Bio-inspired 4D printing of earthen soil composites for structural living wall applications

Rising carbon emissions highlight the need for multifunctional building materials that move beyond inert structural roles toward ecological performance. Vertical greening and living wall systems (LWS) offer climate-responsive façades, yet most rely on synthetic substrates with high embodied carbon, limited biodegradability, and poor long-term moisture retention. This study introduces a 4D printing (4DP) approach for continuous living wall systems, using time as the transformative dimension as printed components evolve biologically. Bio-based earthen composites composed of local Qatari soil, acacia gum, and nanoclay were extrusion-printed to fabricate structural panels capable of supporting germination after printing. 9 formulations were prepared and assessed for rheology, extrudability, buildability, mechanical strength, thermal stability, shrinkage, and germination performance. Germination tests revealed a clear structural–biological relationship: low-binder mixes displayed high shrinkage and rapid moisture loss that inhibited sprouting, while higher acacia-gum dosages improved cohesion, internal moisture retention, and microbial compatibility. Nanoclay enhanced thixotropy, minimized shrinkage, and improved filament stability. The optimal formulation, T3 (90 wt% soil, 6 wt% acacia gum, 0.6 wt% nanoclay), provided stable extrusion, buildability up to 75 layers, and compressive strength above 4 MPa at 28 days. Panels printed using T3 demonstrated rapid biological activation; under controlled humidity, seeds germinated within 54 h while the printed geometry remained dimensionally stable. These results demonstrate that extrusion-based, bio-stabilized earthen composites can act as multifunctional materials that integrate structural integrity, moisture regulation, and biological receptivity. The work establishes a pathway structural element that function as true 4D living structures, evolving over time through controlled germination.