Direct ink writing (DIW) stands as a pioneering additive manufacturing technique that holds transformative potential in the field of hydrogel fabrication. This innovative approach allows for the precise deposition of hydrogel inks layer by layer, creating complex three-dimensional structures with tailored shapes, sizes, and functionalities. By harnessing the versatility of hydrogels, DIW opens up possibilities for applications spanning from tissue engineering to soft robotics and wearable devices. This comprehensive review investigates DIW as applied to hydrogels and its multifaceted applications. The paper introduces a diverse range of printing techniques while providing a thorough exploration of DIW for hydrogel-based printing. The investigation aims to explain the progress made, challenges faced, and potential trajectories that lie ahead for DIW in hydrogel-based manufacturing. The fundamental principles underlying DIW are carefully examined, specifically focusing on rheological attributes and printing parameters, prompting a comprehensive survey of the wide variety of hydrogel materials. These encompass both natural and synthetic variations, all of which can be effectively harnessed for this purpose. Furthermore, the review explores the latest applications of DIW for hydrogels in biomedical areas, with a primary focus on tissue engineering, wound dressing, and drug delivery systems. The document not only consolidates the existing state of DIW within the context of hydrogel-based manufacturing but also charts potential avenues for further research and innovative breakthroughs.

Potential applications of lignin.

In this study, we reported the preparation of self cross-linked oxidized alginate–gelatin hydrogels for muscle tissue engineering. The effect of oxidation degree (OD) and oxidized alginate/gelatin (OA/GEL) weight ratio were examined and the results showed that in the constant OA/GEL weight ratio, both cross-linking density and Young’s modulus enhanced by increasing OD due to increment of aldehyde groups. Furthermore, the degradation rate was increased with increasing OD probably due to decrement in alginate molecular weight during oxidation reaction facilitated degradation of alginate chains. MTT cytotoxicity assays performed on Wharton's Jelly-derived umbilical cord mesenchymal stem cells cultured on hydrogels with OD of 30% showed that the highest rate of cell proliferation belong to hydrogel with OA/GEL weight ratio of 30/70. Overall, it can be concluded from all obtained results that the prepared hydrogel with OA/GEL weight ratio and OD of 30/70 and 30%, respectively, could be proper candidate for use in muscle tissue engineering.

Natural hydrogel-based wound dressings, synthesized from biopolymers such as chitosan, sodium alginate, and cellulose, are gaining recognition in wound care due to their ability to promote healing through biocompatibility, moisture retention, and biodegradability. These materials foster an ideal healing environment by supporting cell proliferation and tissue regeneration while providing a protective barrier against infection. For chronic or infected wounds, enhancing the therapeutic performance of these hydrogels is essential. This review critically evaluates advanced functionalization strategies, including chemical modifications to optimize hydrogel properties, the incorporation of bioactive agents like growth factors and antimicrobial compounds, and the development of stimuli-responsive hydrogels that adjust to environmental cues such as pH, temperature, and enzymatic activity. Furthermore, fabrication techniques-such as solution casting, freeze-drying, electrospinning, and 3D printing-are discussed for their potential to generate tailored dressings with specific mechanical properties and bioactive capabilities. By highlighting key innovations and challenges, this review provides a comprehensive roadmap for the design, functionalization, and fabrication of natural hydrogel-based wound dressings, identifying critical areas for future research and development.