MIT researchers have demonstrated a low-cost design of specialized electronic nozzles, called triaxial electrospray emitters, that could be used to manufacture time-release drug-delivery particles or self-healing materials efficiently and at scale. This groundbreaking advancement, detailed in the journal Virtual and Physical Prototyping, bypasses the costly and time-consuming traditional microfabrication methods, paving the way for wider adoption of advanced materials and therapies.
The core innovation lies in triaxial electrospray emitters, devices that leverage electrical fields to precisely atomize and dispense three distinct liquids from microscopic nozzles. This process generates a stable stream of liquid, forming multilayered droplets. These droplets can then be solidified, resulting in microparticles with distinct, concentric layers. The implications for targeted drug delivery and advanced materials are profound, promising a new era of personalized medicine and enhanced material properties.
A New Paradigm in Microparticle Manufacturing
Traditionally, the creation of such intricate electrospray emitter arrays has been confined to highly specialized semiconductor cleanrooms. These environments necessitate complex, multi-step microfabrication processes that are both expensive and time-intensive, significantly limiting the scalability and accessibility of this technology. The MIT team, led by Luis Fernando Velásquez-García, a principal research scientist at MIT’s Microsystems Technology Laboratories (MTL), and Bryan Ivan Quintanar-Abarca from the Technological Institute of Monterrey in Mexico, has successfully circumvented these limitations through the innovative application of 3D printing.
Their novel approach involves 3D printing arrays of triaxial electrospray emitters, each containing 16 nozzles packed within an area of approximately one square centimeter. This compact design incorporates an intricate network of three-dimensional microchannels that ensure a uniform supply of liquid to each nozzle. The entire fabrication process is a single step, taking only a few hours to produce these complex emitter arrays, a stark contrast to the weeks or months typically required by conventional methods.
When subjected to rigorous testing, the 3D-printed arrays demonstrated their ability to generate highly uniform, three-layered droplets at scale. This uniformity is a critical factor for high-throughput manufacturing, particularly for applications demanding precise control over microparticle properties, such as biosensors designed to detect minute chemical substances or artificial cells engineered for tissue regeneration.
"We couldn’t make a device like this in a semiconductor cleanroom," stated Velásquez-García, the senior author of the study. "This is only possible because they are 3D-printed. The particles these devices generate, whether they are used for a self-healing composite or to deliver medicine, can have a big impact in many applications. We want to democratize this technology so the benefits can touch many more people."
The Precision of Electrospray Technology
Electrospray emitters operate on a fundamental principle: applying a high voltage to a liquid as it exits a nozzle. This electrical charge causes the liquid to break apart into a steady stream of extremely fine droplets, often in the nanometer to micrometer range. The triaxial configuration builds upon this by incorporating three concentric nozzles. Each nozzle dispenses a different, immiscible liquid, allowing for the simultaneous creation of layered droplets. These layered droplets can then be processed to form compound microparticles with distinct, well-defined layers.
The potential applications are vast. For instance, a triaxial electrospray emitter could be employed to create a drug-delivery nanoparticle with a sophisticated release mechanism. The outermost layer might be designed to degrade slowly in the acidic environment of the stomach, exposing a second material that meticulously controls the release of a core substance. This core substance, containing the therapeutic agent, could then be directed to a specific site within the intestines, maximizing efficacy and minimizing systemic side effects.
Beyond pharmaceuticals, this technology holds promise for advanced materials. Imagine a self-healing composite material where microcapsules containing a healing agent are embedded within a matrix. Upon damage, these capsules rupture, releasing the agent to repair the material. Triaxial electrospray could enable the precise fabrication of such capsules with multiple components, enhancing their healing capabilities.
The miniaturization of electrospray devices is paramount. Smaller emitters require lower voltages to generate droplets, leading to more energy-efficient operation and reduced risk of electrical arcing. While a single emitter’s output may be modest, arrays of emitters are essential for achieving high production rates without compromising droplet uniformity.
Overcoming Fabrication Hurdles with Additive Manufacturing
The challenge with multi-emitter electrospray devices has historically been their intricate design and the difficulty of manufacturing them with the necessary precision and complexity. Traditional cleanroom fabrication processes, while capable of producing fine features, often impose constraints on the shapes and sizes of device components. Velásquez-García noted the absence of any reported miniaturized triaxial electrospray arrays in the open literature prior to their work, underscoring the novelty and significance of their breakthrough.
"When you build a triaxial array, you need to find a way to create geometries that have many integrated parts and extremely fine structures in the smallest footprint possible. And you need to ensure the devices will work uniformly," Velásquez-García explained.
To achieve this, the research team employed a 3D printing technique known as vat photopolymerization. This method utilizes light to selectively cure and solidify liquid resin layer by layer, enabling the creation of highly complex geometries. The researchers were able to print layers as thin as 25 micrometers, a fraction of the width of a human hair, allowing them to construct the intricate internal architecture required for the triaxial electrospray emitters.
A Compact Design with Sophisticated Internal Engineering
The fabricated array, slightly larger than a U.S. penny, is a testament to sophisticated internal engineering. It features a network of coiled, helical microchannels that efficiently transport liquid to the 16 nozzles. These coiled channels not only help maintain a uniform spray of microdroplets across all nozzles but also contribute to keeping the overall device footprint as compact as possible.
"In a sense, the emitters in the array never learn they have company, or otherwise there would be cross-talking and causing interference between them," Velásquez-García elaborated. "We achieved uniformity because of the work that went into our designs."
A significant engineering challenge in 3D printing such microfluidic devices is the need to fabricate fine channels without support structures, which could potentially clog the delicate pathways. Furthermore, ensuring the complete removal of uncured resin before the array is put into operation is crucial for its functionality. The microchannels are designed to precisely funnel the liquids to the concentric nozzles, which must be perfectly aligned to ensure consistent and stable emission of microdroplets.
The ability to rapidly iterate on designs through 3D printing was a key advantage for the researchers. "We were able to aggressively optimize the design because we could iterate in a much timelier manner. This ability to exquisitely refine designs is a key advantage of 3D printing," Velásquez-García emphasized.
Optimizing for Stability and Tailored Release
During their experiments, the researchers explored multiple architectural configurations and liquid flow rate combinations to identify the optimal parameters for maximizing the stability and consistency of the emitted microdroplets. A surprising discovery emerged: the viscosity of the middle liquid plays the most critical role in achieving droplet stability. This is because the middle liquid’s viscosity helps maintain the integrity and thickness of each distinct layer within the droplet.
Moreover, the team found that by precisely adjusting flow rates and applied voltages, they could finely tune the thickness of each layer within the microdroplets. This level of control is invaluable for applications like drug delivery, where scientists can design particles with tailored layer thicknesses to ensure that medication is released at precisely the intended time and rate.
Broader Impact and Future Directions
The implications of this research extend beyond immediate applications. By making the fabrication of complex electrospray emitter arrays more practical and cost-effective, the MIT team aims to democratize this powerful technology. "By making such intricate devices more practical, we can empower others to pursue entrepreneurial and scientific advances," Velásquez-García remarked.
The potential for this technology to fuel innovation across various sectors is significant. In medicine, it could lead to more effective and less invasive drug delivery systems, potentially revolutionizing treatments for chronic diseases and cancer. In materials science, it could enable the development of advanced composites with enhanced durability, self-repairing capabilities, and novel functionalities. The field of biosensing could also see a leap forward with the ability to create highly sensitive and specific detection devices.
Looking ahead, the researchers are committed to further refining their fabrication process and device designs. Their future goals include achieving even smaller dimensions for greater precision and integrating conductive or dielectric materials into the devices to enable the development of more sophisticated electrospray emitter arrays with enhanced capabilities. This ongoing work, supported in part by the Tecnológico de Monterrey – MIT Nanotechnology Program, promises to continue pushing the boundaries of what is possible in microscale manufacturing and its transformative applications.
