Microfluidics in Focus: From Design to Device

Microfluidic devices play a pivotal role in the shift towards point-of-care diagnostics. Discover in the third part of our four-part series how to successfully navigate the manufacturing process. This article provides essential information on 3D printing, soft lithography, and lateral flow manufacturing. It also guides you through the key choices and challenges in manufacturing microfluidic devices. 

Overview 

Microfluidic devices are evolving into powerful tools in diagnostics and life sciences, with materials and fabrication methods playing a crucial role in application and device performance. But this is only the beginning. Behind every design choice lies a balance of technical requirements, annual volumes, and economic realities.

3D printing of microfluidic devices

Advantages and challenges

3D printing provides a versatile approach for rapid prototyping and creating complex geometries. Advances in high-resolution 3D printing enable the production of intricate microfluidic structures in a single step.

• Advantages:
  • Flexibility: Designs are easy to modify and custom devices can be produced quickly.
  • Complexity: Capable of fabricating complex and integrated structures that are difficult to achieve with traditional methods.
• Challenges:
  • Surface quality: Ensuring smooth microchannel surfaces is essential for consistent fluid flow, which can be difficult with some 3D printing technologies.
  • Material compatibility: Finding biocompatible and chemically resistant materials suitable for biological assays remains a challenge.
  • Scalability: While excellent for prototyping, scaling up production can be less efficient compared to other methods.
    
    

Several companies entering the field are taking advantage of the rapid development of professional 3D printing engines on the market: Rapid Fluidics, uFluidix, Nanoscribe, UpNano, Microlight3D, and Boston Micro Fabrication(BMF). Combining their capabilities of high-resolution 3D printing, CAD software to design complex channel structures certainly offers a tempting alternative to established technologies to proof concepts and try out new ideas. However, 3D printing materials need to be tested for their applicability, e.g., biocompatibility, surface tension, and others. Also, integration of (structured) surface functionalization has not yet been demonstrated on an industrial scale.

By 3D printing high-resolution molds directly, researchers can bypass cleanroom-intensive processes and rapidly prototype complex geometries for PDMS casting. Post-processing steps – such as solvent extraction to remove residual photo initiators – are often integrated to ensure smooth, defect-free surfaces that enhance PDMS curing and device performance. This approach not only accelerates the development cycle but also enables the creation of three-dimensional features that were previously challenging to realize. Ultimately, 3D-printed molds offer a cost-effective and flexible route for fabricating microfluidic devices, opening the door to scalable production in applications ranging from lab-on-a-chip systems to advanced diagnostic assays (Bazaz et al. 2019, Kamei et al. 2015, and Hollingsworth 2020).

Soft lithography

Advantages and challenges

Soft lithography is commonly used for prototyping microfluidic devices. A master mold is created via photolithography, onto which polymers such as PDMS (polydimethylsiloxane) are cast to form microfluidic structures.

• Advantages: Allows for high-resolution, custom patterns suitable for small-scale production and research applications with extremely fast turnaround times; easy to integrate into fluid control systems.
• Challenges:
  • Scalability: The process is labor-intensive and time-consuming, making it difficult to scale up for mass production.
  • Material limitations: PDMS can degrade when exposed to certain reagents, potentially affecting device longevity and performance. PDMS is a permeable material, a property that excels in innovative applications, e.g., organ-on-a-chip, where gas exchange is beneficial. On the other hand, if air ingress is not desired, e.g., when handling DNA/RNA, it is imperative to use non-permeable materials.

PDMS-based microfluidic devices enable highly stable cell culture thanks to their high permeability to gases. This facilitates long-term stability and cell viability, which could not be achieved using conventional materials, e.g., silicon or glass-based materials. However, despite these many advantages, these polymers still have some limitations. These include optical transparency and thermal, mechanical, and chemical resistance (Aralekallu et al. 2023).

Lateral flow manufacturing

Advantages and challenges

Paper-based lateral flow assays (LFAs) play an intrinsic role in point-of-care diagnostics, particularly in multiplexed immunoassays such as those for cardiac markers. The core manufacturing process – depositing test and control lines of antibodies onto nitrocellulose membranes – is well established and benefits from mature roll-to-roll production techniques.

A typical microfluidic paper-based analytical device setup (a) unfabricated Whatman filter paper, (b) fabricated PAD with distinct hydrophobic barriers and hydrophilic zones, (c) pre-loading of reacting reagents, (d) loading of sample on PAD, and (e) development of color (from Paper-based microfluidic devices: Fabrication, detection, and significant applications in various fields by Das et al. 2022 in Rev. Anal. Chem. 41(1) 112–136).

During the COVID-19 pandemic, lateral flow tests played a crucial role in managing the pandemic due to their speed, accessibility, and ease of use. This enabled widespread self-testing to detect SARS-CoV-2 infection beyond healthcare settings.

Schematic illustration of an antigen detection-based LFIA test (from Recent Advances in Novel Lateral Flow Technologies for Detection of COVID-19 by Wei-Wen Hsiao et al. 2021 in Biosensors 11(9), 295).

Lateral flow test for COVID-19 (photo from Mass testing for COVID-19 using lateral flow tests
by Sarah Bunn, 2021, UK Parliament).

Classic studies in paper microfluidics highlight several factors that become critical as you move beyond simple single-analyte strips.

1. Fluid wicking and channel patterning
   • Martinez et al. 2010 showed that the pore size and fiber structure of nitrocellulose govern capillary flow rate and test-line residence time; uneven flow can lead to signal variability across channels when you place multiple capture lines in close succession.
   • Ballerini et al. 2012 demonstrated that alternative substrates (e.g., glass-fiber or composite laminates) can be used to fine-tune wicking speed and reduce crosstalk in multiplex formats.
2. Reagent deposition and signal development
   • Nilghaz et al. 2016 revealed how precise control of antibody spotting volume, drying conditions, and blocking treatments is essential to maintain consistent line thickness – and thus uniform color development – across multiple test zones.
   • Protein binding kinetics on paper (e.g., diffusion into the porous matrix) set a lower bound on flow velocity. If this is too fast, late zones suffer from incomplete capture.
3. Multiplex geometry and 3D layering
   • Martinez et al. 2008 introduced 3D stacking of patterned paper and tape to route fluids sequentially through multiple reagent fronts, enabling true 3D multiplexing without lateral bleed. This approach illustrates how facility layouts must accommodate lamination equipment and controlled cutting/registration of multilayer stacks.
4. Scaling challenges
   • As the number of test lines increases, variations in membrane width, alignment tolerances, and roller-coater pressure profiles can produce non-uniform channel heights, leading to differential flow rates that skew quantitation.
   • Facilities therefore require tight environmental controls (humidity/temperature), precision spotting robots with micron-scale accuracy, and automated optical QC (line-edge detection) to ensure each batch meets reproducibility specs.
5. Regulatory and cost considerations
   • Incorporating more assay zones demands additional blocking, drying, and lamination steps – and often bespoke tooling for membrane cutting and cassette assembly – which raises the per-unit cost. Careful process mapping (as outlined in AntiTeck’s LFA plant guide and Abingdon Health’s validation roadmap) can help identify bottlenecks and prioritize investments in high-precision equipment over sheer throughput.

By integrating insights from these foundational papers into your production planning – especially for capillary flow control, reagent immobilization, and multilayer device assembly – you can design a facility capable of reliably manufacturing advanced multiplex lateral flow assays without sacrificing the speed or cost-effectiveness that make lateral flow so attractive (Martinez et al. 2010, Nilghaz et al. 2016, Ballerini et al. 2012, and Martinez et al. 2008).

How to choose your manufacturing pathway

Here is a potentially structured approach to selecting materials and fabrication methods for microfluidic devices that balances technical performance, manufacturing costs, and annual consumable volumes – and also includes glass-wafer production.

1. Material-property indicators
   • Score each candidate on optical clarity, autofluorescence, dimensional fidelity, mechanical strength, chemical-barrier performance, biocompatibility, and sterilization resilience.
   • Apply hard thresholds (e.g., ≥90 % transmittance, autofluorescence <0.1 rel. units, transfer ratio >95%, O₂ permeability >50 barrer) to eliminate unsuitable materials early [1 barrer = 10⁻¹⁰ (cm³·cm) / (cm²·s·cm Hg)].
2. Process–material mapping
   • Injection molding and hot embossing serve thermoplastics (COC/COP, PMMA, PC) across different volume bands; laser ablation, soft lithography, and 2PP 3D printing target low-volume, high-resolution prototyping.
   • Glass-wafer microfluidics adds photolithography, wet or dry etching, wafer bonding (thermal or anodic), precision drilling (laser or ultrasonic), and dicing – typically in clean-room facilities.

3. Cost and volume trade-off

• Thermoplastics: Aluminum emboss shims ($5 k) or steel injection molds ($50–200 k) amortized over the expected annual volumes determine per-chip tooling cost. Cycle time and yield set variable costs.
• Glass-wafer: A single patterned 8-inch wafer can cost $500–1000 (including mask, etch, bonding), yielding hundreds of chips but requiring cleanroom run-hours ($50–100/hour), specialized bonding equipment and high-end laser equipment and processes for fluidic access drilling.
• Thresholds: For <10 k chips/year, hot embossing or laser-machined thermoplastics often beat glass per-unit cost. Between $10 k–100 k/year, injection-molded plastics reach $1–5 per chip; glass may justify itself only if optical/chemical demands cannot be met elsewhere. Above $100 k/year, steel-mold injection molding typically outcompetes both embossing and glass-wafer runs.

4. Hybrid roadmap

• Prototype and validate using low-cost methods (PDMS, laser ablation, hot embossing, or single-wafer glass pilots) to prove the fluidic design and surface chemistry.
• Scale to the chosen high-throughput route once annual consumption and target price are set: injection molding for thermoplastics, and (if glass is mandatory), consider batch bonding multiple wafers per run, or outsourcing wafer-level production to a high-volume MEMS foundry to drive per-chip cost down.

Essential criteria for method selection for multiplexed systems

When choosing how to manufacture microfluidic consumables – especially multiplexed devices – you have to balance technical requirements, annual volumes, and economic realities.

1. Quantify annual demand and unit-price goal
   • Build a simple cost model: amortize NRE (non-recurring engineering costs such as tooling, masks, cleanroom time) plus material and wafer costs (e.g., $200–500 per glass wafer; $5–10/kg thermoplastic) and assembly and QC (bonding, port-drilling, inspection: $1–3 per chip).
   • Include indirect costs and margin (R&D recovery, validation, distribution). A common benchmark is COG ≈10% of ASP – your selling price should be ≥2–3× COG.
2. Shortlist materials via quantitative criteria
   • Score candidates on optical clarity, autofluorescence (< 0.1 rel. units), dimensional fidelity (>95% transfer ratio), O₂ permeability (> 50 barrer), biocompatibility, sterilization resistance, etc., eliminating any that miss critical thresholds.
3. Factor in multiplexing value
   • The more assays you perform per chip, the higher per-unit price you can justify. For example, an Illumina SBS flow cell carries millions of sequencing reactions yet costs hundreds of dollars, because each chip carries immense multiplexed value.
   • Embed a “multiplex premium” in your ASP: cost-per-measurement falls even as cost-per-chip rises, so ensure your pricing model allocates value to each parallel readout.
4. Decide on glass only if necessary
   • Glass-wafer microfluidics has higher setup costs (wafer + mask + cleanroom: $200–500) and lower throughput; reserve it for applications whose optical, chemical, or thermal specs cannot be met by plastics at acceptable cost/volume.

Comparison of microfluidic chip manufacturing costs

A) Glass/silicon-based microfluidic chips (200 mm wafer, 2 wafers per chip)

Assumptions:
- Wafer diameter: 200 mm
- Chip size: 15 × 15 mm (225 mm²)
- Chips per wafer: ~100 (after accounting for edge losses and spacing)
- 2 glass wafers per device: 2 × $25 = $50/wafer set
- Cleanroom processing cost: $200/wafer set

Cost breakdown per chip:

B. Thermoplastic-based microfluidic chips

Assumptions:

- Mold size: 100 × 100 mm
- Chip size: 15 × 15 mm
- Chips per cycle: ~40
- Mold amortized over 1 million chips
- Resin cost: $5–10/kg (~0.5 g per chip)

Cost breakdown per chip:

Our key takeaway

Combine quantitative material thresholds with your cost-volume trade-off, and layer in a multiplexing multiplier to reflect the value of multiple simultaneous assays.
Always compare annual demand × amortized COG (including any multiplex premium) against your target ASP.
The best manufacturing route is the one where your per-unit COG (adjusted for multiplex value) falls well below your justified selling price.

Missed the first two parts of this series?
Read Alexis’ insightful articles: "Microfluidics in Focus: Benefits and Breakthroughs" and “Microfluidics in Focus: Materials that Matter”. 

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