Table 1 The spinule dimensions and spacing were almost identical between natural shed gecko skin and polystyrene replicas (n = 4).

Spinule dimensions and geometry (dorsal surface)	Natural	Synthetic Replica	Synthetic BioReplicas	Chitosan	Silk fibroin	Chitosan/αhair keratin blend
Spinule dimensions and geometry (dorsal surface)	Natural shed gecko skin (Strophurus williamsii)	Polystyrene	Alginate/chitosan composite	Chitosan	Silk fibroin	Chitosan/αhair keratin blend
Spinule length/ height from base Fig. 3(a)	3.0 μm (±0.5 μm)	1.0–1.5 μm (±0.5 μm)	2 μm (±0.5 μm)	2 μm (±0.5 μm)	2 μm (±0.3 μm)	3.4 μm (±0.5 μm)
Spinule width (base) Fig. 3(c)	350–450 nm	350–400 nm	350–400 nm	350–450 nm	300–400 nm	350–450 nm
Spinule array spacing (tall and short)	500 nm	600 nm–1 μm	800 nm–1 μm	500 nm–1 μm	500 nm	500 nm–750 nm
Spinule density	8/ μm²	8/ μm²	4/ μm²	8/ μm²	8/ μm²	8/ μm²
Tip radius of curvature Fig. 3(d)	10–50 nm	200 nm	200 nm	10 nm	50–100 nm	50–100 nm
Spinule tip dimension Fig. 3(e)	50 nm	50–80 nm	50–60 nm	50 nm	50–85 nm	50 nm

The tip curvature varied considerably between the replicas (alginate/chitosan (Alg: 2% w/v and Chit: 5% w/v), silk fibroin (8%), chitosan (5% w/v) and chitosan/Keratin blend (Chit: 5% w/v and Ker: 1% w/v), which was related to the substrate fluid mechanics and the way that the substrate flowed and hardened during casting. The spinule length and packing distribution were more strongly correlated with anti-wetting, self-cleaning and anti-fouling effects. However, the correlation of the spinule length and bactericidal activity was not strong. The spinule density, nanotip size and curvature were characteristics directly related to the bacterial killing properties. We measured the length or height of primary spinules. Secondary spinules exist in the packing arrangement with identical tip width and shape of curvature but shorter heights and reduced dimensions (shaft width) by 70–75%. *The physical dimension categories are represented in Fig. 3 (the average measurements were calculated from a total of 20 randomly selected tall and short spinules across the sample surface).

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