**2. Spider silks inspired spindle-knot fibre**

#### **2.1 Biological model and theoretical analysis of spindle-knot fibre**

The ability to harvest fog is determined by the fog-water catching performance at the surface together with the removal efficiency of the captured water from the surface [3, 4]. One-dimensional (1D) fog collectors usually make use of some hydrophilic composition or/and water-loving structure to rapidly seize the water from a humid environment. Subsequently, small droplets hang on the 1D material and accumulated into larger droplets gradually. Finally, large droplets can fall into a container due to growing gravity.

Spider web has been well-known for its excellent mechanical, biochemical and pharmaceutical properties because of its network structure and the composition of biological proteins [5–8]. In the recent decade, a single spider silk was deeply studied and has been deemed as a perfect model for designing artificial water collector. The periodic spindle-knot structure on spider silk (similar to bead-on-string fibre) is useful. This structure can give rise to driving forces on dew based on Laplace pressure difference and probable surface energy gradient. These forces drive the droplet moving to the spindle knot, where the combined droplet is released and collected [9]. The Laplace pressure difference (**Figure 2(a)**) can be calculated by the following formula,

$$F\_L \sim \chi \left(\frac{1}{R\_{1'}} - \frac{1}{R\_{2'}}\right) \frac{\sin \beta}{R\_1 - R\_2} V \tag{1}$$

*An Application of Bio-Inspired Superwetting Surfaces: Water Collection DOI: http://dx.doi.org/10.5772/intechopen.105887*

**Figure 2.** *Sketch of force analysis on the bio-inspired water-collecting units.*

where γ is the surface tension of water droplet, *R*<sup>1</sup><sup>0</sup> and *R*<sup>2</sup><sup>0</sup> indicates the local curvatures of the contact lines at two opposite sides of droplet along the spindle knot, *R*<sup>1</sup> and *R*<sup>2</sup> indicate the local radius of the spindle knot. Angle β is the half apex angle of the knot, which is related to the size and shape of the knot. V is volume of water droplet on knot, which can be approximate estimated by the formula V=π*R*<sup>0</sup> 3 *=*12, where *R*<sup>0</sup> indicates the radius of the droplet.

The spindle knot usually had a higher axial-parallel roughness than the other area on the fibre, contributing to a smaller contact angle. Therefore, the spindle knot is more hydrophilic and has a higher surface energy. The consequent driving force resulting from surface energy gradient (**Figure 2(b)**) can be described as follow,

$$F\_S = \int\_{L\_j}^{L\_k} \chi(\cos \theta\_A - \cos \theta\_R) dl \tag{2}$$

where *θ<sup>A</sup>* and *θ<sup>R</sup>* , respectively indicate the advancing and receding angle of the water droplet on spider silk, and *dl* is the integrating variable from original silk joint (*Lj*) to the spindle knot (*Lk*).

Capillary pressure (result from small holes or probable grooves on spindle-knot) gradient can also be introduced into this system to speed up the water combination or transportation rate. The capillary force *Pc* (**Figure 2(c)**) is estimated by the Young�Laplace formula [10],

$$P\_{\varepsilon} = \mathfrak{D}\chi/\mathbb{R}'\tag{3}$$

where *R*<sup>0</sup> indicates the curvature radius of surface of water in small holes or grooves.

#### **2.2 Featured structures made by multi-methods using various materials**

When aiming at mimicking the biological spindle-knot structure (**Figure 3(a)**) with consequent water-collecting function, diverse biomimetic methods are employed. For example, dip-coating, electro-dynamic, fluid-coating, and microfluidics have been developed or utilised [8, 16]. As shown in **Figure 3**(**b**–**j**), diverse spindle-knot structures can be prepared.

#### **Figure 3.**

*Spider silk and diverse artificial spider silks. (a) SEM image of spider silk in nature [9], scale bar 50 μm. (b) Optical image of artificial spider silk via traditional dip-coating [7], scale bar 200 μm. (c, d) SEM images of micro-porous bead on string fibre via dip-coating with breath figure method [11], scale bar 10 μm. (e) SEM images of gradient-porous bead on string fibre via dip-coating with breath figure method [11], scale bar 10 μm. (f) Optical image and SEM images of helical spindle-knot fibre via deep-coating and sequent calcination [12], scale bar 50 μm. (g) optical image of a gradient-sized spindle knot fibre via fluid-coating at an increasing drawn-out rate [13], scale bar 1 mm. (h) Photo of magnetic assembly integrated artificial spider silks [14], scale bar 1 mm. (i) SEM image of hollow artificial spider silk via microfluidics using nitrogen as inconsecutive phase [15], scale bar 50 μm. (j) Photo of integrated topological artificial spider silk to mimic spider web [15], scale bar 5 mm.*

#### *2.2.1 Dip-coating*

Dip-coating is the earliest-developed, frequently-used, convenient and mature approach to mimic spider silk with fog harvest ability. In a typical process, a Nylon fibre (usually 50 300 μm in diameter) was immersed deeply into a polymer solution which had been prepared in advance. Afterwards, it was drawn out horizontally and a tubular membrane composed of the polymer solution wrapped the fibre uniformly. The membrane broke up into several ellipsoids on account of Rayleigh instability and then turned into a periodic spindle knot on fibre while getting dried (**Figure 3(b)**).

According to the Eqs. (1) and (2) mentioned above, the actuating force for droplet directional transportation depends on the shape and size of the spindle knots together with the interval of each two periodic knots. These factors can be optimised by changing the processing parameters such as the viscosity or volatility of polymer solution, drawn-out velocity, and desiccation condition [17].

*An Application of Bio-Inspired Superwetting Surfaces: Water Collection DOI: http://dx.doi.org/10.5772/intechopen.105887*

In order to improve fog-water capturing efficiency, the spindle knot can be further designed and manufactured into a porous spindle knot due to the high specific surface area and enhanced absorbability. For example, breath-figure method [11, 18] was utilised to achieve steerable micro-pores (**Figure 3(c** and **d)**), and gradient pores (**Figure 3(e)**).

Some spindle knot with ring or helical grooves (**Figure 3(f)**) which show better performance has been developed [12]. To fabricate such periodic ring or helical grooves spindle knot on 1D material, an added procedure of calcination or desiccation under a high temperature (more than 800 K) was essential. Conventional polymer fibres such as Nylon could not stay non-deformation or unshrink at such high temperature. Hence, Glass Fibre or Carbon Fibre (usually 7–15 μm in diameter), with greater high-temperature resistance properties and other mechanical performance, were employed as host fibre material in these studies [11, 12]. Furthermore, rather than the organic polymer solution, some inorganic compounds and materials with ameliorate biocompatibility, such as Titanium salt/oxide [12, 19], would be used if environment conservation or other intention was taken into consideration.

#### *2.2.2 Fluid-coating*

Dip-coating is capable of fabricating fine accurate structure (usually 7 300 μm diameter fibre with minimum feature size about 5 μm) on the fibre. However, it is difficult for dip-coating to realise large scale preparation. Fluid-coating was developed and continuously massive coating comes true [20]. Artificial spider silks in random length are likely to be synthesised via this approach, which is a promising method for the industry.

The principle and operating steps of fluid-coating to prepare spindle knot are similar to dip-coating. As is reported in previous work [21], a Nylon fibre (about 70μm in diameter) was steadily fastened in a polymer solution reservoir (width: 2 cm, length: 3.5 cm, height: 3 cm, with one edge attached to a motor) through two capillary tubes (400 μm inner diameters). The motor drew the fibre out at the desired speed and the polymer solution (parallel to the solution in dip-coating method, **Table 1**) moved with the fibre thanks to the viscous force. Then the fluid covered the fibre to become a cannular polymer membrane and it broke up into droplets on account of Rayleigh instability, which ultimately turned into periodic spindle knots on fibres after evaporation.

The unique feature of fluid-coating lies in the alterable and steerable drawn-out rate. It is reported that by drawing out at a monotone increasing velocity, gradient in thickness of polymer film emerged and gradient-size spindle knots (**Figure 3(g)**) were generated on fibre eventually [13]. Researchers also tried fishing-line fibres, copper fibres and carbon fibres to be the host fibres (**Table 1**) and obtained a satisfactory mechanical property.

#### *2.2.3 Electrodynamic*

Electrodynamic technology is another strategy for large-scale preparation of artificial spider silks. This method is good at preparing slender (200 nm 12 μm in diameter) silk with fat spindle or even ellipsoid knot (usually 3 6 times of the slender silk in diameter, the whole artificial silk namely bead-on-string fibre).

Pervious works [22–24] justified the practicability of a coaxial electrospinning method (namely core-shell electrospinning), which developed from conventional


*DMF: N, N-dimethyl formamide, PMMA: polymethyl methacrylate, PVDF: polyvinylidene fluoride, DMAc: N, N-dimethyl acetamide, DETA: Diethylenetriamine, ER: Epoxy resin-44,TiBO: titania butoxide, P123: HO (CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H.*

#### **Table 1.**

*Materials and polymer solutions are used in dip-coating (DC) and fluid-coating (FC) to make artificial spider silk.*


*a sol-gel: 2 g PVDF-HEP and 1 g FPOSS in 16 mL solvent.*

*b sol-gel: 2 g PVDF-HEP and 1 g FPOSS in 32 mL solvent.*

*PEG: polyethylene glycol, MC: methylene chloride, DMF: N,N-dimethyl formamide, PMMA: polymethyl methacrylate, THF: tetrahydrofuran, PNIPAM: poly(N-isopropylacrylamide), PVDF: polyvinylidene fluoride, PVDF-HSP-FPOSS: poly(vinylidene fluoride-co-hexafluoropropylene) with fluorinate polyhedral oligomeric silsesquioxane, DMAc: N,N-dimethyl acetamide.*

#### **Table 2.**

*Chemical reagents are used for outer and fluid polymer solutions in different electrodynamic studies to make artificial spider silk.*

coaxial electrospraying and electrospinning. In this method, heterogeneous liquids with different viscosity and vapour pressure-filled two different channels of jet. Dilute solution in the outer channel was electro-sprayed into micro-particles (finally acting as spindle knots) due to Rayleigh instability (or combined with wet-assembly technique) [19, 25]. The inner viscous liquid channel was electrospun to micro- or nano-fibres [26].

As is displayed in **Table 2**, multifarious chemical substances were used to prepare inner and outer solutions to meet the request for wettability, finance, or environmentprotecting [27–30]. To exemplify it, poly-l-lactic acid (PLLA) was chosen as a raw material on the basis of being degradable, whose solution (6 wt% PLLA dissolved in Chloroform and DCM: dichloromethane with different volume rates ranging from 95/5 to 65/35 and stirring at room temperature) electrospun bead-on-string fibre exhibited a higher directional transport performance compared with PVA solution (10 wt% polyvinyl alcohol dissolved in hydrothermal water containing 17 wt% glutaraldehyde) [31].

### *An Application of Bio-Inspired Superwetting Surfaces: Water Collection DOI: http://dx.doi.org/10.5772/intechopen.105887*

Albeit electrodynamic approaches can enable relatively large-scale production of bead-on-string hierarchical nanofibres with satisfactory fog-harvesting (normally 5 g water, fog flow 25 mL per hour), mechanical properties such as tensile strength should be improved to prolong the lifespan of fibres. To achieve this, adding postprocessing (e.g., carbonation procedures to transfer it into carbon fibre) can be a decent measure.

### *2.2.4 Microfluidics*

Microfluidics was a relatively recent developed technology to continuously fabricate artificial spider silk. By adjusting the relative location of syringe needles in convention microfluidic devices, artificial spider silk could be synthesised via imitating the authentic spinning process of nature spiders [32].

In a coaxial microfluidic device, there are two flows of liquids (**Table 3**) with different feeding rates. In a typical synthesis [33], a micropipette was employed where an Alginate-based composite solution (ABC solution) served as the continuous phase and liquid Paraffin served as the dispersed phase, respectively consisting of the host fibre and spindle-knot of artificial spider silk after dehydration. The diameter of host fibre and geometry size of periodic spindle knot as well as the distance between two adjacent spindle knots can be altered by optimising the parameter of feeding speed of two flows (and their feeding speed ratio).

To achieve the porous surface of joint or spindle-knot of artificial spider silk to get enhanced fog-captured performance, add a component with distinct solubility (salt such as NaCl or CaCl2) into the liquids and then remove it by washing or soaking in aqueous solution [26]. Analogously, Fe3O4 nanoparticles can be added to the volatile oil drops during microfluidic operation to obtain magnetic property of spindle knot. The synthetic 1D materials can then be assembled or patterned (**Figure 3(h)**) into the multidimensional structure under an external magnetic field [14]. Additionally, gas (e.g., nitrogen [15]) can be used as a dispersed phase and bioinspired cavitymicrofibres (**Figure 3(i** and **j)**) can be fabricated.

### **2.3 Water collecting efficiency of artificial spider silks**

By optimising the geometry of periodic spindle knots, artificial fibres could show enhanced performance in fog-harvesting tests (**Figure 4**). As is demonstrated in **Figure 4(a)**, five small water drops moved and combined rapidly within 4.39 s on a PMMA spindle-knot Nylon fibre. It is recorded that when the vertical downward fog


#### **Table 3.**

*Substance employed in microfluidics to produce artificial spider silks and their corresponding feature.*

flow was 0.75 m/s, the artificial fibre which is 1.5 mm in length could collect 40 nL of water within 12 s in comparison with 17 nL of natural spider silk under a similar circumstance [21].

Distinct types of spindle knot may give rise to different impact on fog-harvesting function. The in-situ observation and optical images of water collecting test on artificial fibre with gradient-size spindle knots (**Figure 4(b)**) showed that droplet coalescence with a special mode could be implemented. Under 90% humidity using ultrasonic humidifier, 7.35 mm fibre with gradient-size spindle knots was capable of harvesting water at a rate of 509.4 μL/h, while 156.7 μL/h of water was collected using similar fibre with uniform sized periodic spindle knots under the same condition [13]. Porous surface morphology also brings in increasing efficiency of water collecting. As illustrated in **Figure 4(c)**, under same 100% high humidity condition using humidifier, water collecting efficiency had the following rank: gradient porous structured > homogenous porous structured > smooth spindle-knot fibre [11]. Additionally, spindle knots do play a significant role in improving the water-harvesting efficiency (**Figure 4(d)**).

Intersection would gain an easier access to capture and harvest fog-water (**Figure 4(e)**). In an updated research, two intersectional silks (8.25 μL in 60 s) tended to harvest more water than two parallel silks (4.57 μL in 60 s) in 60s under a 0.408 mL/min fog flow. Topological network bioinspired by spider web exhibited

#### **Figure 4.**

*Fog-collecting characterisation images of diverse artificial spider silks. (a) In-situ observation of directional fog harvesting of conventional PMMA spindle-knot (by fluid-coating) on Nylon fibre [21], scale bar 100 μm. (b) Optical images of water collecting on gradient sized PMMA spindle-knot (by fluid-coating) on Nylon fibre [13], scale bar 1 mm. (c) Optical images of aggregation of water droplet of ER knots (all by deep-coating) with smooth, homogenous porous and gradient porous structure on carbon fibres [11], scale bar 50 μm. (d) Optical images of PMMA-PS bead on string (right) fibre and smooth PS (left) fibre (all by electrodynamic) [28], scale bar 20 μm. (e) Photos of fog harvesting on Alginic based cavity (all by microfluids) fibres [15], scale bar 5 mm.*

*An Application of Bio-Inspired Superwetting Surfaces: Water Collection DOI: http://dx.doi.org/10.5772/intechopen.105887*

much higher fog-harvesting efficiency: 150 mm topological networks with two radius were able to collect 53.297 μL in 120 s and with three radius, the volume is 68.957 μL in 120 s [15].

By comparing the water collecting performance of diverse artificial spider silks (**Figures 4 and 5**), it can be summarised that appropriate shape (slenderness ratio: 1 10), size (gradient sized spider-knot) and morphology (knot with porous microstructure especially gradient porous microstructure) of artificial spider silks, resulted from optimised processing parameters (**Figure 5(a)**), will contribute to better fogharvesting performance. Diverse materials with different structural or wettability design differs in water-harvesting style and efficiency (**Figure 5(b** and **c)**). Temperature, humidity or flow rate in the water-collecting test may also affect watercollecting volume (**Figure 5(c** and **d)**).

In addition, 2D or 3D intersectional arrangement of artificial fibre to mimic the real topological spider web in nature turns out to own an larger-scale fog-harvesting capacity than 1D spider silk [4]. For example, combined with advanced multidirectional braiding technology, multidimensional integrated spider silks can be acquired with relatively large scale fog harvesting capacity [34, 35]. These 2D or 3D

#### **Figure 5.**

*Various factors which may exert influences on fog-harvesting performance of artificial spider silks. (a) Impacts of spray times, electrospinning times in electrodynamic method slenderness ratio, materials and fibre with/without beads on fog-harvesting [31]. (b) Influences of postprocessing and hierarchical structure of fibre on fog-harvesting [30]. (c) Effects of temperature, materials and core-shell structure on water condensation and water harvesting [29]. (d) Impacts of fog flow rate in water-harvesting [17]. (e) Influence of intersectional angle of two artificial spider silk on fog-harvesting [15].*

network materials were estimated to harvest up to 0.5 � 1.5 ton/day (per square meter of material) fog-water in foggy environment (e.g., industrial cold-water tower). To prepare artificial fibre in industrial scale and to harvest water in much larger scale should be future research topics [36].
