Gradient Hydrogels—Overview of Techniques Demonstrating the Existence of a Gradient

Abstract

Gradient hydrogels are promising future materials which could be usable in tissue engineering (scaffolds), pharmaceutical (drug delivery systems with controlled release) and many others related disciplines. These hydrogels exhibit a more complex inner (gradient) structure (e.g., concentration gradient) than simple isotropic hydrogel. Gradient-structured hydrogels could be beneficial in, for example, understanding intercellular interactions. The fabrication of gradient hydrogels has been relatively deeply explored, but a comprehensive description of the physico-chemical techniques demonstrating the existence of a gradient structure is still missing. Here, we summarize the state-of-the-art available experimental techniques applicable in proving and/or describing in physico-chemical terms the inner gradient structure of hydrogels. The aim of this paper is to give the reader an overview of the existing database of suitable techniques for characterizing gradient hydrogels.

An Integrated Approach to Control the Penetration Depth of 3D-Printed Hollow Microneedles

“…At last, there are currently no biomechanical cues indicative of having surpassed the epidermis thickness other than the stopper height, which itself is subject to printing tolerances and unmeasured shrinkage. Future efforts will include the use of biocompatible composite films [39][40][41][42] with a density gradient to be used as soft sensors to compliment the active control feature of the stopper. Based on the successful incorporation of the stopper into the solid microneedles, the insertion height of this feature may be modulated to reach precisely deeper targeted locations in the skin or tissue.…”

“…It can be crosslinked using a variety of chemical and physical methods, in addition to being integrated in multinetwork hydrogel systems with excellent biocompatibility, specifically the ease of sterilization and mixing [38]. Gradient hydrogels are more complex than homogeneous gels [39], with changing mechanical properties, such as the molecular weight, Young's Modulus, and degradation rate in stiffness-tunable hydrogels, for example [40][41][42]. Gradient hydrogels can serve as skin phantoms because of their capability to capture the complexities of skin, as the skin is an anisotropic and viscoelastic material with mechanical properties varying depending on the skin thickness, age, anatomical site, etc.…”

“…Force data collected during puncture are a summation of stiffness, friction, and cutting forces, with the stiffness force producing elastic deformation in the skin surface, cutting force relating to tissue cutting resistance produced when the tip of the needle pierces the skin, and friction force relating to dynamic friction caused by needle movement [47,49]. Puncture force data are quantified via needle insertion tests [25,39,48,50], as well as computation. Tr ączy ński et al performed needle insertion tests, varying the needle size, insertion speed, and insertion angle to understand the effect of these variables on insertion force, as well as the consistency of force [48].…”

Designing Superlubricious Hydrogels from Spontaneous Peroxidation Gradients

“…To achieve this difficult balance of stiffness and lubricity in synthetic systems, recent investigations have focused on synthesizing and characterizing hydrogels with spatial gradients, where there is a gradual transition in mesh size or cross-linking density with increasing depth ( z ) (Figure a–c) as opposed to discrete layers with abrupt, stepwise changes in structure (Figure d). − There are many established methods used to synthesize hydrogels with spatial gradients, including dip coating, microfluidics, , fluid mixing, , controlling substrate thickness, UV polymerization with photomasks, ,− and electrochemical gradients, although these are often time-consuming and low throughput due to their experimental complexity. , …”