Printed electronics have revolutionized the development of biosensors, electrodes, and drug delivery patches by enabling low-cost, scalable, and flexible solutions. These devices leverage conductive inks, organic semiconductors, and biocompatible substrates to create functional systems for healthcare monitoring, diagnostics, and therapeutic applications. Unlike traditional rigid electronics, printed devices conform to biological surfaces, improving patient comfort and integration with biological tissues. Key considerations include material biocompatibility, sterilization compatibility, and performance stability under physiological conditions.

Printed biosensors are widely used for monitoring biomarkers, such as glucose, lactate, and pH levels, in bodily fluids like sweat, saliva, and interstitial fluid. Electrochemical biosensors dominate this space due to their high sensitivity and compatibility with printing techniques. For example, screen-printed carbon electrodes functionalized with enzymes enable real-time glucose detection. The use of nanomaterials, such as carbon nanotubes or graphene, enhances electron transfer kinetics and detection limits. Biocompatible encapsulation layers, often made of polymers like polydimethylsiloxane (PDMS) or parylene, prevent biofouling and ensure long-term stability.

Sterilization is critical for ensuring the safety of printed biosensors in clinical settings. Common methods include gamma irradiation, ethylene oxide gas, and autoclaving. However, not all materials withstand these processes. For instance, some organic semiconductors degrade under high-temperature sterilization, necessitating the use of low-temperature alternatives. Research has demonstrated that ethylene oxide sterilization maintains the functionality of printed silver nanoparticle-based electrodes, whereas autoclaving can compromise adhesion and conductivity.

Printed electrodes are essential for electrophysiological signal acquisition in electrocardiography (ECG), electromyography (EMG), and electroencephalography (EEG). Unlike conventional gel-based electrodes, dry printed electrodes eliminate skin irritation and improve long-term wearability. Conductive polymers like poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) are commonly used due to their mechanical flexibility and high conductivity. Studies show that printed PEDOT:PSS electrodes achieve comparable signal quality to Ag/AgCl electrodes in ECG monitoring, with skin-contact impedance values below 50 kΩ at 10 Hz.

Biocompatibility is a primary concern for electrodes in direct skin contact. Materials must avoid causing allergic reactions or inflammation. Regulatory standards such as ISO 10993-5 assess cytotoxicity, while in vivo studies evaluate skin irritation. For example, gold and carbon-based inks exhibit excellent biocompatibility, whereas nickel-containing inks may trigger allergic responses. Additionally, breathable substrates like polyurethane or textile-based materials enhance comfort by allowing moisture vapor transmission.

Drug delivery patches represent another major application of printed electronics. These patches use printed circuits to control drug release via iontophoresis, electroporation, or resistive heating. For instance, a printed iontophoretic patch can deliver lidocaine transdermally by applying a low electric current to enhance skin permeability. Studies indicate that such systems achieve drug delivery rates of 0.5–2 mg/cm² per hour, depending on current density and formulation.

The integration of biosensing and drug delivery creates closed-loop theranostic systems. A glucose biosensor coupled with an insulin delivery patch exemplifies this approach. When the sensor detects hyperglycemia, the patch releases insulin in a feedback-controlled manner. Such systems require precise calibration to avoid under- or overdosing. Biodegradable materials, such as poly(lactic-co-glycolic acid) (PLGA), are being explored for transient patches that dissolve after use, eliminating waste and reducing environmental impact.

Flexibility and stretchability are crucial for ensuring reliable performance under mechanical deformation. Conductive elastomers, such as silver flake-embedded silicone, maintain conductivity even when stretched by 50% or more. This property is vital for wearable applications where movement is inevitable. Researchers have demonstrated that stretchable printed electrodes retain functionality after thousands of bending cycles, making them suitable for long-term monitoring.

Environmental stability is another challenge for printed devices. Exposure to humidity, temperature fluctuations, and mechanical stress can degrade performance. Barrier coatings, such as atomic layer-deposited alumina, protect sensitive components from moisture and oxidation. Accelerated aging tests simulate years of use within weeks, helping to predict device lifetimes. For example, some printed biosensors show less than 10% signal degradation after 30 days in 85% relative humidity at 40°C.

Scalability is a key advantage of printed electronics. Roll-to-roll printing enables mass production of disposable biosensors and electrodes at low cost. A single production run can yield thousands of units, reducing per-device expenses to cents rather than dollars. This scalability is particularly beneficial for point-of-care diagnostics in resource-limited settings.

Despite these advances, challenges remain. Achieving high-resolution printing of nanoscale features is difficult with conventional methods. Inkjet printing can achieve resolutions of 20–50 µm, but finer features require advanced techniques like aerosol jet printing. Material compatibility is another hurdle; not all inks adhere well to flexible substrates, leading to delamination or cracking.

Future directions include the development of multifunctional inks that combine conductivity, sensing, and self-healing properties. Self-healing materials can repair minor cracks automatically, extending device lifetimes. Another area of exploration is energy-autonomous patches powered by printed batteries or energy harvesters, such as triboelectric nanogenerators.

In summary, printed biosensors, electrodes, and drug delivery patches offer versatile solutions for modern healthcare. Their success hinges on material selection, sterilization compatibility, and robust performance under physiological conditions. Continued advancements in printing technologies and biocompatible materials will further expand their applications in wearable health monitoring and personalized medicine.