Rapid and precise patterning of functional biomaterials is desirable for point-of-care (POC) tissue engineering and diagnostics. However, existing technologies such as dip-pen nanolithography and inkjet printing are currently unsuitable for POC applications due to issues of cost and portability. Here, we report the development of 'BioPen', a portable tool for continuous, defined and scalable deposition of functional materials with micrometer spatial resolution and nanolitre volumetric resolution. BioPen is based upon the ballpoint pen but with multiple "ink sources" (functional material solutions) and with an apparatus that can be optimized for writing living cells, proteins, nucleic acids, etc. We demonstrate POC detection of human immunodeficiency virus type 1 (HIV-1) nucleic acid by writing on paper with BioPen using "ink" consisting of nucleic acid probes and nucleic acid-modified gold nanoparticles. We also demonstrate POC tissue engineering by writing a continuous pattern of living, functional, interconnected cells with a defined extracellular environment. Because it is simple, accurate, inexpensive and portable, BioPen has broad potential for POC detection of diagnostic biomarkers, and for POC engineering of tissues for a range of healing applications.