Advantage 6 of the Six Major Advantages of Electrostatic Flocking
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Electrostatic flocking structure enhances the mechanical proper ties of materials in two distinct ways. One approach involves directly flocking onto the substrate surface, which results in excellent support performance.[39] The other method involves immersing the fluffed surface into substrates such as epoxy resin or alginate hydrogel (ALG), forming a composite material that strengthens the mechanical properties of the original gel material.[6,25,79]
In the first approach, electrostatic flocking fibers form a support structure that improves the resistance to vertical pressure, outperforming the disordered fiber arrangement typically found in electrospinning processes.[39,46] When a downward force is applied to the flocked surface, the compressive strength of the flocked surface can be described using the stent model developed by Walther et al. (Figure a), as represented in Equation (10)–(12)[39]
Fk =π2EI /4lk² (10)
σsf = Fk/Af (11)
σk = nfAf/Ages σsf (12)
In these equations, Fk is the critical compressive force, E is the elastic modulus, I is the geometric moment of inertia, lk is the length of the fiber not covered by the adhesive, σsf is the compressive strength of a single fiber, σk is the critical compressive strength of the entire stent, Af is the area of a single fiber, Ages is the total area, and nf is the number of fibers. From these equations, it can be seen that shorter fibers and a higher fiber density lead to a higher critical compressive strength.
As shown in Figure b, Walther et al. demonstrated that unseeded electrostatic flocking scaffolds (1 mm polyamide fibers, 15s flocking duration) achieved a Young’s modulus of 250kPa,[39] revealing along with a direct correlation between mechanical strength and flock density/size. However, electrospun scaffolds from prior studies required 42 days of cell culture to reach only 17kPa,[80] which remains significantly lower than the unseeded flocked scaffolds. This comparison underscores the superior mechanical properties of electrostatic flocking technology, even without cellular integration.
In addition, electrostatically flocked materials combined with alginate-based hydrogels have better mechanical qualities than ordinary hydrogels in the application of cartilage engineering and fiber-reinforced hydrogels.[25,79] Elke et al. compared pure ALG and pure chitosan flocculent scaffolds (CFSs) with a composite of ALG and CFSs(CFSþALG).[25]At 50%strain, the compressive strength of the electroflocked scaffolds was significantly higher than that of pure ALG, while the chitosan/alginate flock scaffolds has the highest compressive strength. Drawing inspiration from the rim of the sucker disk on the back of a remora fish, Su et al. fabricated a structure by combining nylon fiber flocking with silicone rubber (Figure c,d). The tensile modulus and compressive modulus of the composite material along the fiber axis were measured to be 1013 and 74kPa, respectively. The result is significantly higher than the pure silicone rubber alone.[79] These experimental results demonstrate that the structures formed by electrostatic flocking can efficiently enhance the tensile and compressive strengths along the fiber axis.
In addition to enhancing the mechanical properties in the normal stress direction, the increased roughness of the electrostatic f locking surface also enhances the stress in the tangential direction. Li et al. demonstrates the enhancement of mechanical prop erties in the radial direction of fibers in electrostatic flocked structures for composite materials. They found that the tensile strength of vertically aligned BN/epoxy resin (7.67 MPa) was significantly higher than that of randomly aligned BN/epoxy resin (1.0 MPa) andpureepoxy resin (1.59MPa).[18] The enhanced friction between the layers of epoxy resin and vertically aligned BN might be the mechanism, leading to the excellent tensile properties. In contrast, the randomly aligned BN/epoxy composite does not possess this characteristic. Conclusively, the regularly vertically aligned surfaces formed by electrostatic flocking enhance the mechanical properties of composite materials in both the axial and radial directions of the fibers, particularly in terms of compressive and tensile properties.
a) Schematic diagram of the force equilibrium on a fiber; reprinted with permission. [113] Copyright © 1976 Springer-Verlag Berlin Heidelberg; reprinted with permission. [39] Copyright © 2012 The Authors; licensee MDPI, Basel, Switzerland.
b) Analysis of the mechanical properties of four different types of flocked scaffolds; reprinted with permission. [39] Copyright © 2012 The Authors; licensee MDPI, Basel, Switzerland.
c) Structural analysis of a flock-based suction cup. i) Schematic diagram of the electrostatic flocking principle; ii) Side view structure and cross-section of nylon fibers on a silicone surface; iii) and longitudinal section; reprinted with permission. [79] Copyright © 2020 Elsevier Inc.
d) Internal vertical fiber structure and super-strong adsorption capability of the electrostatic flocked suction cup; reprinted with permission. [79] Copyright © 2020 Elsevier Inc.
References:
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[18] Li, X., Xu, Q., Lei, Z., Chen, Z., Ceramics International, 2023, Vol. 49
[25] Goslar, E., Bernhardt, A., Törndorf, R., Aibibu, D., Cherif, C., Glinsky, M., International Journal of Molecular Sciences, 2021, Vol. 22
[39] Voigt, A., Hoyer, B., Springer, A., Mrozik, B., Hanke, T., Cherif, C., Pompe, W., Glinsky, M., Materials, 2012, Vol. 5
[46] Törndorf, R., Goslar, E., Cakar, M., Christenn, M., Hund, R.D., Hoffmann, G., Aibibu, D., Glinsky, M., Cherif, C., Textile Research Journal, 2018, Vol. 88
[79] Su, S., Wang, S., Li, L., Xie, Z., Hao, F., Xu, J., Wang, S., Guan, J., Wen, L., Matter, 2020, Vol. 2
Excerpt from the paper "Electrostatic Flocking: Reborn to Embrace Multifunctional Applications"