Editorial: theme issue on complex rheology in biological systems

Recent years have witnessed unprecedented growth in interdisciplinary engagement and collaboration of physical and life sciences, to which the very existence of the Royal Society Journal ‘Interface Focus’ testifies. The subject of rheology itself brings together physics, chemistry, chemical engineering, mathematics and computing. In the biological context, the interdisciplinarity becomes even richer. Cell biology in plants, animals and prokaryotes is usually described in terms of components, biochemical networks and signalling. Yet local flows, and deformations of the entire cell as well as its individual parts [1,2], are essential to function. Questions on such mechanical properties and phenomena are rarely addressed. At the tissue biology level, there are new challenges especially in the highly nonlinear range of deformations [3,4], coupling to smaller structures, and pathologies. Vascular biology (e.g. haematology) is clearly a field where rheology is vital [5], but other key rheological control problems emerge in digestive [6] and reproductive biology [7]. Other biological flows contain rheologically induced structural or phase transitions, and the understanding of how biological fluids and soft solids flow and deform is a key scientific area within this collaboration of physical and life sciences (blood [8], the cytosol, silk protein solutions [9], saliva, mucus [10], synovial fluid, biofilms [11–13], tissue buckling [14], bacterial rheotaxis [15] and E. coli bacteria swimming in media with liquid crystalline order [16] are just some examples [17]). This theme issue on ‘Complex rheology in biological systems’ brings together biorheological work across distinct disciplines in the physics and life sciences. Below,we summarize these contributions and extract common ideas andmethodologies. We hope this will promote their adoption across the field, and thereby accelerate the resolution of outstanding and unresolved problems in biorheology. Beyond that, this theme issue aims to raise awareness of new research questions that have not yet been fully formulated in some of the sub-fields, yet are key to awider understanding. As is generally true in the ‘physics of livingmatter’movement, the biological examples point to new physics and chemistry that is not evident in non-biological systems.