© Fraunhofer IGB
Novel additive manufacturing processes are currently being worked on in a variety of research fields. Among the established printing techniques, inkjet printing offers a highly attractive technique for creating two-dimensional or three-dimensional structures that are previously designed on a computer.
Our work focuses on the development of suitable ink formulations to process diverse functional components such as hydrogels, nano- and microparticles, proteins and cells.
Biological materials for 3D printing
To adjust the viscosity and surface tension of functional inks, the properties of the functional components are adjusted. Depending on the requirements, aqueous or solvent-based inks are produced. For applications in tissue engineering, for example, initiator-free cross-linking biocompatible hydrogels are developed and biomolecules are equipped with cross-linkable groups or functionalities through chemical modification to control the solubility properties and viscosity.
For optimal printing results, different substrate materials are pre-treated wet-chemically or by means of plasma technology in order to obtain an optimal print image.
The following examples of inks are in our focus:
For 2D/3D printing, Fraunhofer IGB develops e.g. initiator-free crosslinking biocompatible hydrogels and equips biopolymers with crosslinkable groups or functionalities by chemical modification. This allows the solubility properties and viscosity to be controlled and an adaptation to the different requirements of additive processes such as inkjet printing or pneumatic dispensing to be made possible.
We want to enable biopolymers such as gelatin, hyaluronic acid and chondroitin sulfate to modify surfaces and build three-dimensional tissue models. To control the physicochemical properties of biomolecules and hydrogels, we couple chemical functions such as crosslinkable methacrylic groups, thiol groups and benzophenones to the biopolymers and mask functional groups responsible for gelling of the materials.
In cooperation with the IGVP at the University of Stuttgart, we are also developing printable and crosslinkable cell-compatible polyethylene glycols.
If the inks contain biological materials such as biomolecules, cells, tissue preparations or biocompatible materials, the printed structures can perform a biological function. The biomaterial in its non-crosslinked, printable form is referred to as a bioink. The composition of these inks depends on the subsequent application. Thus, bioinks can be formulated with and without cells.
Bioinks for tissue engineering applications are optimized for the printing process and at the same time for the promotion of tissue-specific functions through targeted variation of the composition. We have already successfully produced "bone inks" and "vascularization inks" based on the available material construction kit. Both bioinks are dispersions of biomolecules and tissue-specific cells that can be stably formed into a 3D structure via dispensing processes.
Protein-containing bioinks should be processable while maintaining the native functionality of the proteins. We achieve this by using water-soluble and protein-compatible components. These inks can be used, for example, to make specific areas on a substrate attractive for adhesion of different cell types.
Individually or in combination, our bioinks can be used to construct vascularized tissue models.
Bone ink contains a mass fraction of 13 percent of crosslinkable biopolymers and a mass fraction of 5 percent hydroxylapatite (HAp) as a tissue-specific mineral additive. The proportion of HAp is adjusted in such a way that the vitality of the mesenchymal stem cells used and the crosslinking reaction of the hydrogels are not impaired [4]. The increase in the viscosity of the ink due to the addition of the HAp is very desirable: By choosing a suitable ratio of the available gelatin derivatives with different gelling abilities, a gelling temperature of, say, 21.5°C can then be set. Bone ink thus has excellent extrudability at room temperature.
Research has shown that after the ink has been crosslinked to hydrogel, the mineral component promoted the remodeling of the matrix by the cells contained therein: The mechanical strength of the gels increased more markedly during the four-week cultivation when HAp was contained in the matrix than in carrier gels without HAp [5]. Raman spectroscopy suggests that the observed effect is mainly due to increasing mineralization of the matrix. In addition, bone-typical marker proteins indicate that the mesenchymal stem cells differentiated into bone cells in the printed matrices.
The supply of nutrients and oxygen via vascular structures is particularly important for extensive in vitro tissues, as diffusion takes too long. The endothelial cells that line the vessels from the inside play an important role in the formation and growth of new vessels.
Vascularization matrix must have different properties than bone matrix: First and foremost, it must be soft and less strongly cross-linked, so that the endothelial cells can migrate and form capillaries. The vascularization ink developed at IGB therefore contains only 5.75 percent by weight of crosslinkable biopolymers. These also have a low degree of methacrylate and thus crosslink less strongly than bone ink. By the addition of gelatin derivatives with masking, IGB succeeded in manufacturing soft vascularization gels (storage modulus of 2.7 kPa ± 0.31 kPa) with a high water absorption capacity (degree of swelling in equilibrium > 2000 percent). By varying the share of unmodified gelatin (which gels already at room temperature), it was possible to produce a bioink that can be stably printed at room temperature. When microvascular endothelial cells are introduced into these gels, the formation of capillary-like structures takes place.
Nanoparticles have the potential to transport a variety of functions: Both the properties of the particle shell, such as being equipped with certain chemical groups and the loading of the particle core with dyes or active components, which are released after printing, for example in contact with water, can contribute to the systematic design of material properties. We are developing inkjet-compatible formulations at the Fraunhofer IGB, for the targeted and structured application of suspensions of functional nanoparticles. For example, we have developed a water-based ink for coating surfaces with charged nanoparticles. These inks could find their application in the production of sensitive rapid tests based on nucleic acid microarrays.
The generative manufacturing of innovative products already plays a major role in research and development on a laboratory scale. For this, Fraunhofer IGB has at its disposal equipment for upscaling and further development in the area of embossing processes as well as of additive processes.
At Fraunhofer IGB, embossing processes can be carried out in a batch process using a hot press, or in continuous roll-to-roll processes.
Inkjet printers and dispensers are available for additive processes. The available process technology is used to respond specifically to the latest requirements.
The development of biomaterials sometimes requires the flexible functionalization of surfaces, for example the coexistence of cell-adhesive and cell-rejecting areas or the combination of hydrophilic and hydrophobic areas in the production of microfluidic test systems.
With the help of digital printing processes such as inkjet printing or Laser Induced Forward Transfer (LIFT), material layers can be applied to surfaces direct, i.e. without the elaborate manufacturing of masks, in any programmable patterns that are required.
At Fraunhofer IGB we develop inkjet-suitable inks for coating surfaces with biological and biofunctional materials such as proteins or active substance-loaded, degradable particles. The high-precision inkjet printer DMP 3000 (Fujifilm Dimatix, USA) is available to produce functional layers with resolutions in the micrometer range.
For example, we print cross-linkable polymers to produce microstructured hydrogels that form hydrophilic areas and can also fix particles on the surface or serve directly as reservoirs for active substances. FDA-approved dyes are available for inks used to mark medical devices or food products. Cell-adhesive areas can be created by coating them with proteins or simple organic linkers.
Novel additive manufacturing processes are currently being worked on in a variety of research fields. Among the established printing techniques, inkjet printing offers a highly attractive technique for creating surface or three-dimensional structures that are previously designed on the computer.
Digital inkjet printing produces small, uniformly sized droplets that can be used as microcomponents. This allows spatially resolved structures of new, but also known materials to be created in an innovative way. Fraunhofer IGB is therefore focusing its research on inkjet printing as a manufacturing tool for the individualization of production processes.
At Fraunhofer IGB, ink formulations for processing a wide range of functional components such as hydrogels, nanoparticles, proteins and conductive materials are being developed.
The combination of inkjet printing and the well-known polyurethane chemistry has great potential for future production of functional materials. At present, we are focusing on the production of polyurethane foams using two-component reactive inkjet printing. Two inks (an isocyanate-functional and, for example, a hydroxy-functional ink), each containing a reactive component, are printed separately in layers.
We develop and optimize materials and ink formulations with adjustable properties for use in pharmaceutical and cosmetic formulations as well as for additive manufacturing processes. We use biopolymers such as gelatin, collagen, hyaluronic acid, chondroitin sulfate, and heparin, which can be functionalized through targeted chemical modifications—for example, with methacrylic, thiol, or benzophenone groups.
Our materials are characterized by individually adjustable viscosity, gelling behavior, and mechanical properties. They can be used as water-insoluble hydrogels with defined strength or as active ingredient reservoirs with controlled release kinetics. In addition, they offer process-adapted fluid properties and are suitable as cell-compatible, tissue-specific matrices.
In the field of additive manufacturing, we support applications such as inkjet printing and pneumatic and extrusion-based dispensing. Our services include the development of customized materials, comprehensive consulting, and technology transfer to your processes.
We use modern analytical methods for quality assurance and further development. Common (immuno)histological methods and cell culture assays round out our offering.
For crosslinking
For reducing intermolecular interactions (masking)
Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB