Screen-Printing Technology & Substrate Selection
Flexible PET (polyethylene terephthalate, 175 µm, Melinex 401, Dupont) was selected as the substrate for its dimensional stability (thermal expansion 15 ppm/°C, vs. 80 ppm for standard PE) and chemical resistance to common biological matrices. Screen printing was performed on a DEK Horizon 265 printer with a 90-mesh stainless steel screen, emulsion thickness 200 µm, squeegee hardness 75 Shore A. Print-to-print registration accuracy: ±15 µm — sufficient for the 0.5 mm minimum electrode feature size. Conductive silver ink (DuPont 5025) was printed as the current collector, cured at 120°C for 20 minutes, then electrode areas were defined by a dielectric isolation layer (DuPont 5036D) printed overtop.
Gold Working Electrode — Formulation & Electrochemical Characterisation
Gold nanoparticle ink (60% Au by weight, particle diameter 20–40 nm, Electrodag 479SS) was screen-printed over the Ag current collector and sintered at 200°C for 30 minutes. Electrode surface area by CV in 5 mM [Fe(CN)6]³⁻/⁴⁻: 0.083 cm² (Randles-Ševčík). Surface roughness factor: 6.4 vs. polished bulk gold. The gold surface was functionalized with 3,3-dithiodipropionic acid di(N-hydroxysuccinimide ester) (DTSP-NHS) SAM to covalently bind anti-L-lactate dehydrogenase antibody (Abcam ab52488). LoD for lactate by DPV (differential pulse voltammetry, -0.4V to +0.6V, pulse amplitude 50mV): 0.82 ± 0.09 nM — suitable for detection of blood lactate at the 1 mmol/L clinical threshold (x 10⁶ concentration factor in undiluted matrix).
Carbon/Prussian Blue Composite — Glucose Detection
For glucose, a Prussian Blue (PB)-modified carbon electrode was developed. Carbon ink (Electrodag 423SS) was printed and PB electrodeposited by 20 cyclic voltammetry cycles in FeCl₃/K₃[Fe(CN)₆] (0.1M/0.1M, pH 1.5). The PB layer reduces H₂O₂ (produced by glucose oxidase, GOx) at +0.0V vs. Ag/AgCl — 0.7V lower than a bare carbon electrode, eliminating interference from UA, AA, and AP at physiological concentrations. GOx (1000 U/mL in 0.1M PBS pH 7.4) was cross-linked onto the PB-C surface with glutaraldehyde/BSA matrix. Analytical sensitivity: 2.1 µA·mM⁻¹ across 1–20 mM glucose (covering the clinical hypoglycaemia to hyperglycaemia range), r² = 0.9983.
Lot-to-Lot & Run-to-Run Precision
Electrode precision was assessed across 3 production lots (n=20 electrodes per lot). Run-to-run CV for the amperometric glucose response at 10 mM: 2.8% (Lot A), 2.4% (Lot B), 3.1% (Lot C). Lot-to-lot CV: 4.2% — acceptable for a disposable IVD electrode (ISO 17511 target: Lot-to-lot CV < 5%). The primary precision driver was ink viscosity variability: a Brookfield viscosity specification of 8,000–12,000 cP (Spindle 6, 20 rpm) with an in-process check before every print run reduced lot-to-lot variation from 9.8% to 4.2%.