Biological hydrostatic skeletons use an incompressible fluid as the primary load-bearing medium in a cellular, compartmentalised morphology, achieving structural performance per unit mass that rivals many engineered materials while tolerating damage, conforming to irregular substrates, and self-adjusting stiffness. Engineering has never reproduced this combination at infrastructure scale. This paper reports a systematic review of five domains of scholarship in which fluid has been used or theorised as a structural medium: biological hydrostatic skeletons, water-filled barrier systems, pneumatic and inflatable structures, fluid-in-building concepts, and cellular solids theory. Thirty-six sources spanning 1877 to 2025 and nineteen system classes are analysed through a four-discriminator framework—structural role of the fluid, fluid compressibility, internal morphology, and deployment scale—that locates every surveyed system in a common design space. Cross-domain synthesis reveals three structural facts not visible from within any single domain. Primary use of an incompressible fluid is exclusively biological in deployed systems; engineering has not translated this combination. Cellular morphology stops at organism and laboratory scale across all five domains; the distributed redundancy it provides has no engineering counterpart at building or infrastructure scale. The scale barrier is the final uncrossed boundary: pneumatic structures reach building scale, water barriers reach site scale, and cellular incompressible theory is validated at laboratory scale, but no system achieves all three simultaneously. These observations converge on a formally defined gap: no deployed engineering system uses an incompressible fluid as the primary structural medium in a cellular morphology at infrastructure scale for a permanent function. The Adaptive Matrix Ecosystem (AME), based on Natural Rubber Latex direct-mould cellular extrusion and Pressure Differential Architecture, is identified as the first published design to claim all four discriminator positions simultaneously. The three causes of the gap's persistence—disciplinary siloing, absence of a fabrication pathway, and lack of an organising structural concept—are each addressed by the AME programme. Implications for biomimetics methodology, cellular solids theory, and infrastructure engineering are identified, and a four-item empirical validation agenda is proposed.
James Otto Danenberg (Thu,) studied this question.