What Are Fluids and Secretions?

Fluids and secretions are biological liquids produced, transported, and released by living organisms, encompassing everything from circulatory plasma and cerebrospinal fluid to glandular secretions such as saliva, sweat, and tears. In biomedical engineering, the analysis of these substances provides a window into physiological and pathological states without requiring invasive tissue sampling. The field draws on biochemistry, analytical chemistry, microfluidics, and sensor engineering to characterize the composition of biological fluids at the molecular level.

The range of measurable analytes in biological fluids is broad: electrolytes (sodium, potassium, chloride), metabolites (glucose, lactate, urea), proteins, hormones, and nucleic acid fragments can all be detected in fluids accessible at or near the body surface. This compositional richness makes biological fluids a central focus for point-of-care diagnostics, continuous health monitoring, and drug discovery.

Biofluids in Diagnostic and Monitoring Applications

Blood plasma and urine have long served as the primary matrices for clinical chemistry panels, but wearable biosensor research has expanded the diagnostic palette to include sweat, interstitial fluid, saliva, and tears. Sweat is secreted by eccrine glands distributed across the skin and carries electrolytes, metabolites, and heavy metal species that reflect hydration status and metabolic load. Interstitial fluid, which bathes cells in subcutaneous tissue, has a composition closely tracking blood plasma and can be sampled continuously through microneedle arrays without the discomfort of repeated venipuncture.

Tear fluid provides another accessible secretion: lachrymal glands produce it continuously and it contains glucose, lysozyme, and cytokines that correlate with systemic conditions including diabetes and inflammation. Saliva carries amylase, immunoglobulins, and small noncoding RNAs that have diagnostic relevance for oral and systemic diseases.

Microfluidics and Lab-on-Chip Technologies

Handling biological fluids in small volumes requires microfluidic platforms, sometimes called lab-on-chip devices, that manipulate nanoliter to microliter samples through etched channels, valves, and reaction chambers. These platforms integrate fluid transport, mixing, and detection functions on a single substrate, reducing reagent consumption and enabling point-of-care use. Microfluidic electrochemical sensors embedded in flexible substrates can be worn directly on the skin to collect and analyze sweat in real time, transmitting data wirelessly to monitoring systems.

Droplet microfluidics, in which discrete aqueous droplets are formed and manipulated in an immiscible carrier fluid, is widely used for single-cell analysis and digital bioassays. By encapsulating individual cells or reactions in droplets, researchers achieve quantitative measurements of secretion profiles at resolutions unavailable in bulk fluid assays.

Secretion Analysis in Glandular and Neural Research

Glandular secretions, including exocrine secretions from salivary, pancreatic, and sweat glands as well as endocrine hormones released into blood, are studied to understand organ function and to detect disorders such as cystic fibrosis, diabetes mellitus, and hyperthyroidism. The chloride concentration in sweat, for example, remains the diagnostic standard for cystic fibrosis; the NIST Standard Reference Data program supports calibration of the analytical instruments used in such assays.

In neuroscience, the release of neurotransmitters and neuropeptides into the synaptic cleft and extracellular space is a form of secretion that determines signaling between neurons. Electrochemical microsensors implanted near neurons can resolve fast amperometric signals corresponding to individual exocytotic secretion events.

Applications

Fluids and secretions have applications in a range of biomedical and engineering fields, including:

• Wearable continuous health monitoring via sweat and interstitial fluid sensors
• Point-of-care diagnostics for metabolic and infectious diseases
• Drug concentration monitoring through saliva and urine analysis
• Neonatal and pediatric screening using minimal-volume fluid samples
• Neural interface research measuring neurotransmitter release dynamics