Spatially and temporally controlled hydrogels for tissue engineering

Background

Conventional hydrogels provide cells with a chemically and physically defined matrix, but they lack the heterogeneity and the changes over time found in native tissues, and earlier biomaterials research focused mainly on new compositions and static behaviour. The review argues that controlling where cues are placed and when they change is needed to recapitulate the dynamic extracellular matrix (ECM) that directs processes such as stem cell differentiation and tissue regeneration.

How the review is organized

Section 2 treats spatial control: hydrogel surface patterning and hydrogels as implant coatings, microfabrication (replica micromolding, photo-patterning including two-photon polymerization, microfluidics, self-assembly and textile processing of hydrogel fibres) and 3D bioprinting (stereolithography and digital micromirror device printing, shear-thinning and embedded extrusion, sacrificial, microfluidic and multi-material printing, and bioink design). Section 3 treats temporal control: burst, on-demand, sequential and simultaneous release of drugs and growth factors, photoinitiated addition, photocleavage and reversible ligand exchange, and biomechanical changes by stiffening, controlled degradation, reversible stiffness switching and hydrogel actuators. Section 4 discusses multiscale materials and high-throughput screening.

Scope of the review

For each technique the review explains the working principle and summarizes representative studies with the cells or animal models used and the reported outcome, together with the trade-offs the authors identify, such as resolution versus speed or material choice. Nineteen figures reproduce panels from the cited studies, and comparisons are made on spatial resolution, compatible materials, cell viability, the range of stiffness or degradation that can be programmed, and whether a change can be triggered on demand, reversed or applied in vivo.

Main points of the review

For spatial control, the review contrasts resolution and versatility across methods. Stereolithography and digital micromirror printing reach resolutions of a few tens of micrometres but need large bioink volumes and photocrosslinkable materials, whereas extrusion accepts more materials at a resolution of typically a few hundred micrometres; locally cooled gelatin methacryloyl (GelMA) could not be printed below 7% (w/v), and cells in printed GelMA fibres stayed viable for over 14 days. Two-photon polymerization patterns cell-laden gels at high resolution but can take hours to days per structure. Embedded printing in self-healing or granular support baths produced freeform shapes without collapse, sacrificial inks such as agarose, Pluronic (liquefied at 4 °C or lower) and gelatin (liquefied above about 30 °C) left hollow channels inside cast hydrogels, and cells in agarose-templated channel constructs were more viable than in bulk GelMA of similar size, and composite braided fibres allowed mechanical properties to be tuned by three orders of magnitude while preserving cell patterns.

Surface-level examples include hernia meshes coated with ECM hydrogel, which had fewer foreign body giant cells than uncoated meshes by day 35 in rats, and alginate-coated brain microelectrodes, where a thicker coating lowered the foreign body reaction after 4 months of implantation in rats. When aligned fibre topography and photo-patterned RGD peptide cues were combined, cells responded more strongly to topography.

For temporal control of chemistry and release, the review describes light-responsive hydrogel needles that delivered three doses of ibuprofen on programmed light exposure, a construct that released vascular endothelial growth factor (VEGF) within the first 3 days and then bone morphogenetic protein (BMP) in rats, and a chitosan hydrogel delivering doxorubicin with granulocyte-macrophage colony-stimulating factor that reduced tumour growth in mice more than single agents. Photocleavable protecting groups allowed RGD and proteins to be patterned in 3D after gelation, guiding neural cell growth, and a caged RGD peptide was activated in vivo by transdermal UV light.

For temporal control of mechanics, cited systems include a thiolated hyaluronic acid gel that stiffened from 1 kPa to 8 kPa over two weeks, sequentially crosslinked gels in which mesenchymal stem cells (MSCs) kept longer on soft matrix favoured adipogenesis while early stiffening favoured osteogenesis, and stiffer gels (about 24 kPa) that raised alpha-smooth muscle actin and type I collagen in hepatic stellate cells compared with softer gels (about 2 kPa). Hydrolytic degradation could be tuned from weeks to months but not triggered on demand, whereas photodegradable PEG gels allowed on-demand softening and two-photon carving of 3D channels. A pH-responsive triblock copolymer gel changed reversibly from about 1 kPa to 40 kPa between pH 7.0 and 8.0, and hydraulic hydrogel actuators reached response times below 1 s and forces above 1 N.

Open questions

The authors note that two-photon patterning is slow, that textile processes have poor resolution and suit mainly fibrous tissues, that exogenous DNA in DNA-crosslinked gels may affect cell signalling, and that screening of electrical stimuli remains largely unexplored. They also state that no integrated platform yet monitors cell behaviour at the time scales of tissue development and regeneration, and call for standardized high-throughput platforms compatible with conventional culture and analysis.

How it connects to other research

The related photodegradable hydrogel study provides one of the temporal control methods surveyed here; this review places that light-triggered degradation alongside stiffening, reversible switching, timed release and spatial patterning methods.