**3.2 Anemotaxis**

Anemotaxis is the movement towards wind. It is observed in drosophila and some terrestrial mammals such as rats which tend to follow the wind. Drosophilla has been observed to move against the air current [5]. Rats were also observed to follow anemotaxis as air current carries information regarding location and odor content [6].

**149**

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

cell. Neutrophils follows path of least hydraulic resistance.

organism, immune response and cancer metastasis [8].

Barotaxis is the movement towards stimulus that is pressure. Movement due to hydraulic resistance (resistance to flow as a result of liquid) is also termed as barotaxis. This type of movement is observed in the neutrophils, a type of immune

Chemotaxis can be defined as the movement of cells towards the higher chemical concentration gradient [7]. It is directional locomotion of cells and was first described in the bracken fern spermatozoa in 1884 by Pfeffer. Later in 1888, this phenomenon was described by Leber in mammalian leukocytes in response to an injury. Chemotaxis is an important process required for the growth and development of multicellular

Durotaxis is the movement of cells towards more rigid gradient which is a result

of variation in the structural property of the extracellular matrix. This type of motion implies movement towards more stiffness [9]. This type of motion has been observed in various cell types such as human fibroblast cells, mesenchymal cells and cancer cells. Substrate rigidity is the stimulus that initiates the movement in

Electrotaxis is also termed as galvanotaxis and implies movement guided through electric field or current [11]. Living cells have the tendency to sense and follow direct current electric field. This type of movement is observed in both *in vitro* and *in vivo* conditions although the mechanism behind sensing of electric field by cells remains unclear [11]. Its applications are observed in wound healing and development. Disruption of an epithelial layer in wound leads to generation of an endogenous electric field which guides migration of cells towards the wound for

Gravitaxis is characterized by directional movement in response to gravity [13]. This type of movement is observed in the motile microorganisms such as euglena where gravity acts as stimulus to select their niche in environment. It can be both positive and negative. Positive gravitaxis implies movement towards water while negative gravitaxis implies movements towards the surface [14]. This type of motion is observed in *Drosophila melanogaster* and around 18 genes have been identified that mediate this gravitational motion in them [15]. To elucidate the mechanism of this type of motion, asymmetric self-propelled particles were studied for this motion. It was observed that shape anisotropy alone is sufficient to induce

Moisture acts as stimulus in hydrotaxis. Movement of cells, animals or plants towards more moisture is termed as positive hydrotaxis and towards less moisture

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

**3.3 Barotaxis**

**3.4 Chemotaxis**

**3.5 Durotaxis**

durotaxis [10].

regeneration [12].

such type of motion [16].

**3.8 Hydrotaxis**

**3.7 Gravitaxis**

**3.6 Electrotaxis/galvanotaxis**

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

### **3.3 Barotaxis**

*21st Century Surface Science - a Handbook*

microfluidic device.

**3. Types of taxis**

**3.1 Aerotaxis**

**3.2 Anemotaxis**

content [6].

**2. Introduction to taxis**

the basis of the stimulus into various categories.

movement of particles according to some external guiding agent. This agent can be heat, oxygen, pressure, electric field or chemical, etc. Various types of taxis, their applications and microfluidics are discussed in this chapter. Microfluidic is the technology where movement of the particles is on the basis of microenvironment consisting of viscosity, surface tension and pressure. In microfluidics, micro channels are molded or etched over the silicon, glass or various polymer materials such as PolyDimethylSiloxane. These types of devices are vastly being used in all the fields of research, diagnostics and therapeutics. In microfluidics, the micro channels are formed to attain the desired result which can consist of: mixing, sorting, pumping or controlling the biochemical microenvironment. They have the advantage of decreasing the response time and experimental consumables and overall cost. They have the potential to perform large scale experimentation in small scale. Important factors to be considered for fabrication of microfluidic devices are temperature resistance, superior optical transparency of the material, high hardness, excellent electrical isolation, thermal stability, chemical inertness to many fluids, biocompatibility, and surface wettability. The performance of microfluidic device depends majorly upon etched or molded micro channel's surface properties. Therefore surface modification is an important factor to improve overall performance of microfluidic devices. Surface roughness, surface heterogeneity and solution impurity are the key parameter which affects the wettability of

Molecules are always in motion irrespective of their state. Molecules in solid state have least freedom while in gaseous state have maximum freedom. Freedom of movement of molecules in liquid phase lies between molecules in solid and gaseous phases. Heat and temperature are factors that affect the movement of molecules. Enhance temperature increases the translational movement of molecules. Movement of molecules can be random or directed towards certain stimuli. Random movement is termed as "kinesis" while directed movement towards certain stimuli is termed is "taxis". For taxis, there is sensory component to detect the attractant and motor component to enable the movement towards the stimuli [2]. Taxis are classified on

Aerotaxis is the movement of molecules where oxygen acts as stimulant [3]. It has been observed in bacteria and other microorganisms. Active movement of cells is observed along the gradient of oxygen. Aerotaxis plays role in cell survival as optimal concentration of oxygen is required for cell metabolism and growth [4].

Anemotaxis is the movement towards wind. It is observed in drosophila and some terrestrial mammals such as rats which tend to follow the wind. Drosophilla has been observed to move against the air current [5]. Rats were also observed to follow anemotaxis as air current carries information regarding location and odor

**148**

Barotaxis is the movement towards stimulus that is pressure. Movement due to hydraulic resistance (resistance to flow as a result of liquid) is also termed as barotaxis. This type of movement is observed in the neutrophils, a type of immune cell. Neutrophils follows path of least hydraulic resistance.

### **3.4 Chemotaxis**

Chemotaxis can be defined as the movement of cells towards the higher chemical concentration gradient [7]. It is directional locomotion of cells and was first described in the bracken fern spermatozoa in 1884 by Pfeffer. Later in 1888, this phenomenon was described by Leber in mammalian leukocytes in response to an injury. Chemotaxis is an important process required for the growth and development of multicellular organism, immune response and cancer metastasis [8].

#### **3.5 Durotaxis**

Durotaxis is the movement of cells towards more rigid gradient which is a result of variation in the structural property of the extracellular matrix. This type of motion implies movement towards more stiffness [9]. This type of motion has been observed in various cell types such as human fibroblast cells, mesenchymal cells and cancer cells. Substrate rigidity is the stimulus that initiates the movement in durotaxis [10].

#### **3.6 Electrotaxis/galvanotaxis**

Electrotaxis is also termed as galvanotaxis and implies movement guided through electric field or current [11]. Living cells have the tendency to sense and follow direct current electric field. This type of movement is observed in both *in vitro* and *in vivo* conditions although the mechanism behind sensing of electric field by cells remains unclear [11]. Its applications are observed in wound healing and development. Disruption of an epithelial layer in wound leads to generation of an endogenous electric field which guides migration of cells towards the wound for regeneration [12].

### **3.7 Gravitaxis**

Gravitaxis is characterized by directional movement in response to gravity [13]. This type of movement is observed in the motile microorganisms such as euglena where gravity acts as stimulus to select their niche in environment. It can be both positive and negative. Positive gravitaxis implies movement towards water while negative gravitaxis implies movements towards the surface [14]. This type of motion is observed in *Drosophila melanogaster* and around 18 genes have been identified that mediate this gravitational motion in them [15]. To elucidate the mechanism of this type of motion, asymmetric self-propelled particles were studied for this motion. It was observed that shape anisotropy alone is sufficient to induce such type of motion [16].

#### **3.8 Hydrotaxis**

Moisture acts as stimulus in hydrotaxis. Movement of cells, animals or plants towards more moisture is termed as positive hydrotaxis and towards less moisture 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 surface having higher moisture content [18].
