Four-dimensional (4D) materials that incorporate spatial functional gradients enable dynamic, stimulus-responsive shape changes. Traditional gradient-based 4D constructs have largely relied on uniaxial gradients, producing relatively simple, symmetric deformations with uniform curvature. To expand the capacity for spatially coordinated morphogenesis within a single construct, the authors developed a biaxial gradient-engineered hydrogel platform intended to generate programmable, non-uniform shape morphing and heterogeneous curvature distributions.
The rationale is to recreate more complex biomimetic architectures by establishing orthogonal variations in crosslink density so that swelling stresses vary in more than one direction. This allows local differences in mechanical response and volumetric change to drive multi-directional deformation that mimics biological morphogenesis more closely than uniaxial-gradient systems.
A one-step photocrosslinking strategy is used to create orthogonal crosslinking gradients within a single hydrogel construct. The method integrates two photopatterning phenomena: vertical light attenuation through the hydrogel thickness, and horizontal grayscale patterning via a photomask. Vertical attenuation produces a depth-dependent crosslinking gradient, while the grayscale photomask imposes lateral variation in exposure and therefore lateral crosslink density.
By combining these two effects in a single photocuring step, the authors generate crosslinking profiles that vary in orthogonal directions without requiring multiple fabrication steps. The resulting organization of crosslink density underpins spatially heterogeneous swelling behavior and mechanical properties.
Orthogonal crosslinking gradients established by the combined vertical and horizontal patterning produce spatially heterogeneous internal stresses on swelling. Regions with lower crosslink density swell more and generate different local bending moments than more highly crosslinked regions. This spatial heterogeneity in swelling stress is the principal driver of controlled, multi-directional deformation and non-uniform curvature across the construct.
The produced hydrogels display tunable mechanical properties tied to the local crosslink density. By varying exposure patterns and grayscale levels, the degree and distribution of crosslinking—and therefore local stiffness and swelling capacity—can be regulated. The work emphasizes that control over both swelling behavior and mechanical gradients is critical to achieving predictable, programmable morphologies.
The biaxial gradient approach enables tuning of overall swelling and local mechanical responses so that curvature distribution can be precisely regulated. Because gradients act along two axes, the same construct can exhibit non-uniform curvature and complex folding behaviors rather than the uniform curvature typical of uniaxial-gradient systems. The platform therefore supports a wider range of shape transformations and spatially varied morphologies within a single continuous piece of hydrogel.
Control parameters reported include manipulation of light exposure patterns through grayscale masks and reliance on inherent vertical attenuation to establish depth-dependent crosslinking. These parameters together determine local swelling stresses and mechanical gradients, which in turn define shape evolution when the hydrogels are stimulated (for example, by immersion in solvent or another swelling trigger).
Using this biaxial gradient programming, the authors generated a variety of biomimetic architectures to illustrate the method's versatility. Examples reported include constructs resembling a swan-neck, a fiddlehead fern, a sea star, and Euonymus europaeus-like structures. These examples demonstrate that orthogonal gradients can be used to sculpt diverse, non-symmetric forms with spatially varying curvature within a single hydrogel scaffold.
The reported morphologies serve as proofs of principle that the platform can recreate multi-directional, coordinated morphogenetic patterns rather than simple, symmetric bending.
A key aspect of the platform is its compatibility with cell-laden fabrication. The authors encapsulated human mesenchymal stem cells within the gradient-programmed hydrogels and report that the constructs maintained high cell viability following fabrication and morphing.
Furthermore, the cell-laden hydrogels supported chondrogenic differentiation of the encapsulated stem cells while preserving the programmed morphologies. This indicates that the biaxial gradient approach can integrate programmed shape transformation with tissue formation processes, enabling morphogenetic tissue engineering where structural evolution and cell differentiation proceed in tandem.
The biaxial gradient-programmed 4D hydrogel strategy offers a robust route to integrate programmable morphogenesis with tissue formation, expanding the design space for biomimetic scaffolds. By enabling multi-directional, non-uniform deformations within a single construct and supporting viable, differentiating cell populations, the system has potential applications in biofabrication and morphogenetic tissue engineering where coordinated shape change and cell-driven tissue development are desired.
Limitations and caveats noted in the source include that this article is a preprint and has not been certified by peer review. The authors declared no competing interests. Specific experimental details, quantitative performance metrics, and long-term functional outcomes were reported in the full preprint but are not restated here; readers should consult the original manuscript and supplementary material for methods, data, and metrics.
Overall, the work establishes a one-step photocrosslinking method to produce orthogonal crosslinking gradients that drive programmable, non-uniform morphing in 4D hydrogels, and demonstrates the approach's compatibility with stem cell encapsulation and chondrogenic differentiation.