  Drying of biologically-relevant sessile droplets, including passive systems
(like DNA and proteins), as well as active microbial systems comprising
bacteria and algae, have garnered considerable attention over the last decades.
Distinct morphological patterns emerge when bio-colloids undergo drying, with
significant potential in a range of biomedical applications, spanning
bio-sensing, medical diagnostics, drug delivery, and antimicrobial resistance.
This review presents a comprehensive overview of bio-colloidal droplets drying
on solid substrates, focusing on the experimental progress during the last ten
years. We provide a summary of the relevant properties of bio-colloids and link
their composition (constituent particles, solvent, and concentrations) to the
patterns emerging due to drying. We examined the drying patterns generated by
passive bio-colloids (DNA, globular, fibrous, and composite proteins, plasma,
serum, blood, urine, tears, saliva). This article highlights how morphological
patterns are influenced by the nature of the biological entities and the
solvent, micro- and global environmental conditions. Correlations between
emergent patterns and the initial droplet compositions enable the detection of
potential clinical abnormalities when compared with the patterns of drying
droplets of healthy control samples, offering a diagnostic blueprint. Recent
experimental investigations of pattern formation in the bio-mimetic and
salivary drying droplets, relevant to COVID-19 are also presented. Finally, we
summarize the role of biologically active agents in drying process, including
bacteria and algae during the drying process. The review concludes with a
perspective on the next generation of research and applications based on drying
droplets, enabling potential innovations and tools to study this exciting
interface of physics, biology, data sciences, and machine learning.
