Making Self-Healing Materials with 4D Printing

Featured Image: “3D Printing and Mathematics models” from US Embassy South Africa, Public Domain

Title: Self-Healing Four-Dimensional Printing with an Ultraviolet Curable Double-Network Shape Memory Polymer System

Authors: B. Zhang, W. Zhang, Z. Zhang, Y-F. Zhang, H. Hingorani, Z. Liu, J. Liu, and Q. Ge

Journal: ACS Applied Materials & Interfaces

Year: 2019

https://dx.doi.org/10.1021/acsami.9b00359

These days, it seems like everyone on the planet has heard of 3D printing. In case you have somehow missed out, 3D printing creates fully three-dimensional materials from a liquid plastic that is hardened into specific shapes. But now, there’s a new process that goes a step further: 4D printing! This adds another dimension to the material, letting it be controlled over time. Objects that can change their shapes and properties over time could be extremely useful in many different fields, from consumer products to medicine. In this new research, scientists have developed a new material that can not only be printed into specific 3D shapes, but also change its form over time and even heal itself when damaged.

Typically, 3D printed materials are made of plastic, and these new 4D printed materials are no exception. All of these begin with a mixture of small molecules that can be linked together (or “polymerized”) to form long chains called polymers. What makes these 4D materials different is the exact mixture of chemicals used as the starting compounds.

Most 3D printing mixtures have three types of chemicals. The monomer forms long chains of itself, the crosslinker connects those chains together, and the photoinitiatior converts ultraviolet light into energy used to drive the reactions (Figure 1A). In addition to the typical mixture, these authors added a “healing agent,” which is a different polymer that can join separate pieces of the material into one piece, effectively “healing” it. You can think of this almost like Velcro, where the little hooks and loops reach out to touch other pieces of Velcro, joining them together. The final material is made up of a complex network of these polymer chains with the healing agent forming tiny crystals within the structure (Figure 1B). When combined with widely-available 3D printing technology, complex structures can be made with relative ease.

Figure 1: A) Chemicals used as different components of the 4D printing mixture and their roles. B) Diagram showing the formation of the solid plastic with a “high temperature” (h.t.) and “room temperature” (r.t.) step. C) The printed materials can be “programmed” to different shapes with heat and switch back to their starting form with additional heat. Reprinted with permission from B. Zhang, et al. ACS Appl. Mater. Interfaces 2019, 11, 10, 10328-10336. Copyright 2019 American Chemical Society.

One of the interesting properties these materials have is “programming” of their shapes. The material first takes the form of whatever shape the 3D printer made. But by briefly heating the material and forming it into a new shape, staying that way when cooled back to room temperature (Figure 1C, left). But heating it again makes it return back to its original printed shape (Figure 1C, right). This lets the material alternate between different configurations, like open/closed or compressed/expanded.

The self-healing property is the other new and exciting feature of this material. With slight heating, the healing agent polymers can unfold out of their small crystal structures inside the material (Figure 2A). As they stretch out, they can reach into other pieces of the material that are close by, such as two halves of a single broken piece. When the material cools back down, the healing polymers shrink again, pulling the two parts together as if they were the same piece of material (Figure 2B). When they tested the strength of these healed materials, the researchers found that they were just as strong as samples that hadn’t been torn apart (if the material contained at least 20% healing polymer) (Figure 2C).

Figure 2: A) Diagram of how the material heals with heating. B) Scanning Electron Microscope (SEM) images of a cut (top) and healed (bottom) surface. C) The strength (σs) of the whole (“control”) and healed samples for different amounts of healing polymer (PCL) added. Reprinted with permission from B. Zhang, et al. ACS Appl. Mater. Interfaces 2019, 11, 10, 10328-10336. Copyright 2019 American Chemical Society.

Just like 3D printing has made it easier to make many different complex structures, this form of 4D printing could also affect a huge number of other applications. A simple form the authors tested was a small, claw-like grabber. First, they printed a small claw that was almost completely closed (Figure 3). They opened the arms of the claw by tearing them slightly and then healing them back together in the “open” configuration. Then, they placed the claw over a 10 g weight and heated it so it returned back to its starting, “closed” form. The healed material was strong enough that the weight could be lifted and moved. These types of small, adaptable grippers could be used in tiny machines or robots without electronic moving parts.

Figure 3: A 4D printed grabber that is torn (II), healed open (III), and returned to the closed form (V) used to lift a weight. Reprinted with permission from B. Zhang, et al. ACS Appl. Mater. Interfaces 2019, 11, 10, 10328-10336. Copyright 2019 American Chemical Society

These types of small, easy-to-make objects could be complete game-changers in fields such as microtechnology and medicine, especially if they can controllably change their structure over time. Although their properties still need to be fine-tuned and explored completely, these new materials could help us break the limits of current 3D printing technology.

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