A rediscovery of stiff pentmodes. A comment on "High bulk modulus pentamodes: the three-dimensional metal water"

We bring attention to the fact that the claim of Brambilla et.al. [Extreme Mechanics Letters 74 (2025) 102267; arXiv:2406.14502] of discovering a novel design for pentamode materials is incorrect. Back in 2016 Briane, Harutyunyan and myself [Mathematics and Mechanics of Complex Systems 5 (2016) 41–94; arXiv:1606.03305] designed a class of stiff pentamodes, that include the high bulk modulus pentamodes of Brambilla et.al. Our design generalized to three-dimensions, and to full anisotropy, the main aspects of a two-dimensional construction of Sigmund [Journal of the Mechanics and Physics of Solids 48 (2000) 397–428]. It is emphasized that the in depth analysis of Brambilla et.al. goes well beyond our brief treatment.