Mechanical Behavior of 3D Printed Poly(ethylene glycol) Diacrylate Hydrogels in Hydrated Conditions Investigated Using Atomic Force Microscopy

Mechanical properties of soft hydrogels: assessment by scanning ion-conductance microscopy and atomic force microscopy

The growing interest in biomimetic hydrogels is due to their successful applications in tissue engineering, 3D cell culturing and drug delivery. The major characteristics of hydrogels include swelling, porosity, degradation rate, biocompatibility, and mechanical properties. Poor mechanical properties can be regarded as the main limitation for the use of hydrogels in tissue engineering, and advanced techniques for its precise evaluation are of interest. The current research aims to demonstrate the suitability of scanning ion conductance microscopy (SICM) for assessing the stiffness of various hydrogels - Fmoc-FF peptide hydrogel, polyacrylamide and gelatin, - which differ by two orders of magnitude in Young's modulus (E). We provide a direct comparison between SICM measurements and atomic force microscopy (AFM) data, the latter being a widely used method for assessing the mechanical properties of scaffolds. The results of these methods showed good agreement, however, for materials with various stiffness two SICM-based approaches - application of hydrostatic pressure and application of intrinsic force - should be used. For hydrogels with Young's modulus of more than 2.5 kPa the application of SICM using hydrostatic pressure is recommended, whereas for soft materials with E ∼ 200-400 Pa the technique using intrinsic force can also be applied. We have shown that SICM and AFM methods can be used for the evaluation of the mechanical properties of soft hydrogels with nanometer resolution, while SICM is a completely non-invasive method, which requires a minimum influence on the sample structure.

Injectable Hydrogels in Cardiovascular Tissue Engineering

Heart problems are quite prevalent worldwide. Cardiomyocytes and stem cells are two examples of the cells and supporting matrix that are used in the integrated process of cardiac tissue regeneration. The objective is to create innovative materials that can effectively replace or repair damaged cardiac muscle. One of the most effective and appealing 3D/4D scaffolds for creating an appropriate milieu for damaged tissue growth and healing is hydrogel. In order to successfully regenerate heart tissue, bioactive and biocompatible hydrogels are required to preserve cells in the infarcted region and to bid support for the restoration of myocardial wall stress, cell survival and function. Heart tissue engineering uses a variety of hydrogels, such as natural or synthetic polymeric hydrogels. This article provides a quick overview of the various hydrogel types employed in cardiac tissue engineering. Their benefits and drawbacks are discussed. Hydrogel-based techniques for heart regeneration are also addressed, along with their clinical application and future in cardiac tissue engineering.

3D printing of polyvinyl alcohol hydrogels enabled by aqueous two-phase system

The synthesis of PVA hydrogels (PVA-Hy) requires a highly basic environment (e.g., an aqueous solution of sodium hydroxide, NaOH, 14% w/w, 4.2 M), but the rapid crosslinking of PVA due to high pH makes it challenging to perform layer-by-layer three-dimensional (3D) printing of PVA-Hy. This work demonstrated 3D printing of PVA-Hy in moderate alkaline conditions (e.g., NaOH, 1% w/w, 0.3 M) assisted by aqueous two-phase system (ATPS). Salting out of PVA to form ATPS allowed temporal shape retention of a 3D-printed PVA structure while it was physically crosslinked in moderate alkaline conditions. Crucially, the layer-to-layer adhesion of PVA was facilitated by delayed crosslinking of PVA that required additional reaction time and overlapping between the layers. To verify this principle, we studied the feasibility of direct ink write (DIW) 3D printing of PVA inks (5-25% w/w, μ = 0.1-20 Pa s, and MW = 22 000 and 74 800) in aqueous embedding media offering three distinct chemical environments: (1) salts for salting out (e.g., Na2SO4), (2) alkali hydroxides for physical crosslinking (e.g., NaOH), and (3) a mixture of salt and alkali hydroxide. Our study suggested the feasibility of 3D-printed PVA-Hy using the mixture of salt and alkali hydroxide, demonstrating a unique concept of embedded 3D printing enabled by ATPS for temporary stabilization of the printed structures to facilitate 3D fabrication.

Multifunctional hydrogels with spatially controlled light activation with photocaged oligonucleotides

Recreating tissue environments with precise control over mechanical, biochemical, and cellular organization is essential for next-generation tissue models for drug discovery, development studies, and the replication of disease environments. However, controlling these properties at cell-scale lengths remains challenging. Here, we report the development of printing approaches that leverage polyethylene glycol diacrylate (PEGDA) hydrogels containing photocaged oligonucleotides to spatially program material characteristics with non-destructive, non-ultraviolet light. We further integrate this system with a perfusion chamber to allow us to alter the composition of PEGDA hydrogels while retaining common light-activatable photocaged DNAs. We demonstrate that the hydrogels can capture DNA functionalized materials, including cells coated with complementary oligonucleotides with spatial control using biocompatible wavelengths. Overall, these materials open pathways to orthogonal capture of any DNA functionalized materials while not changing the sequences of the DNA.

Additive Manufacturing and Physicomechanical Characteristics of PEGDA Hydrogels: Recent Advances and Perspective for Tissue Engineering

In this brief review, we discuss the recent advancements in using poly(ethylene glycol) diacrylate (PEGDA) hydrogels for tissue engineering applications. PEGDA hydrogels are highly attractive in biomedical and biotechnology fields due to their soft and hydrated properties that can replicate living tissues. These hydrogels can be manipulated using light, heat, and cross-linkers to achieve desirable functionalities. Unlike previous reviews that focused solely on material design and fabrication of bioactive hydrogels and their cell viability and interactions with the extracellular matrix (ECM), we compare the traditional bulk photo-crosslinking method with the latest three-dimensional (3D) printing of PEGDA hydrogels. We present detailed evidence combining the physical, chemical, bulk, and localized mechanical characteristics, including their composition, fabrication methods, experimental conditions, and reported mechanical properties of bulk and 3D printed PEGDA hydrogels. Furthermore, we highlight the current state of biomedical applications of 3D PEGDA hydrogels in tissue engineering and organ-on-chip devices over the last 20 years. Finally, we delve into the current obstacles and future possibilities in the field of engineering 3D layer-by-layer (LbL) PEGDA hydrogels for tissue engineering and organ-on-chip devices.