PROJECT SUMMARY This R01 application is responsive to the NIH initiative PAR-19-253 “Focused Technology Research and Development”. Assays for measuring receptor-ligand affinity are valuable in many areas of biomedical research. The “gold standard” surface plasmon resonance assay is limited to recombinant soluble receptors fixed on solid surfaces. The emerging adhesion frequency assay (AFA) techniques can measure the receptor-ligand affinity on their native cellular membranes. However, existing AFA methods can neither resolve the non-uniform distribution of receptors on single cells nor measure the rolling cell adhesion under shear forces. In addition, currentAFAapproaches are generally bulky and low throughput, which require tedious operation. Recently, we have invented a light-driven microrobot (LDM) platform as a non-invasive, programmable, and multimodal cell-manipulation technology. Based on this versatile LDM platform, we propose to develop a paradigm- shift four-dimensional (4D) AFA (i.e., integrated 3D translational AFA and 3D rotational AFA) to overcome these key obstacles in the existing assays. In this R01 project, we will develop and validate our 4D AFA with the following features: (1) measuring receptors on their native cell membrane environments, (2) resolving the non-uniformly distributed receptors on single cells, (3) enabling both translational and rotational AFAs on an integrated platform, (4) investigating cell adhesion under both shear force and tensile force, and (5) allowing on-chip multiplexed cell adhesion measurements. With such features, the proposed 4D AFA has the potential to exceed current lab standards, address unmet needs in the field, and enable high-throughput full profiling of receptor-ligand interactions at sub-cellular resolution. We will validate and improve the 4D AFA performance using well-studied receptor-ligand pairs with variable affinities. We will further package and apply the validated assay to investigate the binding of SARS-CoV-2 virus to angiotensin-converting enzyme 2 receptor and to screen T cells for immunotherapy for cytomegalovirus infection. In this regard, we aim to demonstrate the far-reaching potential of 4D AFA to enable improved research in areas ranging from clinical immunotherapy to fundamental biology.