Steady cone-jet mode of electrospray for single-cell deposition

This paper proposes and demonstrates a single-cell deposition method using the electrospray cone-jet mode near its minimum-flow-rate stability limit, which enables high spatial resolution placement of individual cells while maintaining their viability.

The Gist

The Big Idea: The "Cellular Sniper"

Imagine you are trying to build a living city (like a tissue or an organ) out of tiny bricks (cells). The problem with current construction methods is that they are like using a firehose to water a single flower. You spray a lot of water (liquid) and hope the flower gets wet, but you can't control exactly where the water lands, and you often drown the flower or wash it away.

This paper introduces a new way to build: The "Cellular Sniper."

The researchers developed a method to shoot out one single cell at a time, with extreme precision, into a specific spot. They call this the "Steady Cone-Jet Mode."

How It Works: The Magic Cone

Think of the liquid containing the cells as a drop of water hanging from a faucet.

1. The Electric Spark: The researchers apply a high-voltage electric charge to this drop.
2. The Taylor Cone: Instead of dripping slowly, the electric charge pulls the drop into a perfect, sharp point, like a tiny, spinning ice cream cone.
3. The Jet: From the very tip of this cone, a stream of liquid shoots out.
4. The Magic Trick: Usually, this stream is thick. But the researchers figured out how to turn the "flow" down to the absolute minimum. The stream becomes so incredibly thin (thinner than a human hair) that it looks like a needle.

Why This is a Game-Changer

1. Seeing the Invisible
Because the stream is so thin, the cells (which are much bigger than the stream) stick out like rocks in a fast-flowing river.

• Analogy: Imagine a parade where the marchers are huge, but the road is a tiny crack in the sidewalk. You can clearly see every single marcher passing by.
• Result: The researchers can actually see and count the cells as they fly through the air. They know exactly when a cell is coming.

2. The "One-by-One" Delivery
Because the liquid flow is so slow, there is a long pause between each cell.

• Analogy: Imagine a mailman delivering letters. Old methods were like throwing a whole sack of mail into a house. This new method is like the mailman walking up to the door, handing you one letter, waiting for you to sign for it, and then walking away to the next house.
• Result: They can place Cell A in Spot X, wait a moment, move the target, and place Cell B in Spot Y. This allows for building complex structures with "micrometer accuracy" (super precise).

3. Are the Cells Hurt?
You might worry that shooting a cell through an electric field and a thin needle would kill it.

• The Test: They used human breast cancer cells (MCF-7) mixed in a special gel. They shot them through the machine and then watched them grow in a petri dish.
• The Verdict: The cells took a little "shock" (like getting a mild sunburn), but most of them bounced back. About 30–40% survived the process immediately, and the ones that survived started growing and dividing normally within two days. The damage wasn't permanent; the cells recovered.

Why This Matters

Currently, scientists struggle to build tissues because they can't arrange cells precisely.

• Old Way: "Here is a pile of cells, mix them up and hope they form a heart."
• New Way: "I am placing this specific heart cell here, and this specific blood vessel cell right next to it."

This technique opens the door to:

• Custom Organs: Building tissues layer by layer with perfect cell placement.
• Drug Testing: Testing how a single cell reacts to a medicine without interference from its neighbors.
• Cloning: Isolating one perfect cell to grow a pure colony.

The Bottom Line

The researchers found a way to turn a "firehose" of cells into a "syringe" that delivers one cell at a time. It's fast, precise, and surprisingly gentle on the cells, making it a powerful new tool for the future of medicine and biology.

Technical Summary

1. Problem Statement

Current bioprinting and single-cell manipulation technologies face significant limitations in achieving micrometer-scale spatial resolution while maintaining cell viability and handling high cell concentrations.

• Resolution vs. Throughput: Existing methods like contact printing, laser-based printing, and inkjet bioprinting struggle to position individual cells with micrometer accuracy without damaging them or clogging nozzles. Inkjet printing, for instance, is limited to low-viscosity or low-concentration bioinks.
• The Conductivity-Viability Trade-off: Electrospray (electrohydrodynamic printing) offers high resolution via a "steady cone-jet" mode, but this mode requires low electrical conductivity. However, cell culture media and cell suspensions typically have high conductivity, which destabilizes the Taylor cone required for electrospray.
• Lack of Single-Cell Control: Previous electrospray attempts for biology (e.g., coaxial nozzles) achieved resolutions around 50 µm but could not deposit individual cells at distinct, user-defined locations due to flow rate constraints and jet instability.

2. Methodology

The authors propose a novel approach using the steady cone-jet mode of electrospray operated near its minimum-flow-rate stability limit.

• Liquid Formulation:
  • Cell Suspension: MCF-7 human breast adenocarcinoma cells suspended in 10% (w/v) Polyethylene Glycol (PEG) 35 kDa in ultrapure water.
  • Rationale: PEG 35 kDa provides the necessary viscosity and low electrical conductivity ($K \approx 6.1$ mS/m) to sustain a stable Taylor cone, while the low osmolality is managed to minimize osmotic shock.
  • Target: The suspension is deposited into a standard cell culture medium (DMEM with FBS) which has high conductivity and cannot be electrosprayed directly.
• Experimental Setup:
  • A syringe pump delivers the cell suspension through a metallic capillary ($D_i = 150$ µm).
  • A DC high-voltage potential (1.7–3.6 kV) is applied to the capillary relative to a grounded plate with an 800 µm orifice.
  • A co-flowing air stream prevents liquid accumulation on the plate.
  • Imaging: A high-speed camera (10,000 fps) visualizes the jet and cell ejection events.
• Deposition Strategy:
  • The system operates at an extremely low flow rate ($Q = 0.05$ ml/h), close to the theoretical minimum stability limit ($Q_{min} \approx 0.024$ ml/h).
  • This low flow rate ensures a large time interval ($\Delta t \approx 40$ ms) between consecutive cell ejections, allowing a robotic stage to move the substrate between drops.
  • Cells are deposited one-by-one onto a millimeter-sized droplet of culture medium.

3. Key Contributions

• Sub-Cellular Resolution: The technique achieves a jet diameter ($d_j \approx 3$ µm) significantly smaller than the cell diameter. This allows individual cells to be visualized and detected during the ejection process, enabling true single-cell resolution.
• Overcoming the Conductivity Barrier: By using a low-conductivity carrier fluid (PEG) for the electrospray and depositing it into a high-conductivity culture medium, the authors bypass the instability issues that usually prevent electrospraying cell suspensions directly.
• Deterministic Single-Cell Placement: The method allows for the placement of individual cells at distinct, user-defined locations, even at relatively high cell concentrations ($2 \times 10^6$ cells/ml), a feat previously difficult with electrospray.
• Validation of Viability: The study provides a comprehensive assessment of cell damage, distinguishing between reversible stress (osmotic shock) and irreversible damage (membrane rupture).

4. Key Results

• Jet Dynamics:
  • The jet velocity was measured at $\approx 4$ m/s, with a diameter of $\approx 3$ µm.
  • Cell ejection causes a transient destabilization of the Taylor cone, which re-stabilizes within $\approx 0.18$ ms. This is much faster than the time between cell ejections ($\approx 20-40$ ms), ensuring the process is effectively "instantaneous" and stable for continuous operation.
  • The ejection frequency ($\approx 49$ cells/s) matched the theoretical prediction based on flow rate and concentration, confirming no cell accumulation or clogging.
• Deposition Capability:
  • The system successfully deposited single cells onto a culture medium droplet.
  • The time between ejections ($\Delta t \approx 40$ ms) is sufficient for a robotic platform (e.g., using a Miniature Ultrasonic Motor) to reposition the substrate by 100 µm, enabling precise spatial patterning.
• Cell Viability and Proliferation:
  • MTS Assay: Immediately after exposure to the PEG solution, viability dropped to $\approx 55\%$. However, cells showed significant recovery, reaching $\approx 75\%$ viability at 48 hours and stabilizing around $62\%$ at 72 hours. This suggests the initial damage was largely due to transient osmotic shock (hypoosmotic environment) rather than irreversible cytotoxicity.
  • Confocal Microscopy: Staining with Hoechst (nuclei) and Propidium Iodide (membrane integrity) showed that while processed samples had a slightly lower percentage of viable cells ($31.26\%$) compared to controls ($40.10\%$), the difference was not statistically significant ($p > 0.05$). Crucially, a substantial fraction of cells maintained membrane integrity.
  • Conclusion on Damage: The induced damage is largely reversible; surviving cells retain metabolic activity and can proliferate.

5. Significance

This work represents a significant advancement in biofabrication and single-cell analysis:

• Scalability and Flexibility: Unlike microfluidic droplet generators that rely on Poisson statistics (random encapsulation) and biphasic oil/water systems, this open-architecture electrospray system allows for deterministic, on-demand single-cell deposition.
• Applications: The technique is highly applicable for:
  • Single-cell cloning: Isolating a single cell to generate a pure clone.
  • Omics studies: Precise deposition for proteomics, transcriptomics, and genomics.
  • Tissue Engineering: Building complex tissues with controlled cell density and spatial arrangement, mimicking native tissue gradients.
• Technological Bridge: It demonstrates that electrospray can be optimized to handle biological fluids without the need for complex coaxial setups, offering a robust, low-cost, and high-resolution alternative to laser-based or inkjet bioprinting.

In summary, the authors have demonstrated a viable pathway to single-cell resolution bioprinting by operating electrospray at its physical stability limits, balancing the conflicting requirements of fluid stability and cell viability.