Boutinguiza et al. Nanoscale Research Letters 2011, 6:255 http://www.nanoscalereslett.com/content/6/1/255 Page 3 of 5 Figure 2 XRD patterns of calcined fish bones compared with commercial stoichiometric HA (JCPDS 1993). which can lead to material fracture into more or less chunks [21]. As the use of CW laser is dominated by thermal regime, both conditions can be estimated in our work in the case of pulsed laser when thermal diffusivity (α = 3.1 · 10−3 cm2 s) and the speed of sound (vs = 1801 m/s) for spongy bone [22,23] are assumed valid for fish Figure 3 HRTEM micrograph showing crystalline nanoparticles obtained from fish bones by laser ablation in water using pulsed laser and their corresponding fast Fourier transform (inset). Laser irradiance: 8 × 5 × 106 W/cm2. bones. Calculations for the used laser beam diameter ( = 0.14 mm) confirm that thermal confinement condi- tion is fulfilled for laser pulse durations in our experi- ments. The characteristic time tch for heat dissipation in fish bones can be estimated according to ttc = d2 , 4 · α where d is the smallest dimension of the heated volume (beam diameter) and a is the thermal diffusivity, result- ing in ttc = 18 ms, which is considerably longer than the used laser pulses. On the other hand, stress confinement d characteristic can be estimated as tsc = , giving tsc = 83 vs ns, which is orders of magnitude shorter than the used pulse durations. This corroborates that thermal confine- ment is the only mechanism responsible for material explosive ejection and subsequent particles formation, which is consistent with the size as well as the spherical shape of the obtained particles. As the irradiance when using the pulsed laser is higher than when using the CW laser, the particles obtained in the latter conditions are Table 1 The experimental inter-planar spacing of crystalline nanoparticles obtained from fish bones by pulsed laser ablation in water (laser irradiance: 8 × 5 × 106 W/cm2) compared to the correspondence to HA and b-TCP Experimental (dhkl nm) 0.238 0.242 0.250 (dhkl nm) JCPDS_ICDD(1993) 0.230 (HA) 0.242 (b-TCP) 0.253 (HA)