**4. Microfluidic devices: an application of Chemotaxis**

Microfluidics is the technology based upon behavior of fluids in the microenvironment. Fluids tend to behave very differently in micrometric scale as compared to macro scale. These characteristics of fluids are now been used for various studies based upon taxis. In macroscopic system, pressure, volume and temperature are the key players whereas viscosity, surface tension, high shear rate and geometric effects (high surface to volume ratios, constriction, and bifurcation) are the key drivers

**151**

*Microfluidic Devices: Applications and Role of Surface Wettability in Its Fabrication*

of the microfluidic system [29]. Microfluidics is the integration of fluids physically restricted to sub-millimeter dimensions with micro/nanostructures and devices [30]. Microfluidics is an emerging interdisciplinary field consisting of engineering, physics, chemistry, microtechnology, biotechnology and material sciences [31]. The reason for its emergence is miniaturization of operational unit in the microfluidic devices. Miniaturization is preferred as all operations can be packed in small form that can be automated and is portable [32]. Low amount of materials and chemicals are required for development and samples required is also less. Automation enables widespread use of the system without any special training requirements. Easy disposals, low cost, reduction of cross-contamination and fast response time are

The global size of microfluidic devices was USD 13.5 billion in 2019 and is supposed to have a compound annual growth rate of 11.3%. The large market size is due to its multi-application and ease of usability. Basic layout of microfluidic devices consist of incorporated fluid channels in at least one direction. These channels provide high surface to volume ratio which is useful in applications such as biochemical analysis, antimicrobial susceptibility test and heat exchange modules. This field started with applications in chromatography and electrophoresis [34]. With time it has evolve and currently it has vast applications due to development of new fabrication materials and technologies [31]. Its applications include environmental sensing, biomedical applications, drug discovery, drug delivery, micro scale energy systems, artificial organs, micro scale chemical testing and production, micro propulsion, combinatorial synthesis and assays. These applications have been classified under broad categories

Microfluidics can be used in biomedical field as analytical arrays, gradients, separators, microdiluters, gel structures, droplets, painting cells and devices [35]. In arrays, a set of multiple microchannels is used to study the relationship between different cells with proteins or chemicals within a combinatorial system. This type of system can be used for detection of specific proteins in large number of samples, antibiotic resistance testing, etc. Microfluidics can be used for generation of very steep gradients that cannot be created using other macro techniques [35]. These gradients are useful in the study of macromolecules and cells in response to their varying environment. Biochemical gradients are useful in dictating physiological processes such as proliferation, differentiation and migration. These gradients play an important role in tissue generation as well. They are used for organ on chip techniques also. Phil et al. used drug gradients for activity measurement over CHO cells [36]. Migration and behavior of neutrophils according to protein gradient has also been studied [37]. Chung et al. used growth factor gradient to study the

Microfluidics can be used as diluters where solution is passed through series of controlled dilutions to be used in a specific assay. Ainla et al. have shown use of pulse width flow modulation based designing of microdiluter [39]. They used this microfluidic diluter for analyzing the effect of Ca(2+) concentration over phospholipid bilayer spread onto a SiO2 surface. Microdiluters can also be used as immunoassays for detection of multiple antigens at a same time [40]. Microfluidics can be used in conjunction with gels or microchannels can be made in gels using soft lithography technique. Various types of gels in which microfluidic can be fabricated are agarose, agar and calcium alginate. These types of systems can be used to study complex microenvironment of cells. Takeuchi et al. used microchannels fabricated in agarose to grow *Escherichia coli* in presence of various molecules that can alter

*DOI: http://dx.doi.org/10.5772/intechopen.93480*

other benefits of the microfluidic system [33].

differentiation of human neural stem cells [38].

for discussion in this chapter.

**4.1 Biomedical applications**

#### *Microfluidic Devices: Applications and Role of Surface Wettability in Its Fabrication DOI: http://dx.doi.org/10.5772/intechopen.93480*

of the microfluidic system [29]. Microfluidics is the integration of fluids physically restricted to sub-millimeter dimensions with micro/nanostructures and devices [30]. Microfluidics is an emerging interdisciplinary field consisting of engineering, physics, chemistry, microtechnology, biotechnology and material sciences [31]. The reason for its emergence is miniaturization of operational unit in the microfluidic devices. Miniaturization is preferred as all operations can be packed in small form that can be automated and is portable [32]. Low amount of materials and chemicals are required for development and samples required is also less. Automation enables widespread use of the system without any special training requirements. Easy disposals, low cost, reduction of cross-contamination and fast response time are other benefits of the microfluidic system [33].

The global size of microfluidic devices was USD 13.5 billion in 2019 and is supposed to have a compound annual growth rate of 11.3%. The large market size is due to its multi-application and ease of usability. Basic layout of microfluidic devices consist of incorporated fluid channels in at least one direction. These channels provide high surface to volume ratio which is useful in applications such as biochemical analysis, antimicrobial susceptibility test and heat exchange modules. This field started with applications in chromatography and electrophoresis [34]. With time it has evolve and currently it has vast applications due to development of new fabrication materials and technologies [31]. Its applications include environmental sensing, biomedical applications, drug discovery, drug delivery, micro scale energy systems, artificial organs, micro scale chemical testing and production, micro propulsion, combinatorial synthesis and assays. These applications have been classified under broad categories for discussion in this chapter.

### **4.1 Biomedical applications**

*21st Century Surface Science - a Handbook*

surface having higher moisture content [18].

motion is observed in the tissue model [21].

in three-dimension in open water [22].

Crustaceans and American lobsters [25].

**3.9 Magnetotaxis**

**3.10 Phototaxis**

**3.11 Rheotaxis**

**3.12 Thermotaxis**

female body [28].

is termed as negative hydrotaxis. Hydrotaxis is observed in the *C. elegans* as they move towards their preferred water content for mating, geographical distribution and reproduction [17]. It is also observed in the cyanobacterium in desert crusts. Cyanobacteria colonies are observed 1.5–2.0 mm deep into the desert crust but when crust surface is saturated with water, cyanobacterium moves towards the

Magnetotaxis is the movement due to magnetic field. This type of movement is a character of diverse group of gram-negative bacteria that perform their orientation and coordination movements according to earth's magnetic field [19]. They are majorly aquatic and swim along the geomagnetic field lines. These types of bacteria are also termed as magnetotactic bacteria [20]. Supramolecular adaptive nanomoters have been developed that exhibit magnetotactic behavior and their guided

Phototaxis is the movement towards or away from the light source. This type of movement is characteristic of phototrophic organisms and is also observed in plants. Prokaryotes use type-I sensory rhodopsin photoreceptors for phototaxis and it allows them movement towards steep light gradient. Cyanobacteria can also perform phototaxis but they also can perform it in two-dimension only through gliding on the surface. Eukaryotes have the ability to navigate through light vector

Rheotaxis is the movement in response to water or air current. This type of motion is observed in aquatic animals where their movement occurs in response to water current [23]. When movement is towards oncoming water current, it is termed as positive rheotaxis while movement opposite of oncoming water current is termed negative rheotaxis [24]. This type of motion is observed in zebrafish,

Thermotaxis is the movement towards or away from temperature gradient. In this motion, organism move towards temperature source. Slime molds and nematodes are known to move along shallow temperature gradient [26, 27]. Mammalian sperm is also observed to perform theromtaxis to reach towards the oviduct in the

Microfluidics is the technology based upon behavior of fluids in the microenvironment. Fluids tend to behave very differently in micrometric scale as compared to macro scale. These characteristics of fluids are now been used for various studies based upon taxis. In macroscopic system, pressure, volume and temperature are the key players whereas viscosity, surface tension, high shear rate and geometric effects (high surface to volume ratios, constriction, and bifurcation) are the key drivers

**4. Microfluidic devices: an application of Chemotaxis**

**150**

Microfluidics can be used in biomedical field as analytical arrays, gradients, separators, microdiluters, gel structures, droplets, painting cells and devices [35]. In arrays, a set of multiple microchannels is used to study the relationship between different cells with proteins or chemicals within a combinatorial system. This type of system can be used for detection of specific proteins in large number of samples, antibiotic resistance testing, etc. Microfluidics can be used for generation of very steep gradients that cannot be created using other macro techniques [35]. These gradients are useful in the study of macromolecules and cells in response to their varying environment. Biochemical gradients are useful in dictating physiological processes such as proliferation, differentiation and migration. These gradients play an important role in tissue generation as well. They are used for organ on chip techniques also. Phil et al. used drug gradients for activity measurement over CHO cells [36]. Migration and behavior of neutrophils according to protein gradient has also been studied [37]. Chung et al. used growth factor gradient to study the differentiation of human neural stem cells [38].

Microfluidics can be used as diluters where solution is passed through series of controlled dilutions to be used in a specific assay. Ainla et al. have shown use of pulse width flow modulation based designing of microdiluter [39]. They used this microfluidic diluter for analyzing the effect of Ca(2+) concentration over phospholipid bilayer spread onto a SiO2 surface. Microdiluters can also be used as immunoassays for detection of multiple antigens at a same time [40]. Microfluidics can be used in conjunction with gels or microchannels can be made in gels using soft lithography technique. Various types of gels in which microfluidic can be fabricated are agarose, agar and calcium alginate. These types of systems can be used to study complex microenvironment of cells. Takeuchi et al. used microchannels fabricated in agarose to grow *Escherichia coli* in presence of various molecules that can alter

their phenotype [41]. Cabodi et al. used alginate based microchannels for study of mass transfer in channels [42]. Complex Microfluidic systems are now being highly researched and commercialized to develop point of care/lab-on-chip (LOC) devices and organ on chip. These devices have high potential as they can provide the customer with the easy of usability, less sample requirement, time and cost efficiency.

Point of care devices are diagnostic measures that are directly used by patients and without requirement of medical staff. A simple paper based microfluidic that can be used as point of care device are known as lateral flow test (LFT). Porous material such as glass fiber, nitrocellulose and cellulose paper can be used for fabrication of LFT. The components on microfluidic LFT device are sample collection pad, a dried conjugate pad followed by a reaction area and an absorbent wicking pad. This is incorporated within a plastic housing and plastic barriers throughout to maintain one dimensional flow. The best example of LFT is dipstick pregnancy test kit. This test works on the principle of an immunoassay. Sample which is urine is applied to the sample pad and rehydrates the goldnanoparticles conjugated detection antibodies [43]. These rehydrates antibodies bind to the target antigen present in the sample. Together they flow to the capture region which consists of control and test line. At the test line, non-labeled antibodies specific for the detection antigen are immobilized. When rehydrated labeled antibodies conjugated with the sample antigen reaches test line, it binds to the non-labeled antibodies specific for that same antigen. This interaction gives visual color change thereby making test line visible in case of positive results. This process is depicted through **Figure 1a** and **b**. The wicking pad in the device performs function of attracting the sample through LFT. After reaction membrane is completely wetted, the capture region functions through capillary action.

Paper microfluidics has also been used to provide point of care diagnostics for non-communicable diseases such as cardiovascular disease and cancer. In this

#### **Figure 1.**

*(a) Non-wetting phenomenon, (b) wetting phenomenon, (c) larger contact angle (non wetting), (d) wetting, (e) angle close to zero complete (wetting).*

**153**

film based microreactor [56].

*Microfluidic Devices: Applications and Role of Surface Wettability in Its Fabrication*

tures that contain genetic information) in the fetus [46].

work, synthetic urinary biomarker is used which is detected through paper microfluidics [44]. These types of devices are also being used for saliva based detection of oral diseases. In the research work by Amy et al. point of care diagnostic device for oral diseased was developed using monolithic disposable cartridge. It was designed in a compact analytical device. This device combined sample pre-treatment procedure of filtering, enrichment and mixing of sample with electrophoretic immunoassays. It can efficiently and quickly measure analyte concentration in the minimally treated and very low volume (20 μl) of saliva sample [45]. Microfluidic devices are also used for digital polymerase chain reaction (PCR) which is a very powerful gene expression analytical tool. Christina et al. showed use of microfluidic based digital PCR for prenatal detection of fetal aneuploidy. Fetal Aneuploidy is the presence of an abnormal number of chromosomes (struc-

Organ on chip is the new class of laboratory models that have advantages of both

The development in the field of integrated microfluidics was successfully laid by its incorporation with the optical elements such as plasmonic surfaces [51] and waveguides [52]. In 2000s, the development of liquid-crystal switchable gratings, microfluidically tunable photonic crystal fibers and bubble switch laid the foundation of using microfluidics as an essential part of the photonic devices. During mid-2000s a new field of "optofluidics" was evolved from the existing technologies in the field of photonics and microfluidics. Using microchannels and photonic elements, optofluidics has the strength of having precise control over light and fluidics at small scale [53]. Microfluidic systems are being used for development of photocatalytic microreactor. A planar microfluidic reactor was developed by Lei et al. It consisted of the small planar chamber where two TiO2 coated slides were used as top cover and bottom substrate. Microstructured UV-cured NOA81 layer was used as the sealant and flow input/output. This reactor has advantages of microfluidics such as easy control of flow, rapid fabrication and large surface/ volume ratio. It is the key to more efficient photocatalytic water treatment [54]. TiO2 based microreactor has been developed by Matic et al. for photocatalytic applications. This system was fabricated on metal-titanium foil. Titania nanotubes were mechanically engraved in the substrate foil. Using anodization & hydrothermal treatment TiO2 anatase film was immobilized over the inner layer of these tubules. An additional TiO2 anatase layer was added on top of the film to provide larger photocatalytic area. This microreactor depicted enhanced durability and efficiency [55]. Meng et al. also developed microfluidic based photocatalytic microreactor. They used nanofibrous TiO2 through electrospun to develop this photocatalytic microreactor. It depicted enhance efficiency as compared to TiO2

*in vivo* and *in vitro* models. These chips are microfluidic devices in which tissue of interest is cultured in the favorable microenvironment simulating the actual physiological conditions efficiently. These types of devices can also be used in the field of personalized medicine. For personalized medicine, cells from specific donor patients and healthy patients can be studied under the same environment. Various examples of such devices are lung on a chip [47], atherosclerosis on a chip that made study of physiological functions of an organ and its response to various stimuli feasible [48]. Other examples are bacteria inhabited gut on a chip [49] and blood brain barrier on a chip [50]. This field of organ on chip is emerging rapidly and showcasing various organs' culture and their physiological microenvironment

*DOI: http://dx.doi.org/10.5772/intechopen.93480*

simulation on these microfluidic chips.

**4.2 Energy applications**

*Microfluidic Devices: Applications and Role of Surface Wettability in Its Fabrication DOI: http://dx.doi.org/10.5772/intechopen.93480*

work, synthetic urinary biomarker is used which is detected through paper microfluidics [44]. These types of devices are also being used for saliva based detection of oral diseases. In the research work by Amy et al. point of care diagnostic device for oral diseased was developed using monolithic disposable cartridge. It was designed in a compact analytical device. This device combined sample pre-treatment procedure of filtering, enrichment and mixing of sample with electrophoretic immunoassays. It can efficiently and quickly measure analyte concentration in the minimally treated and very low volume (20 μl) of saliva sample [45]. Microfluidic devices are also used for digital polymerase chain reaction (PCR) which is a very powerful gene expression analytical tool. Christina et al. showed use of microfluidic based digital PCR for prenatal detection of fetal aneuploidy. Fetal Aneuploidy is the presence of an abnormal number of chromosomes (structures that contain genetic information) in the fetus [46].

Organ on chip is the new class of laboratory models that have advantages of both *in vivo* and *in vitro* models. These chips are microfluidic devices in which tissue of interest is cultured in the favorable microenvironment simulating the actual physiological conditions efficiently. These types of devices can also be used in the field of personalized medicine. For personalized medicine, cells from specific donor patients and healthy patients can be studied under the same environment. Various examples of such devices are lung on a chip [47], atherosclerosis on a chip that made study of physiological functions of an organ and its response to various stimuli feasible [48]. Other examples are bacteria inhabited gut on a chip [49] and blood brain barrier on a chip [50]. This field of organ on chip is emerging rapidly and showcasing various organs' culture and their physiological microenvironment simulation on these microfluidic chips.

#### **4.2 Energy applications**

*21st Century Surface Science - a Handbook*

region functions through capillary action.

their phenotype [41]. Cabodi et al. used alginate based microchannels for study of mass transfer in channels [42]. Complex Microfluidic systems are now being highly researched and commercialized to develop point of care/lab-on-chip (LOC) devices and organ on chip. These devices have high potential as they can provide the customer with the easy of usability, less sample requirement, time and cost efficiency. Point of care devices are diagnostic measures that are directly used by patients and without requirement of medical staff. A simple paper based microfluidic that can be used as point of care device are known as lateral flow test (LFT). Porous material such as glass fiber, nitrocellulose and cellulose paper can be used for fabrication of LFT. The components on microfluidic LFT device are sample collection pad, a dried conjugate pad followed by a reaction area and an absorbent wicking pad. This is incorporated within a plastic housing and plastic barriers throughout to maintain one dimensional flow. The best example of LFT is dipstick pregnancy test kit. This test works on the principle of an immunoassay. Sample which is urine is applied to the sample pad and rehydrates the goldnanoparticles conjugated detection antibodies [43]. These rehydrates antibodies bind to the target antigen present in the sample. Together they flow to the capture region which consists of control and test line. At the test line, non-labeled antibodies specific for the detection antigen are immobilized. When rehydrated labeled antibodies conjugated with the sample antigen reaches test line, it binds to the non-labeled antibodies specific for that same antigen. This interaction gives visual color change thereby making test line visible in case of positive results. This process is depicted through **Figure 1a** and **b**. The wicking pad in the device performs function of attracting the sample through LFT. After reaction membrane is completely wetted, the capture

Paper microfluidics has also been used to provide point of care diagnostics for non-communicable diseases such as cardiovascular disease and cancer. In this

*(a) Non-wetting phenomenon, (b) wetting phenomenon, (c) larger contact angle (non wetting), (d) wetting,* 

**152**

**Figure 1.**

*(e) angle close to zero complete (wetting).*

The development in the field of integrated microfluidics was successfully laid by its incorporation with the optical elements such as plasmonic surfaces [51] and waveguides [52]. In 2000s, the development of liquid-crystal switchable gratings, microfluidically tunable photonic crystal fibers and bubble switch laid the foundation of using microfluidics as an essential part of the photonic devices. During mid-2000s a new field of "optofluidics" was evolved from the existing technologies in the field of photonics and microfluidics. Using microchannels and photonic elements, optofluidics has the strength of having precise control over light and fluidics at small scale [53]. Microfluidic systems are being used for development of photocatalytic microreactor. A planar microfluidic reactor was developed by Lei et al. It consisted of the small planar chamber where two TiO2 coated slides were used as top cover and bottom substrate. Microstructured UV-cured NOA81 layer was used as the sealant and flow input/output. This reactor has advantages of microfluidics such as easy control of flow, rapid fabrication and large surface/ volume ratio. It is the key to more efficient photocatalytic water treatment [54]. TiO2 based microreactor has been developed by Matic et al. for photocatalytic applications. This system was fabricated on metal-titanium foil. Titania nanotubes were mechanically engraved in the substrate foil. Using anodization & hydrothermal treatment TiO2 anatase film was immobilized over the inner layer of these tubules. An additional TiO2 anatase layer was added on top of the film to provide larger photocatalytic area. This microreactor depicted enhanced durability and efficiency [55]. Meng et al. also developed microfluidic based photocatalytic microreactor. They used nanofibrous TiO2 through electrospun to develop this photocatalytic microreactor. It depicted enhance efficiency as compared to TiO2 film based microreactor [56].

Recently, applications of microfluidics have been developed in the form of microfluidic fuel cells. In these cells, all the systems such as fluid delivery, removal, etc. is confined to the microfluidic channel only. These cells do not require a physical barrier for separation of fuel and oxidant species and therefore they operate in co-laminar flow mode. Whereas, in conventional fuel cell a physical barrier such as proton exchange membrane is required. They can be used to power microsystems, generate on-chip power and in consume electronics as well [31]. Microfluidic fuel cells have attracted huge researchers as they are portable power sources with short startup time and environment friendly nature. Microfluidic fuel cell using laminar air flow had been developed by Eric et al. (**Figure 2a**). It was made through a Y-shaped microchannel consisting of two catalyst covered electrodes on opposite walls. Through these channels, fuel and oxidant merge and flow laminarly parallel between these two electrodes without turbulent mixing. They showed that this type of system can be effectively used to generate microscopic power source for room temperature [57]. There is patented microfluidic fuel cell system for portable energy applications. In this system, microfluidic container, substrate for catalytic composition, a liquid/gas separator, a fuel cell consisting of anode and cathode and electrical connections were all assembled to form this portable energy system [58]. The design of the system and fuel cell components is depicted through **Figure 2b** and **c**, respectively [58]. Luke et al. also developed these microfluidic cells based on microbial fuel that can be used to provide power supply to integrated biosensors. This system was developed in polydimethylsiloxane. Here, two carbon cloth electrodes and proton exchange membrane was used. *Shewanella oneidensis* MR-1 was used in anode chamber as electrogenic bacterial strain and ferricyanide was used in cathode chamber (**Figure 2d**). Maximum current of 2.59 μA was generated using this miniature microbial fuel cell [59]. Svetlana et al. developed a microfluidic cell for energy conversion. They developed hydrogen and oxygen based microfluidic cell using polydimethylsiloxane (PDMS) device. In this

#### **Figure 2.**

*(a) The reaction of antibiotic dish with bacteria, (b) glass microchannel without bacterial coating, (c) reaction of antibiotic with bacteria in microchannel, and (d) the spreading of chemical reaction in microchannel with different antibodies.*

**155**

*Microfluidic Devices: Applications and Role of Surface Wettability in Its Fabrication*

temperature with the maximum power density of 700 μW/cm2

device Pt/quartz electrodes in the form of thin film were embedded into the device. The PDMS microchannel network containing liquid electrolyte was used for immersion of electrode array into it. This also performs the function of thin glass permeable membrane for feeding reactants to the electrodes. This fuel cell operated at room

this cell was comparatively higher to the exiting higher surface electrodes based fuel

Recent growth in the field of microfluidics has been observed in the field of environmental assessment. Microfluidics is advantageous as multiple processes such as pre-treatment, pre-concentration, separation and detection are incorporated at the same platform. It is used for trace analysis of materials as less risk of contamination is there due to preclusion of sample transportation process. Microfluidics play role in the development of subsurface energy based technologies in the future. Mark et al. developed a microfluidic system based upon high temperature and pressure. Within geo-material micromodels such as rock, cement, clay, etc., direct observations for flow and transport can be made using this system and that too in reservoir conditions. In this micromodel fabrication method, 3D tomography images of real fractures were used as micromodel template. This provided better representation of the pore space and fracture geometries in subsurface formations [61]. Several microfluidic devices can be used for detection and analysis based upon electrochemistry, surface enhanced Raman spectroscopy, chemiluminescence, absorbance and laser-induced fluorescence. These electrochemical and optical based systems can be conjugated on a single micro platform to perform environmental monitoring. These labs on chip systems can be used for real time tracking of pollutants in the environment. Major advantages of these systems are portable compact size, better process control, low-cost production, real-time analysis, low sample consumption and fast response. LOC is used for real-time analysis of pollutants in wastewater. Combining it with the wireless communication, make it a strong tool for modifying

Microfluidic systems are being used for detection of formaldehyde as well. Formaldehyde is the organic volatile compound found in many household products. It is associated with health risk factors and is also a cause of sick building syndrome. Therefore its detection at real-time in the surroundings is essential for a healthy living. Liu et al. developed a paper based microfluidic system for detection of formaldehyde. Acetoacetanilide reagent is used to implant paper-based chip at reaction site. Concentration of formaldehyde is detected using UV light which induces fluorescence intensity in the dihydropyridine. Dihydropyridine is the complex of formaldehyde with acetoacetanilide. This method was used to detect formaldehyde in the commercial food samples and proved to be an efficient method for detection of formaldehyde concentration [63]. Similarly, Czugala et al. developed a fully integrated microfluidic device to provide wireless and portable analytical platform. This system can be used for detection of nitrite anions in the water. Nitrite anions are one of the water contaminants along with lead, cadmium and nitrate. In this system detection is done through analysis of color intensity of complex formed between nitrite anions and Griess reagent. This color intensity was assessed using low cost Paired Emitter Detector Diode. Biomimetic photoresponsive ionogel microvalve controlled by LED was used for manipulation of on-chip fluid. This system was one of its type that conjugated fully functional microfluidics with photobased valving and photo detection [64]. Microfluidic devices along with porous plugs have also been developed. This device can be used

. The overall lifetime of

*DOI: http://dx.doi.org/10.5772/intechopen.93480*

**4.3 Chemical and environmental applications**

data acquisition parameters and data transfer [62].

cells [60].

*Microfluidic Devices: Applications and Role of Surface Wettability in Its Fabrication DOI: http://dx.doi.org/10.5772/intechopen.93480*

device Pt/quartz electrodes in the form of thin film were embedded into the device. The PDMS microchannel network containing liquid electrolyte was used for immersion of electrode array into it. This also performs the function of thin glass permeable membrane for feeding reactants to the electrodes. This fuel cell operated at room temperature with the maximum power density of 700 μW/cm2 . The overall lifetime of this cell was comparatively higher to the exiting higher surface electrodes based fuel cells [60].

#### **4.3 Chemical and environmental applications**

*21st Century Surface Science - a Handbook*

Recently, applications of microfluidics have been developed in the form of microfluidic fuel cells. In these cells, all the systems such as fluid delivery, removal, etc. is confined to the microfluidic channel only. These cells do not require a physical barrier for separation of fuel and oxidant species and therefore they operate in co-laminar flow mode. Whereas, in conventional fuel cell a physical barrier such as proton exchange membrane is required. They can be used to power microsystems, generate on-chip power and in consume electronics as well [31]. Microfluidic fuel cells have attracted huge researchers as they are portable power sources with short startup time and environment friendly nature. Microfluidic fuel cell using laminar air flow had been developed by Eric et al. (**Figure 2a**). It was made through a Y-shaped microchannel consisting of two catalyst covered electrodes on opposite walls. Through these channels, fuel and oxidant merge and flow laminarly parallel between these two electrodes without turbulent mixing. They showed that this type of system can be effectively used to generate microscopic power source for room temperature [57]. There is patented microfluidic fuel cell system for portable energy applications. In this system, microfluidic container, substrate for catalytic composition, a liquid/gas separator, a fuel cell consisting of anode and cathode and electrical connections were all assembled to form this portable energy system [58]. The design of the system and fuel cell components is depicted through **Figure 2b** and **c**, respectively [58]. Luke et al. also developed these microfluidic cells based on microbial fuel that can be used to provide power supply to integrated biosensors. This system was developed in polydimethylsiloxane. Here, two carbon cloth electrodes and proton exchange membrane was used. *Shewanella oneidensis* MR-1 was used in anode chamber as electrogenic bacterial strain and ferricyanide was used in cathode chamber (**Figure 2d**). Maximum current of 2.59 μA was generated using this miniature microbial fuel cell [59]. Svetlana et al. developed a microfluidic cell for energy conversion. They developed hydrogen and oxygen based microfluidic cell using polydimethylsiloxane (PDMS) device. In this

*(a) The reaction of antibiotic dish with bacteria, (b) glass microchannel without bacterial coating, (c) reaction of antibiotic with bacteria in microchannel, and (d) the spreading of chemical reaction in* 

**154**

**Figure 2.**

*microchannel with different antibodies.*

Recent growth in the field of microfluidics has been observed in the field of environmental assessment. Microfluidics is advantageous as multiple processes such as pre-treatment, pre-concentration, separation and detection are incorporated at the same platform. It is used for trace analysis of materials as less risk of contamination is there due to preclusion of sample transportation process. Microfluidics play role in the development of subsurface energy based technologies in the future. Mark et al. developed a microfluidic system based upon high temperature and pressure. Within geo-material micromodels such as rock, cement, clay, etc., direct observations for flow and transport can be made using this system and that too in reservoir conditions. In this micromodel fabrication method, 3D tomography images of real fractures were used as micromodel template. This provided better representation of the pore space and fracture geometries in subsurface formations [61]. Several microfluidic devices can be used for detection and analysis based upon electrochemistry, surface enhanced Raman spectroscopy, chemiluminescence, absorbance and laser-induced fluorescence. These electrochemical and optical based systems can be conjugated on a single micro platform to perform environmental monitoring. These labs on chip systems can be used for real time tracking of pollutants in the environment. Major advantages of these systems are portable compact size, better process control, low-cost production, real-time analysis, low sample consumption and fast response. LOC is used for real-time analysis of pollutants in wastewater. Combining it with the wireless communication, make it a strong tool for modifying data acquisition parameters and data transfer [62].

Microfluidic systems are being used for detection of formaldehyde as well. Formaldehyde is the organic volatile compound found in many household products. It is associated with health risk factors and is also a cause of sick building syndrome. Therefore its detection at real-time in the surroundings is essential for a healthy living. Liu et al. developed a paper based microfluidic system for detection of formaldehyde. Acetoacetanilide reagent is used to implant paper-based chip at reaction site. Concentration of formaldehyde is detected using UV light which induces fluorescence intensity in the dihydropyridine. Dihydropyridine is the complex of formaldehyde with acetoacetanilide. This method was used to detect formaldehyde in the commercial food samples and proved to be an efficient method for detection of formaldehyde concentration [63]. Similarly, Czugala et al. developed a fully integrated microfluidic device to provide wireless and portable analytical platform. This system can be used for detection of nitrite anions in the water. Nitrite anions are one of the water contaminants along with lead, cadmium and nitrate. In this system detection is done through analysis of color intensity of complex formed between nitrite anions and Griess reagent. This color intensity was assessed using low cost Paired Emitter Detector Diode. Biomimetic photoresponsive ionogel microvalve controlled by LED was used for manipulation of on-chip fluid. This system was one of its type that conjugated fully functional microfluidics with photobased valving and photo detection [64]. Microfluidic devices along with porous plugs have also been developed. This device can be used

for size based separation of particles including microorganisms and therefore have implications as miniature filter for analysis of water samples. Living radical photopolymerization technique using wide range of polymers was used for fabrication of these devices. Salt-leaching technique was used for placement of porous plug in the microfluidic channels. Pore size of the porous plug in this device was determined using flow field-flow fractionation. It is a new and cost efficient simple tool for water assessment [65]. Research is moving at a fast pace for development and commercialization of such paper based microfluidic devices that can be conjugated with other existing techniques.
