Encapsulate bacteria in implantable hydrogels for living therapeutics

Abstract

Recent advances in implantable living materials have highlighted their potential for autonomous therapeutic delivery by integrating mechanically robust hydrogels with encapsulated therapeutic bacteria. By enabling long-term biocontainment, environmental sensing, and genetically programmed drug release within physiologically stable scaffolds, these living materials establish a versatile foundation for next-generation personalized therapeutics and closed-loop disease management.

Engineered microbes are receiving growing attention as living therapeutics because of their ability to sense physiological signals and dynamically produce bioactive payloads in vivo.^1–3 In particular, bacteria can colonize diverse biological environments, including tumors, inflamed tissues, and infected sites, making them promising candidates for localized and stimulus-responsive therapy. However, uncontrolled microbial dissemination and off-target toxicity remain major barriers to clinical translation, especially for long-term therapeutic applications. Although genetic containment strategies have been developed to reduce escape risks, evolutionary instability and prolonged bacterial residence continue to raise biosafety concerns. Recent advances in implantable biomaterials have enabled mechanically compliant platforms for physiological sensing, ambulatory monitoring, and adaptive biomedical interfacing.^4–9 Implantable hydrogels offer a promising physical confinement strategy for localized therapeutic delivery. However, existing systems generally suffer from limited mechanical robustness and insufficient long-term bacterial containment under physiological conditions.

A recent Science¹⁰ study described an implantable living material platform that enables long-term bacterial biocontainment and autonomous therapeutic delivery in vivo (Figure 1A). The system integrates engineered bacteria within mechanically reinforced hydrogels featuring simultaneously high stiffness and high toughness, allowing resistance to bacterial proliferation and physiological fracture (Figures 1B and 1F). Encapsulated bacteria remain viable while maintaining stable confinement under prolonged culture and mechanical loading conditions. Through genetically engineered sensing circuits, the living materials further achieve pathogen-responsive therapeutic release and autonomous antimicrobial treatment in implant-associated infection models. Together, this work establishes a mechanically guided framework for clinically deployable living therapeutics.

Figure 1. Implantable living materials with bacteria for autonomous therapeutic delivery.

(A) Schematic illustration of ILMs integrating engineered bacteria with mechanically robust hydrogels.

(B) Matrix stiffness suppresses bacterial proliferation within hydrogels. Scale bars, 50 μm.

(C) Ashby plot comparing the elastic modulus and fracture toughness of representative hydrogel systems.

(D) Sequential mechanical reinforcement of PVA hydrogels through freeze–thawing, dry annealing, and salting-out treatments.

(E) Fabrication strategy of implantable living materials by embedding bacteria-containing gelatin microgels within a mechanically robust PVA hydrogel matrix.

(F) Expansion dynamics of bacterial colonies confined within the PVA matrix over time. Scale bars, 25 μm.

(G) Engineered genetic circuit enabling pathogen sensing, bacterial self-lysis, and therapeutic payload release in response to quorum signals from P. aeruginosa.

(H) Dynamic activation and lysis behavior of engineered bacteria within ILMs following quorum induction.

(I) Experimental design for evaluating in vivo biocontainment and biosafety of implantable living materials.

(J) Representative X-ray image showing ILM implantation adjacent to a prosthetic device in the murine femur.

(K) In vivo bacterial confinement following implantation of ILMs.

(L) Long-term bacterial viability and biocontainment within ILMs after implantation.

(M) Preservation of engineered genetic circuit functionality after in vivo implantation.

(N) Experimental design for evaluating therapeutic ILMs in a prosthetic joint infection model.

(O) Bioluminescence imaging of P. aeruginosa infection following treatment with therapeutic ILMs.

(P) Quantification of infection progression during therapeutic treatment with ILMs.

(Q) Reduction of P. aeruginosa burden following autonomous therapeutic treatment by ILMs.

Adapted from Harimoto et al.¹⁰ Reuse governed by the AAAS Science journal license policy.

More specifically, engineered Escherichia coli ClearColi were first encapsulated and confined within sacrificial gelatin microgels generated through a water-in-oil emulsion process and subsequently embedded into polyvinyl alcohol (PVA) hydrogels to construct implantable living materials (ILMs) (Figure 1E). To simultaneously achieve bacterial confinement and mechanical durability, the PVA hydrogel network was reinforced through sequential freeze-thaw cycling, dry annealing, and Hofmeister-effect-mediated salting-out treatments (Figures 1C and 1D). Systematic optimization of polymer molecular weight, freezing conditions, annealing duration, and salt concentration enabled the formation of densely crosslinked crystalline domains within the hydrogel matrix. The resulting PVA hydrogels achieved elastic moduli approaching ~3 MPa together with fracture energies exceeding 20 MJ m⁻³, positioning the material in a distinct mechanical regime compared with previously reported bacterial encapsulation hydrogels (Figures 1C and 1D). Pure shear testing further demonstrated fracture toughness values exceeding 5 kJ m⁻², whereas cyclic crack propagation measurements revealed fatigue thresholds of ~457 J m⁻². Notably, bacterial proliferation was progressively suppressed with increasing matrix stiffness and became nearly arrested above ~1.5 MPa, establishing a mechanical threshold for bacterial confinement.

The mechanically reinforced ILMs enabled long-term bacterial viability while maintaining robust biocontainment under physiologically relevant loading conditions. Encapsulated bacteria remained confined within the hydrogel matrix for at least 6 months without detectable leakage, while preserving >97% viability after the initial growth phase. Under compression, tensile deformation, crack-defect loading, and 10,000-cycle fatigue tests, the PVA hydrogels maintained structural integrity and prevented large-scale bacterial leakage that readily occurred in conventional agarose controls. To further establish programmable therapeutic functionality, the authors engineered quorum-sensing genetic circuits responsive to Pseudomonas aeruginosa-derived acyl-homoserine lactone (AHL) signals (Figures 1G and 1H). Upon activation, the engineered bacteria underwent self-lysis and released antimicrobial chimeric pyocin (ChPy) payloads, enabling autonomous pathogen-responsive therapy (Figures 1I–1M). The ILMs successfully suppressed P. aeruginosa growth both in vitro and in murine prosthetic joint infection models, demonstrating the feasibility of mechanically guided, closed-loop living therapeutic systems for localized and responsive in vivo treatment (Figures 1N–1Q).

The key contribution of this work is the establishment of a mechanically guided framework for implantable living therapeutics by integrating robust hydrogel mechanics with genetically engineered bacterial systems. Using sequentially reinforced PVA hydrogels and programmable microbial circuits, the authors developed implantable living materials capable of long-term bacterial biocontainment, environmental sensing, and autonomous therapeutic release. This mechanically robust living platform not only advances the fundamental understanding of microbial confinement within physiological environments but also enables practical in vivo applications, including closed-loop antimicrobial therapy and localized infection treatment.

Looking forward, two major directions may further advance implantable living materials toward clinically deployable autonomous therapeutic systems. First, although the current platform demonstrated robust bacterial confinement for 6 months in vitro and short-term functionality in murine implantation models, substantial challenges remain for true long-term in vivo applications. Future studies will likely require month-scale implantation experiments, together with systematic investigations of fibrosis, immune remodeling, and chronic foreign-body responses under physiologically relevant conditions, potentially extending toward large-animal validation. Second, the current system primarily relies on a single engineered bacterial strain performing sensing and therapeutic functions simultaneously. Expanding toward multicellular or synthetic microbial ecosystems with distributed sensing, computation, and therapeutic roles may substantially enhance system complexity, adaptability, and robustness. Such developments could ultimately transform implantable living materials from passive localized drug-delivery systems into autonomous, adaptive therapeutic platforms capable of long-term physiological integration and closed-loop disease intervention.