Towards Tumor pH Detection Using Plain Radiography: An Injectable pH-Responsive Polyacrylic Acid Based Hydrogel Biosensor*

A pH responsive polyacrylic acid hydrogel based injectable biosensor was developed to measure local tumor pH using plain radiography in order to study cancer pathophysiology, track tumor progression, and determine effectiveness of drugs in vivo. Although plain radiography is unable to detect local chemical concentrations directly, a chemically responsive hydrogel was synthesized to move a tantalum bead and thus report the local concentration. The hydrogel was synthesized by free radical co-polymerization of acrylic acid and n-octyl acrylate and a 13 mm disk of the gel with embedded radiopaque wire had a range of pH 4–8 with a precision of 0.065 pH units. A preliminary study was carried out to miniature this hydrogel sensor and fit it into an injectable porous metal sleeve, all fitting within a breast cancer biopsy marker needle. The 10 mm sensor showed repeatable response to pH cycling in the range of pH 4–8. We are optimizing the sensor for sensing in the pH 6.5 to 7.5 range most relevant to tumor acidosis in order to track tumor physiology and assess response to chemo/radiotherapy.