How Is Ultrasonic Spray Coating Used to Fabricate CGM Sensing Layers? From Preserving Enzyme Activity to Uniform Coating on PET Films

In the development of continuous glucose monitoring (CGM) sensors, reliably depositing enzyme-containing solutions onto flexible substrates may seem like a basic task. In practice, however, it involves materials science, fluid dynamics, interfacial phenomena, and biochemistry.

For electrochemical sensors that use glucose oxidase (GOx), the coating must do more than cover the target area. It must also maintain adequate enzyme activity throughout deposition, drying, immobilization, and subsequent use, while forming a functional layer that meets the requirements for glucose diffusion and electrochemical detection.

As substrates shift from rigid materials to flexible PET films, and experimental goals expand from individual sensing sites to consistency across multiple sites or continuous production, conventional coating processes may encounter new challenges. Ultrasonic spray coating is a candidate process worth evaluating. Its core value lies in controllable liquid atomization and deposition, along with the ability to adjust spray paths and deposited amounts.

1. What Are the Main Challenges in Fabricating CGM Enzyme Layers?

1.1 An Enzyme Solution Is More Than an Ordinary Coating Liquid

Glucose oxidase is a protein with biocatalytic activity. Its activity can be affected by temperature, pH, the solvent environment, shear and interfacial effects, drying, and storage conditions.

Enzyme layer fabrication therefore requires attention to more than whether the solution has been successfully deposited. Key considerations include:

  • Whether enzyme activity changes after spraying;
  • Whether the enzyme is distributed uniformly across the substrate;
  • Whether immobilization affects enzyme activity and accessibility;
  • Whether the coating contains pinholes, localized buildup, or areas of insufficient coverage;
  • Whether the resulting electrochemical signal is stable and the response is reproducible.

Enzyme activity is not determined solely by what happens during spraying. Stabilizers, crosslinkers, and buffer systems in the formulation, as well as subsequent drying and immobilization steps, can all affect the final outcome.

From Preserving Enzyme Activity to Uniform Coating on PET Films

1.2 Wetting and Film Formation on Flexible PET Films

PET offers flexibility and relatively good dimensional stability, but its surface wetting behavior depends on surface treatment, contamination, storage conditions, and the specific material formulation.

If an enzyme solution does not adequately wet the PET surface, droplets may retract, leaving locally exposed substrate or an uneven coating. Excessive liquid deposition, on the other hand, may cause localized buildup, uneven drying, or variations in film thickness.

For miniature sensing sites, enzyme loading may vary from one site to another even when the coated area appears uniform overall. Development work should therefore evaluate substrate surface condition, liquid wettability, and deposition parameters together.

1.3 Moving from Single-Site Samples to Batch Fabrication

Laboratory research often begins by determining whether a single sensing site can generate a detectable signal. Process development must then address additional questions:

  • Is enzyme loading consistent across multiple sensing sites?
  • Are sample responses reproducible across batches?
  • Is there unnecessary material deposition outside the target coating area?
  • Can the process accommodate sheet substrates or roll-to-roll flexible films?
  • Do equipment operation, cleaning, and restart procedures meet research or production requirements?

This means the coating method must evolve from a sample preparation technique into a quantifiable, repeatable process.

2. How Ultrasonic Spray Coating Works

Ultrasonic spray coating uses high-frequency mechanical vibration to break a liquid into fine droplets, which are then deposited onto a target substrate through controlled delivery and spraying.

Compared with certain spraying methods that atomize liquids through direct impingement by high-pressure gas, ultrasonic atomization can provide a relatively gentle approach to droplet generation. The deposition process can be controlled through parameters such as liquid flow rate, nozzle motion, spray distance, spray path overlap, and substrate motion.

For enzyme solutions, this approach is worth considering for three main reasons:

  1. Reduced reliance on high-pressure atomizing gas. This does not mean that the overall spraying process is free from shear, interfacial effects, or other factors that may affect enzyme activity.
  2. Adjustable deposition per unit area. Parameter optimization and multiple coating passes can be used to explore ways of controlling film thickness and enzyme loading.
  3. Precise spray path control. For PET substrates with defined sensing regions, nozzle motion and masking designs can be used to evaluate the feasibility of localized coating.

Actual droplet characteristics, deposition uniformity, and enzyme activity retention still depend on the equipment configuration, solution properties, and process conditions, and must be established experimentally.

3. Four Research Directions for Ultrasonic Spray Coating of CGM Enzyme Layers

Direction 1: Optimizing the Process to Preserve Enzyme Activity

For biologically active materials, equipment selection should not be based solely on whether spraying is stable. Enzyme activity should also be compared before and after spraying.

Development teams should use enzyme solution from the same batch and include an unsprayed control, a sprayed group, and, where necessary, additional controls for drying or immobilization. Results should be compared using appropriate enzyme activity assays, with temperature, holding time, and sample storage conditions controlled.

If the research objective is to improve enzyme layer stability, the effects of spraying should be investigated separately from those of enzyme immobilization, protective layer design, and storage conditions. This helps avoid attributing every performance change to the atomization method.

Direction 2: Improving the Spatial Consistency of Enzyme Loading

In electrochemical sensors, variations in enzyme loading can affect local catalytic reactions, signal magnitude, and response reproducibility. However, enzyme loading is not the only determining factor: enzyme activity, mass transport through the coating, and electrode condition are also important.

An evaluation framework can include the following metrics:

  • Enzyme loading per unit area or per sensing site;
  • Coating thickness and coverage at different locations;
  • Enzyme activity retention after spraying;
  • Differences in electrochemical response between sensing sites;
  • Within-batch and between-batch reproducibility.

If a project targets a coefficient of variation (CV) below 1% for enzyme loading, the CV calculation, sample size, measurement method, and applicable conditions should be clearly defined, and the target should be validated using representative samples. A CV below 1% should be treated as a process target requiring validation, not as an equipment performance guarantee that can be made without testing.

Direction 3: Controlling PET Surface Wetting and Film Morphology

For substrates with low surface energy or insufficient wettability, development can begin by measuring how the enzyme solution spreads on PET, followed by comparisons of film formation under different substrate surface conditions.

Variables to investigate include:

  • PET surface cleanliness and pretreatment method;
  • Enzyme solution viscosity, surface tension, and solids content;
  • Liquid flow rate and nozzle travel speed;
  • Spray distance and spray path overlap;
  • Drying temperature, drying time, and ambient humidity.

Maximizing the spreading area does not necessarily improve sensor performance. An effective process should balance uniform coverage, enzyme activity, film structure, and mass transport.

Direction 4: Moving from Sheet Coating to Roll-to-Roll Processing

For laboratory development, sheet-format PET samples can first be used to establish relationships between material properties and process parameters. Once coating morphology, enzyme activity, and electrochemical response meet expectations, continuous roll-to-roll coating can be evaluated.

Roll-to-roll processing introduces additional considerations, including web tension, line speed, coating registration, drying section design, web edge quality, and batch traceability. Parameters established with sheet samples should not be transferred directly to continuous production without validation.

4. How to Design a Meaningful CGM Spray-Coating Trial

A stepwise approach is recommended:

Step 1: Define the materials and structure. Provide the PET thickness, surface treatment details, sensing site dimensions, basic information about the enzyme solution formulation, and the target coating area.

Step 2: Establish a baseline. Record enzyme activity, coating morphology, enzyme loading, and electrochemical response achieved with the current process as a basis for comparison.

Step 3: Conduct small-scale parameter screening. Adjust liquid flow rate, nozzle travel speed, spray distance, and drying conditions systematically, avoiding changes to too many variables at once.

Step 4: Evaluate multiple performance dimensions. Combine microscopy, film thickness or enzyme loading measurements, enzyme activity assays, and electrochemical testing to determine whether the process meets the requirements.

Step 5: Verify reproducibility. Confirm the results across multiple sensing sites, multiple substrate sheets, and different batches before assessing the potential for scale-up and continuous processing.

5. What Should You Evaluate When Selecting Equipment?

For PET substrates coated with glucose oxidase solutions, key considerations include:

  • Whether the target enzyme solution can be delivered consistently and reproducibly;
  • Whether the deposited amount, motion path, and spray coverage can be adjusted;
  • Whether wetted components are compatible with the solution formulation;
  • Whether temperature and humidity in the spray zone can be controlled or monitored;
  • Whether the equipment is easy to clean to prevent enzyme solution residues or cross-contamination;
  • Whether the equipment supports sheet samples and allows for the evaluation of future roll-to-roll options.

A temperature- and humidity-controlled chamber may be worth considering. However, its settings should be determined by the enzyme solution formulation, enzyme stability data, and process validation results. Lower temperatures or higher humidity should not automatically be assumed to be beneficial.

6. Frequently Asked Questions

Q1: Does ultrasonic spray coating always preserve enzyme activity better than conventional spraying?

Not necessarily. Ultrasonic spray coating can reduce reliance on high-pressure gas atomization, but enzyme activity is also affected by temperature, formulation, interfacial effects, drying, and immobilization conditions. Comparisons should be based on enzyme activity measurements before and after spraying.

Q2: Can ultrasonic spray coating directly solve poor wetting on PET?

Not necessarily. Spray parameters can help optimize deposition, but the PET surface condition and the wetting characteristics of the enzyme solution are equally important. Different surface treatments and formulation conditions should be compared where necessary.

Q3: Does a uniform enzyme layer guarantee a stable CGM sensor signal?

No. A uniform enzyme layer helps reduce one potential source of process variability, but electrode performance, enzyme activity, mass transport, layer immobilization, and the testing environment also affect signal stability.

Q4: Can ultrasonic spray coating directly extend the operating life of a CGM sensor?

This conclusion cannot be drawn from the coating process alone. Further validation is needed to assess enzyme activity retention, long-term electrochemical drift, in vitro performance, and compliance with applicable biocompatibility and safety requirements.

Ultrasonic Spray Coating Used to Fabricate CGM Sensing Layers

7. Conclusion: Moving Enzyme Layer Fabrication from Experience-Based Practice to Data-Driven Development

The key to developing CGM enzyme layers is not simply depositing an enzyme solution onto PET. It is establishing an interpretable relationship between liquid atomization, substrate wetting, film deposition, enzyme activity, and electrochemical performance.

Ultrasonic spray coating offers a process option that can be evaluated for this purpose. Through controlled experiments and validation against multiple metrics, development teams can determine whether it is suitable for their enzyme formulations, sensing site structures, and production strategies.

If you are developing glucose oxidase sensing layers on flexible PET films, contact Cheersonic to discuss your material properties, substrate structure, and target process, and to arrange ultrasonic spray-coating trials using your samples.

Email: market2@cheersonic.com

When submitting samples, please also provide basic enzyme solution properties, the target coating area, expected coating specifications, and available electrochemical testing conditions to support a more targeted evaluation of process suitability.