Finding the sweet spot: a library of hydrogels with tunable degradation for tissue model development.

In vitro models are valuable tools for applications including understanding cellular mechanisms and drug screening. Hydrogel biomaterials facilitate in vitro models by mimicking the extracellular matrix and in vivo microenvironment. However, it can be challenging for cells to form tissues in hydrogels that do not degrade. In contrast, if hydrogels degrade too much or too quickly, tissue models may be difficult to assess in a high throughput manner. In this paper, we present a poly(allylamine) (PAA) based synthetic hydrogel system which can be tuned to control the mechanical and chemical cues provided by the hydrogel scaffold. PAA is a polycation with several biomedical applications, including the delivery of small molecules, nucleic acids, and proteins. Based on PAA and poly(ethylene glycol) (PEG), we developed a synthetic non-degradable system with potential applications for long-term cultures. We then created a second set of gels that combined PAA with poly-L-lysine (PLL) to generate a library of semi-degradable gels with unique degradation kinetics. In this work, we present the hydrogel systems' synthesis, characterization, and degradation profiles along with cellular data demonstrating that a subset of gels supports the formation of endothelial cell cord-like structures.

Screen Printing Tissue Models Using Chemically Cross-Linked Hydrogel Systems: A Simple Approach To Efficiently Make Highly Tunable Matrices.

In vitro models provide a good starting point for drug screening and understanding various cellular mechanisms corresponding to different conditions. 3D cultures have drawn significant interest to mimic the in vivo microenvironment better and overcome the limitations of the 2D monolayered cultures. We previously reported a technique based on the screen printing process to pattern live mammalian cells using gelatin as the bioink. Even though gelatin is an inexpensive scaffolding material with various tissue engineering applications, it might not be the ideal hydrogel material to provide various mechanical and chemical cues to the cells. In this paper, we discuss the synthesis and characterization of two synthetic chemically cross-linked hydrogel systems based on poly(ethylene glycol) (PEG) and poly-l-lysine (PLL) to be used as the bioink in the screen printing process. These hydrogels are suitable as the bioinks for the screen printing process and serve as the barebone materials that can be tuned mechanically and augmented chemically to create a suitable in vitro microenvironment for the cells. This paper presents the synthesis, mechanical testing, and characterization of the hydrogel systems and their applications in the screen printing process.

Screen Printing to Create 3D Tissue Models.

3D printing has revolutionized making tissue models, but the instruments are often quite expensive, and the approach can involve heat and/or shear forces that can damage cells. As a complement to more traditional 3D printing approaches, we looked at screen printing. Screen printing is an additive manufacturing technique used to pattern inks through screens supporting patterns onto different surfaces. It has a wide range of applications ranging from traditional printing to printing electric circuit boards. Taking cues from this we have developed a process of screen printing live cells along with a suitable scaffold on to different surfaces to generate in vitro models. The process is not only inexpensive and simple to use, but it also offers a wide range of advantages like the ability to use a range of bioinks limited only by their gelation time, printing on different surfaces, and the ability to autoclave all of the major components. In this paper, we present the screen assembly and the setup we used to print the cells along with the resolution and limits of features printed and the effect of the printing on the cells.