Nature has a remarkable way of creating materials that are both strong and highly functional. Bones, bamboo, and wood are excellent examples. Their internal structures are not uniform; instead, they change gradually from one region to another. These changes, known as gradient structures, allow natural materials to perform different functions in different areas.
Now, researchers led by Huawei Qu have introduced a new 3D printing strategy that could make it much easier to manufacture such complex structures. Called filament diameter-adjustable 3D printing (FDA-3DP), the approach allows conventional extrusion-based 3D printers to create one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) gradient structures by continuously changing the diameter of printed filaments.
The development could open new possibilities in tissue engineering, flexible electronics, advanced materials, soft robotics, and other fields where precisely controlled structures are important.
Why Gradient Structures Matter
Many natural materials owe their unique properties to hierarchical structures. These structures contain features of different sizes and arrangements that work together.
Bone provides a good example. Natural bone contains two major structural regions. The porous inner region, known as cancellous or trabecular bone, is associated with bone marrow and helps support biological functions. The denser outer cortical bone provides much of the body's mechanical support.
Similarly, bamboo and wood contain carefully organized structures that help them combine low weight with strength and durability.
Scientists have increasingly tried to reproduce these natural designs using advanced manufacturing technologies. Computer modeling, optimization methods, and machine learning can help design complicated gradient structures. However, turning these designs into physical objects remains difficult.
Traditional manufacturing and many existing 3D printing technologies often struggle to produce structures in which pore size, filament thickness, or other characteristics gradually change across multiple directions.
The Challenge with Traditional 3D Printing
Direct ink writing, or DIW, is an extrusion-based 3D printing technology that has attracted significant attention for nearly three decades.
In DIW, an ink or paste is pushed through a nozzle and deposited along a programmed path. The process is relatively simple, inexpensive, and compatible with a wide range of materials.
As a result, DIW has been explored for many applications, including porous materials, aerogels, tissue engineering, cell-containing structures, flexible electronics, batteries, soft robotics, concrete structures, and 4D-printed devices.
However, conventional DIW generally uses the same printing speed and printing height throughout each layer. Because the extrusion flow rate and nozzle size are also typically fixed, the resulting printed filament has a relatively constant diameter.
This works well when researchers want a uniform porous structure. But it becomes a major limitation when they want a material whose pore sizes gradually change from one location to another.
Previous approaches have attempted to create gradients by changing filament diameter, spacing, or the angle at which filaments intersect. While these methods can produce some gradient structures, controlling the size, resolution, and three-dimensional shape of the gradient remains challenging.
A Simple Idea with a Major Impact
The FDA-3DP strategy developed by Qu and his team takes a different approach.
Instead of relying on a special nozzle or complicated printing equipment, the researchers control the diameter of the printed filament by changing two important printing parameters: printing velocity (V) and printing height (H).
The basic principle is straightforward. When the printing speed changes while the amount of extruded ink remains controlled, the amount of material deposited along a particular section of the printing path also changes. This makes it possible to produce filaments with different diameters using the same nozzle.
The researchers developed a design-to-fabrication workflow that connects computer-based parametric design with extrusion printing. This allows the filament diameter to be programmed at different locations within a structure.
In other words, instead of producing a structure with identical filaments throughout, the printer can create thicker or thinner filaments exactly where they are needed.
This provides much greater control over the resulting pore sizes and internal architecture.
Maintaining Shape Fidelity
Producing variable-diameter filaments is only part of the challenge. A 3D structure must also remain stable during printing.
When filament diameters change continuously, gaps in height can appear between different printed layers. In some cases, this could cause parts of the structure to collapse or lose their intended shape.
To address this problem, the researchers introduced a thickness compensation mechanism.
They selectively added supporting filaments at locations where additional structural support was required. These supporting layers helped maintain the overall shape and stability of the printed object.
This approach allowed the researchers to achieve complex structures while using conventional extrusion-based 3D printing equipment.
From 1D to 3D Gradients
One of the most important advantages of FDA-3DP is its ability to create gradients in multiple directions.
The researchers demonstrated 1D, 2D, and 3D gradient pore structures. They also produced horizontal, radial, and axial gradients, showing that the technique is not limited to a single type of pattern.
The strategy can therefore create structures in which pore sizes or filament dimensions change gradually across different directions.
This level of control is important because many real-world applications require materials to perform different functions in different locations.
For example, a tissue scaffold may need one region to be relatively dense and another to be more porous. A flexible electronic device may require different structural properties across its surface. Gradient structures could help designers achieve these variations within a single printed object.
Potential Applications
The researchers demonstrated several applications of their approach.
These included letter-embedded structures, metastructures, tissue-mimicking scaffolds, flexible electronic structures, and time-driven or 4D-printed devices.
Tissue engineering is particularly promising. Natural tissues often contain gradual changes in structure and properties rather than having perfectly uniform architectures. The ability to control pore size could therefore help researchers create scaffolds that better reproduce certain characteristics of biological tissues.
The technology could also benefit flexible electronics, where carefully designed structures can influence mechanical and electrical behavior.
In metastructures, changing the internal architecture can produce unusual properties that are not found in conventional materials. FDA-3DP could provide another tool for designing these advanced structures.
A Cost-Effective Route to Complex Materials
A major strength of FDA-3DP is that it does not require highly specialized printing equipment.
The strategy can work with traditional DIW extrusion 3D printers, potentially making gradient fabrication accessible to a much wider research and engineering community.
The researchers also developed a workflow that can convert parametric designs into printable G-code files. This could simplify the transition from a computer-generated design to a physical object.
Such an approach could make the fabrication of complex gradient structures more practical and repeatable.
Challenges and Future Improvements
Although FDA-3DP represents an important advance, the researchers acknowledge that the method can still be improved.
One limitation is the use of a constant printing velocity for the supporting layers. This simplifies the process, but it may sometimes create non-uniform or non-monotonic changes in pore gradients.
Future research could investigate continuous printing methods in which the filament diameter changes throughout the entire printing process without relying on a constant minimum diameter for support.
Another challenge is accurately predicting the shape of extruded filaments. Viscoelastic inks do not always produce perfectly circular filaments. Their cross-sections can become elongated or oblong.
More advanced computational fluid dynamics models could help researchers predict the actual cross-sectional shape of the filament more accurately. This could further improve printing precision.
A Step Toward Nature-Inspired Manufacturing
The FDA-3DP strategy represents an important step toward bringing the complexity of natural materials into manufactured structures.
Nature rarely relies on completely uniform materials. Instead, biological and structural materials often use gradual changes in architecture to achieve a combination of strength, flexibility, porosity, and functionality.
By allowing the diameter of printed filaments to change precisely along a printing path, Huawei Qu and his team have demonstrated a practical way to reproduce some of this complexity using conventional extrusion 3D printing.
The approach could ultimately support the development of functionally graded and multi-material heterogeneous structures for applications across science and engineering.
Most importantly, the technology shows that creating complex 3D gradients does not necessarily require expensive or highly specialized equipment. With better control of printing parameters, ordinary extrusion printers can become powerful tools for producing sophisticated, nature-inspired architectures.
As research continues to improve filament modeling, printing stability, and continuous gradient control, FDA-3DP could become a valuable platform for the next generation of advanced 3D-printed materials.
Reference: Qu H, Gao C, Liu K, Fu H, Liu Z, Kouwer PHJ, Han Z, Ruan C. Gradient matters via filament diameter-adjustable 3D printing. Nat Commun. 2024 Apr 4;15(1):2930. doi: 10.1038/s41467-024-47360-y. PMID: 38575640; PMCID: PMC10994943.
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