  Advanced photonic tools may enable researchers and clinicians to visualize,
track, control and manipulate biological processes at the single-cell level in
space and time. Biological systems are complex and highly organized on both
spatial and temporal levels. If we are to study, perturb, engineer or heal
biological entities, we must be able to visualize key players in such systems
and to track, control and manipulate them precisely and selectively. To achieve
this goal, the engineering of non-destructive tools will allow us to
interrogate and manipulate the function of proteins, pathways and cells for
physicians, enabling the design of 'smart materials' that can direct and
respond to biological processes on-demand. Among the potentially exploitable
non-destructive tools, light-based actuation is particularly desirable, as it
enables high spatial and temporal resolution, dosage control, minimal
disturbance to biological systems and deep tissue penetration. Here, we
overview existing approaches toward the engineering of light-activated tools
for the interrogation and manipulation of single-cell processes, and list the
types of studies and types of functions that can be controlled by light. Timely
applications, such as studies of inflammation and of crossing brain barrier
systems - via super-resolution imaging and optomechanical manipulation - are
two representative examples of emerging applications so far never addressed.
