Flat-plate foil (with rounded leading and trailing edges) covered with skin from the lateral midline area of a male shortfin mako shark (left). Dark skin color indicates skin from the lateral shark surface, whereas a whitish color indicates more ventral skin. The foil is 7.22 cm in chord length (width) and 19 cm in height (whereof 17.5 cm is covered with shark skin). Distribution of the skin structure on the surface of the mako flat-plate foil (right). Environmental scanning electron microscope (ESEM) images from parts of a top (A), a top middle (B), a middle (C), a middle bottom (D) and a bottom (E) area. Images were taken from skin pieces extracted 0.5 cm downstream of the right foil edge at each location. Scale bars, 200 μm. The leading edge of the denticles is on the left, and the natural water flow pattern would thus be left to right in this figure.

Flexible shark skin foil mako membrane no. 1 (left). Bonded skin pieces (6.4 cm in length and 9.2 cm in height) of both lateral sides from 10 cm above the midline of a male shortfin mako shark are clamped 2.5 cm above the lower end in a sandwich bar holder. Right: distribution of skin structure on the surface of this mako membrane foil. ESEM images from parts of an upper (A), a middle (B) and a lower (C) area. Image samples were taken from skin pieces extracted 0.5 cm downstream of the foil edge of this foil at each location. The blue arrow indicates the direction of water flow during testing. Scale bars, 100 μm.

Close-view ESEM image of denticles from the surface of the mid-body region in a bonnethead shark (Sphyrna tiburo) to show details of typical denticle structure with the three surface ridges and three posteriorly pointing prongs. Such denticle structure is common on the body, fins and tail, although denticles of this species on the head have a different morphology. Scale bar, 50 μm.

An ESEM image of a part of the mako membrane no. 1 foil (Fig. 3) after the process of sanding. The full denticles are almost completely removed, with only small stubs remaining (compare with the intact denticle surface shown in Fig. 3). These stubs could not be removed without damaging the underlying collagen surface framework as denticles are embedded in the skin. Scale bar, 200 μm.

ESEM images of Speedo® Fastskin FS II fabric. (A) Surface image of the underside (non-biomimetic) surface of the fabric. (B) Surface image of the outside (biomimetic surface) of the Speedo® fabric at the position of V-shaped printing. (C) Image of a cross-section of the Speedo® fabric, showing the dents on the biomimetic side (red arrows, ‘d’) in the fabric that generate the ‘ribbed’ surface. Scale bars, 500 μm.

ESEM images of the biomimetic riblet silicone material. (A) Front side of riblet surface with clearly visible height peaks (white lines). (B) Image of a cross-section showing the riblet structures with a height (h) of 87 μm and a spacing (s) of 340 μm. Scale bars, 200 μm.

Histogram of the mean self-propelled speed results (mean from N=9 trials for each test) for Speedo® membrane foils. Error bars are ± 1 s.e.m. The motion program settings below each group of similarly colored bars show the programmed foil movement, defined by frequency (Hz), amplitude (cm) and pitch (deg). Speedo® fabric was tested on two different foil orientations (vertical and horizontal – see Materials and methods) and with the biomimetic surface ridges parallel to the free-stream flow, perpendicular to the free-stream flow and ‘inside’ or reverse orientation, with the biomimetic surface on the inside glued to the foil and the non-biomimetic surface exposed to the water. Within each group of similarly colored bars, bars with # symbols are not significantly different from each other (P>0.05). All other comparisons have a level of significance between P<0.05 and P>0.0001.

Histogram of the mean self-propelled speed (mean from nine trials for each test) for the silicone riblet material applied to a NACA 0012 foil surface. Error bars are ± 1 s.e.m. The motion program settings below each group of similarly colored bars show the programmed foil movement, defined by frequency (Hz), amplitude (cm) and pitch (deg). Silicone riblet material was tested with the biomimetic surface ridges parallel to the free-stream flow, perpendicular to free-stream flow and ‘inside’ or reverse orientation, with the riblet surface on the inside glued to the foil and the non-biomimetic (smooth) surface exposed to the water. Within each group of similarly colored bars, bars with # symbols are not significantly different from each other (P>0.05). All other comparisons have a level of significance between P<0.05 and P>0.000. On average, the foils with the ‘inside’ orientation propel at a 7.2 % slower self-propelled swimming (SPS) speed than the foils with parallel-oriented ridges.

Histogram of the mean self-propelled speed results (mean from nine trials for each test) for the mako shark skin attached to a rigid flat plate. Error bars are ± 1 s.e.m. The motion program settings below each group of similarly colored bars show the programmed foil movement, defined by frequency (Hz), amplitude (cm) and pitch (deg). Mako shark skin on the flat plate was tested with the denticle surface oriented parallel to the free-stream flow in the same direction as on a living shark (‘in flow’), oriented opposite to the in vivo shark denticle orientation (‘against flow’) and sanded, where most of the denticle surface had been removed (Fig. 5). Within each group of similarly colored bars, bars with # symbols are not significantly different from each other (P>0.05). All other comparisons have a level of significance between P<0.05 and P>0.0001. On average, the sanded rigid foils propel at a speed 13.4% higher than that of the foils with denticles, with a maximum difference of 18.03% (P<0.001) at a motion program of 2 Hz, 1.5 cm and 10 deg.

Histogram of the mean self-propelled speed results (mean from nine trials for each bar) for the flexible moving shark skin membranes. (A) mako membrane no. 1, (B) mako membrane no. 2 and (C) porbeagle membrane. Error bars are ± 1 s.e.m. The motion program settings below each group of similarly colored bars show the programmed foil movement, defined by frequency (Hz), amplitude (cm) and pitch (deg). Shark skin membranes were tested intact, with denticles oriented as in live sharks, and sanded, where most of the denticle surface had been removed (Fig. 5). All paired comparisons within similar motion programs had a level of significance between P<0.001 and P>0.0001. The sanded foils swim at an average speed 12.3% lower than that of the intact foils.

Time series of flow velocities over a whole motion cycle of the normal mako shark skin membrane no. 2 foil, swimming at its average self-propelled speed of 0.2 m s^–1 (motion program: 2 Hz, 2 cm heave, 0 deg pitch). For the cycle period of 500 ms, images (above) and plots (below) of 0, 100, 200, 300, 400 and 500 ms are shown. A cycle begins and ends at 0 ms and 500 ms, respectively. Yellow arrows in DPIV images show velocity vectors. Colored contours indicate the velocity in the x-direction (V_x) of a downstream (red) and upstream (green) moving fluid. The foil shape at each time is overlaid on each image. Black scale bar, 2 cm. The orange scale vector represents 1 m s^–1 and indicates the flow direction from left to right. Values for V_x are taken along the blue lines and plotted against the distance from the foil edge in the V_x-plots. The blue line starts 3–4 mm above the lower foil edge. In this area, V_x values approximate zero, and so the actual foil edge begins at the point where the V_x-plots shows the first slope. Note that negative velocities, where the flow travels upstream, occur near the foil edge. White areas above the foil membrane indicate areas that were in shadow, and so no vectors were calculated in these regions.