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Reply to ‘Water content, not stiffness, dominates Brillouin spectroscopy measurements in hydrated materials’

Nature Methods volume 15, pages 562–563 (2018)Cite this article

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The recent applications of Brillouin microscopy to tissues and cells following publication of our work1,2 have renewed interest in what physical quantity this technique measures. The Correspondence by Wu et al.3 focuses on water content and questions the biomechanical relevance of Brillouin measurements. The physics of Brillouin light scattering has firm roots in the thermodynamic and elastic properties of materials. The dependence of Brillouin scattering on solid constituents and water content has been extensively studied for hydrated materials4.

The Brillouin frequency shift (ωb) is proportional to the square root of the longitudinal modulus (M). In hydrated samples, compressibility β(~1/M) is approximated to a concentration-weighted sum, β = εfβf + εsβs, where εf,s and βf,s are the volume fraction and compressibility of fluid and solid constituents, respectively. Brillouin shift is sensitive to changes in water content, Δεf, and solid constituents, Δβs, via the following equation:

$$\frac{{\Delta \beta }}{\beta } = \left( {1 - \frac{{\beta _s}}{\beta }} \right)\frac{{\Delta \varepsilon _f}}{{\varepsilon _f}} + \left( {1 - \varepsilon _f} \right)\frac{{\Delta \beta _s}}{\beta }$$

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Authors and Affiliations

  1. Fischell Department of Bioengineering, University of Maryland, College Park, MD, USA

    Giuliano Scarcelli

  2. Wellman Center for Photomedicine and Harvard Medical School, Boston, MA, USA

    Seok Hyun Yun

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  1. Giuliano Scarcelli
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  2. Seok Hyun Yun
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Correspondence to Giuliano Scarcelli or Seok Hyun Yun.

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Competing interests

The authors declare the following competing interests: G.S. is a scientific advisor for and A.Y. is the scientific founder of Intelon Optics, Inc.

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Scarcelli, G., Yun, S.H. Reply to ‘Water content, not stiffness, dominates Brillouin spectroscopy measurements in hydrated materials’. Nat Methods 15, 562–563 (2018). https://doi.org/10.1038/s41592-018-0075-2

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