Rx Tattoo

Overall Design Concept

Imagine a thin, flexible patch—akin to a temporary tattoo—that can adhere tightly to the skin without significant discomfort. It has:

1. Microneedles embedded or protruding from the patch surface, designed to painlessly penetrate the stratum corneum (the top layer of the skin).

2. Sensing elements integrated into or onto these microneedles to measure glucose and cardiac signals.

3. Flexible circuits that connect the microneedles with on-patch electronics for signal processing and wireless data transmission.

4. Possible power source (micro-battery, printed battery, or energy-harvesting system) and a wireless communication module (Bluetooth Low Energy, NFC, or similar).

The patch could be fabricated via 3D printing or additive manufacturing of microneedles and flexible circuit layers, enabling a design that is both anatomically conformable and highly integrated.

Monitoring Glucose with Microneedles

Rationale for Microneedle-Based Sensing

Interstitial fluid (ISF) in the upper dermis contains biomarker concentrations (e.g., glucose) that correlate closely with blood levels.

Microneedles of sufficient length (generally 100–1000 micrometers) can penetrate only the outermost skin layer without reaching deeper nerve endings—making the process minimally painful or even imperceptible.

The small sample volume in the microneedle tip can be used for continuous or semi-continuous monitoring of glucose.

Microneedle Fabrication

Materials: Biocompatible polymers (e.g., poly(lactic-co-glycolic acid), polyvinylpyrrolidone), silicones, or metals (e.g., stainless steel, nickel, gold-coated steel) can be used. Often, polymer microneedles with conductive coatings or embedded nanoparticles are leveraged for biosensing.

3D Printing / Additive Manufacturing: High-resolution 3D printing methods (e.g., two-photon polymerization, micro-stereolithography) can create microneedles with precise geometries that maintain mechanical integrity while being sharp enough to penetrate the skin.

Glucose Sensing Mechanism

Enzymatic Electrochemical Sensors:

Glucose oxidase (GOx) is a common enzyme that selectively reacts with glucose, producing hydrogen peroxide or other by-products that can be electrochemically detected.

A typical setup includes a working electrode (often platinum, gold, or carbon-based), a reference electrode, and sometimes a counter electrode.

When glucose in the ISF diffuses into the enzyme layer on/within the microneedles, an electrochemical reaction occurs. The resulting current is proportional to the glucose concentration.

Non-Enzymatic Electrodes:

Uses metal catalysts (e.g., copper, nickel, gold nano-structures) to oxidize glucose. Less enzyme stability issues but may require more robust calibration to manage interfering species.

Microfluidic Integration:

Some designs incorporate microfluidic channels that wick ISF into small reaction chambers. This can help ensure that the sensing mechanism is stable and the volume tested is well-defined.

However, simple direct contact between the microneedle electrode and the ISF can also be sufficient if the electrode surface is optimized.

Challenges in Glucose Sensing

Fouling & Biocompatibility: Proteins and other biomolecules in the ISF can accumulate on the sensor surface over time, degrading performance. Sensor coatings and antifouling chemistries (e.g., PEG-based coatings) are often used.

Long-term Stability: The glucose oxidase enzyme can degrade or lose activity over days/weeks. Packaging the enzyme in stable polymer matrices, or using non-enzymatic catalysts, can extend sensor life.

Calibration & Lag: Real-time glucose monitoring needs calibration. The correlation between blood glucose and ISF glucose can lag by a few minutes. Advanced algorithms can account for this delay.

Monitoring Heart Function

What Cardiac Signals To Measure?

ECG (Electrocardiogram): Captures the electrical activity of the heart. Typically requires electrodes placed in specific orientations on the body. For a single patch, you might get a limited-lead ECG or local electrical signals that can still provide heart rate and some measure of cardiac rhythm.

PPG (Photoplethysmography): Uses LEDs and photodiodes to measure changes in blood volume in the microvasculature. Commonly seen in pulse oximeters. May be integrated if you can incorporate tiny light sources and sensors onto the patch.

Impedance Cardiography: Measures changes in the electrical impedance of the thorax as blood volume changes with each heartbeat. However, this typically requires at least a pair of electrodes.

Methods for Embedding Heart Monitoring Into the Patch

Flexible Electrodes for ECG:

The patch could have two or three flexible electrodes arranged such that they can pick up the heart’s electrical signals from the chest or a local site.

These electrodes might be conductive inks (silver nanowires, graphene-based, or gold patterns) printed onto a stretchable polymer substrate (like PDMS or TPU).

Small electronics on the patch amplify and filter the ECG signals before sending them out wirelessly.

Microneedle-Assisted ECG:

If the patch is placed near the chest region, microneedles might help reduce skin-electrode impedance. Penetration through the stratum corneum can provide more direct contact with bodily fluids/tissues, thus decreasing noise.

However, for ECG, full penetration is not always strictly necessary—often just good contact with conductive hydrogel or a mildly invasive approach improves signal quality.

Embedded PPG:

A tiny LED and photodiode integrated into the patch, shining light into superficial blood vessels, measuring reflectance. Fluctuations correspond to the pulsatile flow of blood.

This technique might require more power than a purely passive electrode approach, so it depends on the patch’s power budget.

Data Processing and Transmission

Signal Processing Chip: Onboard amplifiers and analog filters handle the very small ECG/PPG signals.

Microcontroller: A low-power microcontroller digitizes these signals.

Wireless Transmission: A Bluetooth Low Energy (BLE) or NFC module sends data to a smartphone or dedicated receiver in real time.

Power Management: Could use a thin-film battery or energy harvesting (e.g., through thermoelectric or piezoelectric materials) depending on complexity.

3D Printing and Layer-by-Layer Construction

1. Substrate Layer: A flexible polymer film (e.g., polyimide, TPU) that can bend with the skin.

2. Conductive Traces Layer: Printed metallic or carbon-based inks forming electrode interconnects and sensor contacts.

3. Sensor Integration:

   • Microneedle array is printed or affixed.

   • The microneedle tips may be coated with or incorporate the glucose sensing chemistry (e.g., enzyme layers, conductive metals, protective polymers).

   • Separate regions for ECG electrodes or PPG modules could be added in adjacent (non-microneedle) areas.

4. Encapsulation Layer: A thin protective layer to seal in the electronics, leaving only the microneedle tips exposed and the electrode surfaces open to skin contact.

This layer-by-layer approach can be realized through advanced additive manufacturing or screen-printing hybrid processes. Some research groups already do multi-layer printing of flexible electronics and microneedle arrays.

Practical Considerations

Adhesive: The patch must remain attached during normal activities (sweating, movement) without significant irritation.

Sterilization & Regulatory: Microneedles for clinical use must be sterile, and the device would likely require FDA/CE approval for continuous glucose monitoring and cardiac monitoring. Materials and adhesives must be biocompatible.

Wear Duration: How often it needs to be replaced depends on microneedle integrity, enzymatic stability (if used), and adhesive lifespan.

Data Security & Processing: Since it’s medical data, encryption protocols for wireless transmission are crucial.

A Hypothetical Use Scenario

1. The user applies the patch on their upper arm or chest area (depending on the best location for both glucose ISF extraction and decent ECG signal—though more commonly glucose patches go on the arm, ECG electrodes on the chest, so one might compromise or build two separate modules).

2. The microneedles painlessly penetrate the outer skin layer.

3. The user’s smartphone automatically connects to the patch, receiving two data streams:

   • Glucose Trend: Real-time glucose readings, with warnings if levels rise or drop too quickly.

   • Cardiac Signals: Heart rate, possibly basic rhythm data (e.g., possible arrhythmias) if ECG analysis is sophisticated enough.

4. The patch’s battery is designed to last one week. After that, the user disposes of the patch responsibly, and applies a new one.

Looking Ahead

Longer-Term Implants? If one envisions even longer monitoring, microneedles might evolve into biodegradable or dissolvable arrays that deposit sensors under the skin. However, that’s a more invasive approach than a removable patch.

Additional Biomarkers? Beyond glucose, future patches might measure lactate, cortisol, or other metabolic/cardiovascular indicators.

Smart Algorithms & Telemedicine: The data can feed into an AI-driven platform, offering predictive alerts or integration with telehealth systems.

Conclusion

A 3D-printed microneedle patch that continuously measures both cardiac signals (via ECG/PPG or impedance) and glucose levels is a plausible marriage of several existing research innovations. The key scientific and engineering pillars are:

1. Microneedle design for minimally invasive sampling of interstitial fluid.

2. Biosensing chemistry (enzymatic or non-enzymatic for glucose).

3. Flexible electronics and biocompatible adhesives to allow comfortable, skin-conforming placement.

4. Embedded electrodes for heart signal monitoring.

5. Low-power circuits and wireless communication for real-time data transmission.

Though challenging in practice—especially regarding long-term stability, sensor fouling, and regulatory hurdles—the concept is well within the realm of emerging wearable medical technology. Such a patch could provide continuous, on-demand insights into a person’s metabolic and cardiovascular status, bridging convenience and clinical-grade data in a single device.