Protein Binding and Molecular Size Govern Molecular Transport into Dermal Interstitial Fluid

Dermal interstitial fluid (ISF) has emerged as the leading frontier for real-time molecular monitoring due to its rapid equilibration with blood and its role in carrying biomarkers that may reflect health status. To access ISF, we and others have developed microneedle sensor arrays, composed of microscopic probes that penetrate the epidermis painlessly to reach the dermis. These sensors, combined with wearable electronics, enable wireless, real-time molecular monitoring in the body. Previous studies have used enzyme-based microneedle sensors to monitor metabolites like glucose and lactate, while aptamer-based sensors have been applied for therapeutic drug monitoring. In this work, we expanded the application of aptamer-based microneedle sensors to evaluate the effect of protein binding on molecular transport from blood to ISF. Specifically, we monitored two antibiotics (vancomycin and tobramycin), the amino acid metabolite phenylalanine, two antineoplastics (irinotecan and doxorubicin), and a protein biomarker (platelet-derived growth factor, PDGF). To enhance measurement accuracy, we developed a multichannel sensor platform with magnetic attachment for reliable sensor placement in rodents. Our measurements reveal that small molecules with a significant "free" fraction in plasma (<70% protein binding), like vancomycin, tobramycin, and phenylalanine, transport efficiently into the dermis ISF and are detectable by microneedle sensors. Conversely, molecules that exist mostly protein bound, such as irinotecan and doxorubicin, reach ISF concentrations well below the limit of detection of benchmark aptamer-based sensors. Additionally, we demonstrate the first successful real-time measurement of PDGF-BB transport (∼25 kDa for the homodimer) from blood to ISF, highlighting the potential of microneedle sensors for tracking larger biomolecules and broadening the scope of real-time molecular health monitoring.