds, there are several limitations in using the material in biomedical research. 226 The control of uid ow over space and time with sufficient accuracy was the fundamental challenge in designing MFDs. 227 The initial fabrication of MFDs required clean-room facilities for silicon or glass devices which were eliminated by polymers and elastomers devices. However, PDMS was limited by its ability to withstand high temperature, difficulty in developing complex multi-layered 3D-congured devices, and incompatibility with organic solvents and low molecular weight organic solutes due to their surface chemistry. 228 The usage of inert chemicals and utilization of highthroughput methods for manufacturing and permeability limit the application of PDMS devices in a few cases. 229 Evaporation affects the application of microuidics in various domains. 230 This could be overridden by maintaining a uniform surface throughout the entire channel area. In pressure-driven techniques, variation in pressure is noted due to the difference in channel length and utilization of different ngers at the inlet. 119,127 This unbalanced pressure may lead to backow, followed by clogging or the entire destruction of the device. Hence, a uniform pressure needs to be maintained for a long period. This can be achieved by osmosis-driven MFDs. However, in osmosis-driven MFDs, the osmotic reagent had to be refreshed at regular intervals, which restricted their applications. 145 In addition to that, selection of a suitable membrane is necessary to maintain the pH and to avoid clogging when biological cells or biomolecules are used. 143 Hence, capillary-based passive ow techniques are used as they depend only on the design and material characteristics of the MFD. On the other hand, in some instances, due to a drop in ow rate, essential pressure prediction at each step decreased sensitivity in capillary-based passive ow techniques. 151,165 This could be averted by selection of suitable materials with the ability of spontaneous sample lling in the column. 231 Because of this, under many circumstances, gravity-driven techniques are adopted, but they generate an unstable or decreasing ow rate, which was further eliminated by maintaining an appropriate reservoir volume and proper inlet pressure. 22,198 Whereas hydrostatic-pressure-driven techniques are limited by the development of Laplace pressure at the air-liquid interface due to the affinity between the liquid, the atmosphere, and the reservoir parameters. 232 Another listed shortcoming was the linear pressure drop with time. These errors could be avoided, if the pressure difference was obtained by varying the altitude of the liquid to the atmospheric interface in the proper way. 197 They could be eliminated by using a vacuum MFD; nevertheless evacuation is necessary to drive the uid in order to reduce the pressure within the channel. Due to the presence of high pressure initially, the suction of the uid is higher and gradually decreases as the pressure decreases. Hence a syringe pump is used to maintain a constant ow in the device. This additional requirement can be neglected by the use of simple capillary force. 211 Based on the study performed, the vacuumdriven techniques with complex construction were not suitable for continuous operation due to the evaporation effect and the loss of sample in a few cases. 75,208,216 Besides the above-mentioned limitations, these methods found numerous clinical applications, because they use ultra-low volumes of biouids for processing and can be accomplished quickly and efficiently.

Industrial perspectives
One of the promising types of assistance of microuidics is found in fast, reliable, and accurate POC diagnostic devices. Detection and diagnostics of communicable diseases can decrease the mortality rate in many developing and less developed countries. As reported by the World Health Organization (WHO), nearly 46% of new tuberculosis case are not detected, causing over 3 million incidences to be missed annually. 233,234 Also, according to a recent report by WHO, the lack of proper diagnostic capacity is a signicant challenge in monitoring effective service coverage. As an example, cholera was estimated to have infected 1.3-4 million people annually and to have caused 21 000-143 000 deaths each year during 2008-2012. But the average annual numbers of cases and deaths reported to WHO were only 313 000 and 5700, respectively. One of the major issues is the lack of diagnostic capacity, which results in the exact burden of the disease being unknown, thus making it difficult to take preventive measures. 235 Although many researchers in the microuidic domain are focusing on developing diagnosis and detection devicessearching for a few related keywords such as diagnosis, detection, and pathogen detection in LOC, bio-microuidics, and biomedical microdevices resulted in nearly 3000 papersmany of the products do not end up on the market. One major issue in this regard is transforming common expensive sample delivery systems in laboratories, such as syringe pumps, into passive and low-cost delivery systems to meet the needs of the market in developing and less developed countries. To analyse the status of current commercialized MFDs, we delved into the circle database of uFluidix Inc (http://circle.uuidix.com). The database includes a list of registered start-ups offering microuidic products. Nearly 85% of the analysed start-ups in the diagnostics industry were utilizing a passive method for sample delivery. This clearly emphasizes the importance of passive techniques for industrial sectors.

Conclusion and future directions
Microuidics is articulated as a multidisciplinary research eld that requires basic knowledge in uidics, micromachining, electromagnetics, materials, and chemistry to nd their relevance in the pharmaceutical industry, diagnosis, healthcare, and life science research. LOC is one of the essential applications of microuidics, and is also a revolutionary tool for many varieties of applications in chemical and biological analyses due to its fascinating advantages (speed, low cost, simplicity, and self-testing) over conventional chemical or laboratory equipment.
Microuidics covers the science of uidic behaviours at micro/nanoscales employed in design engineering, simulation, and fabrication of uidic devices. It is the backbone of biological or biomedical microelectromechanical systems (BioMEMS) and the LOC concept, as most biological analysis involves uid transport and reaction. MFDs have been operated in inkjet printing, blood analysis, biochemical detection, chemical synthesis, drug screening/delivery, protein analysis, DNA sequencing, and so on. Several different MFDs have been developed with basic structures analogous to macroscale uidic devices. Such devices include microuidic valves, microuidic pumps, and microuidic mixers. Active devices are usually more expensive, due to their functional and fabrication complexities. However, it has been very challenging to implement these actuation schemes fully at the microscale, owing to the requirements for high voltages, electromagnets, etc. Typically, passive MFDs do not require an external power source, where control is instituted by the energy drawn from the working uid, or based purely on surface effects, such as surface tension or uid pressure, with high reliability due to the lack of mechanical wear and tear. Hence, upcoming industrial sectors rely on passive methods to achieve better results in a composed way.
MFDs offer very repeatable performance, once the underlying phenomena are well understood and characterized. They are well suited to bioMEMS applications, as they can handle several microuidic manipulation sequences. They are also well suited to the low-cost mass production of disposable MFDs to specically work with blood. In addition, inexpensive and simple fabrication techniques can promote the use of paperbased MFD in various research elds, such as drug testing and viral detection.
Passive MFDs with high throughput, high ow rate, reduced operation time and easy equipment handling are expected to have protable outcomes. Therefore, MFDs with passive methods are designed in such a way as to offer a wide range of optimizations that can be performed within the channel and reservoir dimensions for employment to certain specic applications with better ow rates. In the future, device progress will focus on developing new materials for substrates in such a way as to overcome the drawbacks of currently existing devices. Passive systems cannot maintain constant ow rates. Hence, systems with constant ow rates are expected to be developed as an advantageous method. In addition, a low operation time remains an essential parameter. A reduction in running time can be demonstrated to be the crucial factor on which to focus during upcoming developments. Future work is expected to focus on exploring these listed areas, so that microuidics can nd application in many other elds to satisfy growing demands and needs.

Conflicts of interest
There are no conicts of interest to declare.