Accurate prediction of nanoparticle (1~100 nm) deposition in the rat nasal cavity is important for assessing the toxicological impact of inhaled nanoparticles as well as for potential therapeutic applications. A quasi-steady assumption has been widely adopted in the past investigations on this topic, yet the validity of such simplification under various breathing and sniffing conditions has not been carefully examined. In this study, both steady and unsteady computational fluid dynamics (CFD) simulations were conducted in a published rat nasal model under various physiologically realistic breathing and sniffing flow rates. The transient airflow structures, nanoparticle transport and deposition patterns in the whole nasal cavity and the olfactory region were investigated and compared with steady state simulation of equivalent flow rate. The results showed that (1) the quasi-steady flow assumption for cyclic flow was valid for over 70% of the cycle period during all simulated breathing and sniffing conditions in the rat nasal cavity, or the unsteady effect was only significant during the transition between the respiratory phases; (2) yet the quasi-steady assumption for nanoparticle transport was not valid, except in the vicinity of peak respiration. In general, the total deposition efficiency of nanoparticle during cyclic breathing would be lower than that of steady state due to the unsteady effect on particle transport and deposition, and further decreased with the increase of particle size, sniffing frequency, and flow rate. In the contrary, previous study indicated that for micro-scale particles (0.5~4μm), the unsteady effect would increase deposition efficiencies in rat nasal cavity. Combined, these results suggest that the quasi-steady assumption of nasal particle transport during cycling breathing should be used with caution for an accurate assessment of the toxicological and therapeutic impact of particle inhalation. Empirical equations and effective steady state approximation derived in this study are thus valuable to estimate such unsteady effects in future applications.

中文翻译：

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不同呼吸和嗅探条件下大鼠鼻子中的气流和纳米颗粒沉积——非稳态和湍流效应的计算评估

准确预测大鼠鼻腔中纳米颗粒 (1~100 nm) 的沉积对于评估吸入纳米颗粒的毒理学影响以及潜在的治疗应用非常重要。在过去关于该主题的调查中，准稳态假设已被广泛采用，但尚未仔细检查这种简化在各种呼吸和嗅探条件下的有效性。在这项研究中，稳态和非稳态计算流体动力学 (CFD) 模拟都在已发表的大鼠鼻模型中进行，并在各种生理真实的呼吸和嗅探流速下进行。研究了整个鼻腔和嗅觉区域的瞬态气流结构、纳米颗粒传输和沉积模式，并与等效流速的稳态模拟进行了比较。结果表明：（1）在大鼠鼻腔所有模拟呼吸和嗅探条件下，循环流的准稳态流假设在循环周期的 70% 以上是有效的，或者非稳态效应仅在呼吸阶段；(2) 然而，纳米粒子传输的准稳态假设是无效的，除非在峰值呼吸附近。一般来说，循环呼吸过程中纳米颗粒的总沉积效率会由于对颗粒传输和沉积的不稳定影响而低于稳态，并且随着颗粒尺寸、嗅探频率和流速的增加而进一步降低。相反，先前的研究表明，对于微米级颗粒（0.5~4μm），不稳定效应会增加大鼠鼻腔的沉积效率。综合起来，这些结果表明，在准确评估颗粒吸入的毒理学和治疗影响时，应谨慎使用循环呼吸期间鼻腔颗粒传输的准稳态假设。因此，本研究中得出的经验方程和有效稳态近似值对于估计未来应用中的这种不稳定效应很有价值。