3D Printing Hydrogels to Mimic Biological Tissues

3D printing has exploded over the last few years and is now seen as a manufacturing tool that can produce complex and intricate parts for high performance applications. In recent years, 3D bioprinting has taken off where the aim is to directly print living biological tissues that can perform functions as if they were grown naturally. There is also an area of 3D printing which bridges these aspects and that is the 3D printing of synthetic materials that mimic the behaviours and characteristics of natural, biological materials. One promising option of achieving 3D printed pseudo-biological systems is to use hydrogels.

What are Hydrogels?

Hydrogels have attracted a lot of attention in medical settings, and most recently in nanomedicines and as tissue scaffolds. Now, there’s the potential for hydrogels to be used alongside 3D printing methods, further increasing their potential in biological and medical scenarios.

Hydrogels are a polymeric material. They are essentially a network of cross-linked hydrophilic (water loving) polymer chains. These polymer chains form a colloidal gel in water. Because the chains are held together strongly by the cross-links, hydrogels are 3D solid materials that contain water inside their internal structure.
Hydrogels utilise either physical interactions (hydrogen bonds, hydrophobic interactions, and chain entanglements) or direct chemical bonds to link the polymer chains together. These cross-links are the backbone of hydrogels, leading to the polymer chains not being dissolved in water. So, instead of being dispersed in water, the water gets absorbed into the polymer network. Because of this, hydrogels can be up to 90% water.

Self-Healing Properties of Hydrogels

Alongside their ability to mimic soft tissues, one of the biggest benefits of using hydrogels for medicine—especially regenerative medicine—is their self-healing properties. This is one of the key reasons why hydrogels have been trialled for many applications involving repairing damage to biological tissues, as the ability to self-heal not only further mimics how the natural tissue might repair, but it also ensures that they are long-term medical solution (rather than having to be replaced due to irreversible deformations etc).

There are typically two self-healing mechanisms that hydrogels exhibit. These are extrinsic healing and intrinsic healing mechanisms. In extrinsic healing mechanisms, there is usually a reservoir of unreacted monomers (polymer building blocks) embedded in the network that acts as a sealant when the hydrogel takes damage and forms cracks. On the other hand, intrinsic healing mechanisms require specific functional groups to be present in the polymer network. These functional groups establish new bonds between the polymer chains when a crack occurs, sealing and healing the hydrogel in the process.

Aside from the traditional approaches, there has been a way to obtain 3D shaping, printability and self-healing properties that are more suitable for 3D printing approaches (as the process is quite a bit different than the standard wet chemistry route for producing hydrogels). This has been done by designing hydrogels with an interpenetrated network (IPN). These networks have a rigid and robust frame containing non-reversible chemical bonds, but a weaker network based on reversible physical bonds.

This enables IPN to possess both toughness and flexibility properties for 3D printing approaches. Up to now, extrusion-based 3D printing technologies have been the common approach, including with IPNs. This has typically been because the extrusion process is highly compatible with the characteristics of the hydrogel. However, these approaches can also be tailored by changing the viscosity of the printing ink and therefore interactions of the macromolecules within the ink.

This makes hydrogels very tuneable for 3D printing processes, so there is a lot of potential for 3D printing. However, extrusion methods have some issues. While extrusion processes can introduce both robust and self-healing properties to the hydrogel, they cannot do both simultaneously because the two properties require different viscosity inks.

The self-healing mechanisms of many 3D printed hydrogels rely on long chain mobility, but for robustness, you need highly cross-linked polymers. Unfortunately, when the network is heavily cross-linked, it hinders the migration of the macromolecules throughout the hydrogel, and this reduces the amount of healing that the hydrogel can undergo. So, you either get a robust hydrogel or a self-healing one, but new approaches are being trialled that can add both features to 3D printed hydrogel structures.

Printing via Digital Light Processing (DLP)

A recent study has built on existing research in this area, and to overcome the limitations of other extrusion-based methods (especially around the inability to produce self-healing hydrogels and be robust simultaneously), a vat photopolymerization (VP) method was implemented using a digital light processing (DLP) printer. The hydrogel was also created using commercially available materials and used a range of Poly (vinyl alcohol) (PVA) acrylic acid (AAc), and Poly (ethylene glycol) diacrylate (PEGDA) polymer materials.

The process can create hydrogels with very complex architectures at room temperatures without extra stimuli. Some of the architectures possible include overhanging and hollow features and sharp edges, and these types of architectures are not possible with conventional extrusion-based printing methods.

Creating these complex architectures enabled the researchers to 3D print hydrogels that had both robustness and self-healing properties. The cross-linking density vs large chain mobility typically leads to issues for obtaining both properties in the same hydrogel. The DLP process used a water-based photocurable formulation that created a physical-chemical semi-IPN.

While remaining structurally robust, the semi-IPN could also self-repair itself. The self-restoration process occurs rapidly and is completely autonomous— so it doesn’t require an external trigger to heal itself. After healing themselves, the 3D printed samples tested could withstand deformation and recovered up to 72% of their initial strength after 12 hours.

The research in this latest study builds on the 3D printing methods already developed but solves some of the key issues enabling a true self-healing hydrogel to be 3D printed that is functional from a structural perspective. While there is a lot of potentials here for mimicking biological tissues in regenerative medicine approaches, such hydrogels could also extend beyond the medical industry and be useful in soft robotics and energy storage applications.