4D-DLP Foldable Scaffolds

• 4D-DLP-printed foldable scaffolds are stimuli-responsive constructs created via high-resolution, layerwise photopolymerization to enable programmed shape transformation.
• They leverage precise geometric patterning and differential swelling of hydrogels to transition from flat films to complex 3D architectures for tissue applications.
• The integration of additive manufacturing with tunable material systems allows scalable, modular assembly of constructs for cardiac, tracheal, and neural tissue engineering.

A four-dimensional (4D) digital light processing (DLP)-printed foldable scaffold is a class of architected, stimuli-responsive soft construct fabricated by layerwise photopolymerization, designed to undergo programmed, temporally controlled shape transformation after printing, typically to enable advanced functionalities in tissue engineering contexts. These scaffolds integrate precise geometric patterning with spatiotemporally encoded actuation, leveraging shape-morphing material systems—predominantly hydrogels or elastomers—that translate environmental triggers (most commonly aqueous swelling or thermal response) into large-scale, often topologically nontrivial, shape changes. The paradigm is distinguished by both its additive manufacturing (DLP-based, pixel-resolved photo-crosslinking) and its capacity for post-fabrication shape evolution (the "fourth dimension"), allowing the formation of structures such as cylinders, tubes, shells, or deployable lattices from initially planar or compact geometries, with applications in cardiac, tracheal, neural, and bone tissue engineering (Hosseinabadi, 27 Aug 2025, Bonetti et al., 13 Jan 2025).

1. Fundamental Principles and Motivations

The driving motivation for 4D-DLP-printed foldable scaffolds is to circumvent intrinsic limitations of traditional, static, three-dimensionally printed or cast biomaterials—namely, their inability to adapt, conform, or reconfigure after fabrication, particularly in demanding tissue-mimetic or minimally invasive deployment scenarios (Bonetti et al., 13 Jan 2025, Zadpoor, 2021). In engineered human myocardium (EHM) paradigms, such as those addressing myocardial infarction, conventional thin-film, randomly organized constructs fail to reproduce both the hierarchical anisotropy and the physiologically relevant tissue thickness of native myocardium, limiting contractility and integration potential (Hosseinabadi, 27 Aug 2025).

By starting from a flat, DLP-printed, geometrically resolved hydrogel film and inducing a programmed, time-dependent transformation (e.g., flat-to-tube or planar-to-curved morphogenesis), 4D DLP scaffolds enable:

• High-resolution cell-instructive topography pre-folding (easier planar patterning, grooving, porosity)
• Controlled spatial distribution of encapsulated or seeded cells, with post-transformation inner-wall localization
• Formation of anisotropic, volumetrically thick, modular "bricks" amenable to assembly into large, functional tissue constructs (Hosseinabadi, 27 Aug 2025)

These constructs are typically envisioned as modular, scalable, and potentially biobankable living building blocks, directly addressing the bottlenecks of current EHM production (thinness, random orientation, poor vascularization potential, manual instability) (Hosseinabadi, 27 Aug 2025).

2. Fabrication Methods: DLP-Based 4D Printing Platforms

DLP-based 4D fabrication employs projection photopolymerization: a digital mask, typically encoded via a digital micromirror device (DMD), selectively cures a photocurable bioresin layer-by-layer with high (sub-100-µm) feature resolution (Hosseinabadi, 27 Aug 2025, Bonetti et al., 13 Jan 2025). For 4D foldable scaffolds, the design involves:

• Initial geometric encoding: A planar film patterned with grooves (parallel/perpendicular to planned fold axis), meshes (e.g., 3×4 micropore arrays), or anisotropic architectures for directed folding and cell alignment (Hosseinabadi, 27 Aug 2025).
• Photocurable hydrogel/bioresin: Material systems include PUPEGDA (polyurethane-PEG-diacrylate), Sil-MA (methacrylated silk fibroin), PEGDA, GelMA, HA-MA, AA-MA, and composite hydrogels, formulated for cytocompatibility, controlled swelling, and mechanical robustness (Hosseinabadi, 27 Aug 2025, Bonetti et al., 13 Jan 2025).
• Sterile/near-GMP process: Custom automated setups with robotic arms, sterile hood integration, and fluidic-chip DLP modules provide process reproducibility and scale-up potential.

Key reported metrics include theoretical lateral resolutions of 7 µm (limited in practice to ~40 µm), light intensities of ~20 mW/cm², and modular cylinder sizes in the hundreds-of-microns range (Hosseinabadi, 27 Aug 2025). Photoinitiators (LAP at 0.1% w/v; Ru:SPS at 3:30 mM) enable cytocompatible crosslinking chemistry.

4D Activation Workflow

Upon immersion in aqueous culture media (the most common stimulus in tissue-engineering implementations (Hosseinabadi, 27 Aug 2025, Bonetti et al., 13 Jan 2025)), the printed planar film undergoes self-actuated folding—via differential swelling, crosslink density gradients, or intrinsic material anisotropy—forming a tubular or cylindrical scaffold that acts as the cellular template. The actuation is a one-way, time-dependent transformation; in most hydrogel-based systems, the folding is moisture-triggered and not thermally reversible (Hosseinabadi, 27 Aug 2025).

3. Material Systems and Folding Mechanisms

4D-DLP foldable scaffolds are typically built from shape-morphing hydrogels (SMHs) or, less commonly, shape-memory polymers (SMPs) (Bonetti et al., 13 Jan 2025). Material selection is driven by the need for:

• High-fidelity photocurability (to enable DLP patterning)
• Robust, cytocompatible, and, ideally, biodegradable crosslinked networks
• Tunable swelling behavior or thermal responsiveness for controlled folding

Actuation Mechanisms

• Swelling mismatch: Differential water uptake in bilayers, crosslinking gradients, or patterned domains induces controlled curvature, rolling, or tube formation (Bonetti et al., 13 Jan 2025). Typical constructs exploit programmed anisotropies (bilayers, grooves, patterned porosity).
• Thermal actuation: For SMPs, transition temperatures can be tuned to physiological (body) temperatures, though most systems in tissue contexts exploit swelling due to the cytocompatibility of aqueous triggers (Bonetti et al., 13 Jan 2025).
• Magnetic and photothermal programming: Advanced approaches allow director alignment (in LCEs) or region-selective activation, though typical actuation temperatures (>100°C) in LCE systems preclude direct applicability in standard tissue engineering (Wang et al., 2024).

Quantitative shape-change is a function of mismatch strain, mechanical gradient, geometry, and thickness. While some systems offer reversible actuations (e.g., certain LCEs or reversible Ca²⁺/EDTA crosslinking), most foldable hydrogels for cell-instructive use are designed for one-way, stable transformations (Bonetti et al., 13 Jan 2025). No explicit curvature or time-dependent folding equations are reported for most hydrogel-based DLP constructs in tissue applications (Hosseinabadi, 27 Aug 2025).

4. Biological Integration and Modular Assembly

The integration of cells into 4D DLP-printed scaffolds follows several strategies:

• Pre-folding cell seeding (4D-B/4D-C): Cells (e.g., iPSC-derived cardiomyocytes and stromal fibroblasts in 70:30 ratio) are seeded or embedded on the planar scaffold, ensuring homogeneous distribution and optimal surface interaction prior to folding (Hosseinabadi, 27 Aug 2025).
• Matrix environment: Collagen I matrices are used to suspend cells, often with ROCK inhibitor supplementation during seeding to enhance viability.
• Guided tissue architecture: Scaffold grooves direct sarcomere alignment (ACTN2-citrine confocal imaging confirms anisotropic organization), achieving more physiologically relevant myocardium structure and, conceptually, contractile performance (Hosseinabadi, 27 Aug 2025).

Post-folding, engineered "cardiac bricks" display spontaneous, synchronous contraction, with homogeneous cell density and improved oxygen/nutrient access compared to cast or direct-seeded 3D tubes. Modular assembly of these cylindrical units—potentially aligned by external fields or ultrasound—enables scalable patch formation, with the prospect of improved contractility, force generation, and conduction velocity (Hosseinabadi, 27 Aug 2025, Bonetti et al., 13 Jan 2025).

5. Design Paradigms, Optimization, and Theoretical Underpinnings

Shape transformation in 4D-DLP foldable scaffolds can be engineered by manipulating:

• Bilayer/multilayer architecture: Differential swelling or modulus across the film thickness enables programmable bending (Timoshenko-type and thick-beam finite deformation models are theoretical foundations for maximizing post-folding hinge stiffness and curvature (Manen et al., 2021)).
• Geometry and feature scaling: Thickness, groove dimensions, porosity, and panel–hinge proportion are critical for balancing foldability and structural function (energy scaling: bending ∼ $E t^3$, stretching ∼ $E t$; curvature inversely related to thickness).
• Actuation kinetics: While explicit time–curvature models (universal bi-exponential law for multi-material actuation) are available in the general 4D literature (Momeni et al., 2018), most current cell-compatible hydrogel platforms rely on design and empirical optimization.

Optimal post-folding mechanical performance requires nontrivial combinations of active–passive layer thickness and modulus, often with the active layer an order of magnitude softer than the passive layer (Manen et al., 2021). However, explicit values for tissue-engineering photopolymer systems are typically absent; design is guided by the interplay of shape-morphing amplitude, structural integrity, and biological compatibility (Bonetti et al., 13 Jan 2025, Zadpoor, 2021).

6. Translational and Manufacturing Implications

4D-DLP-printed foldable scaffolds are positioned as enabling platforms for:

• Scalable, reproducible cardiac tissue manufacturing: Robotic DLP fabrication, modular unit assembly (bricks), and high-throughput potential directly address the throughput, standardization, and handling bottlenecks of current EHM production (Hosseinabadi, 27 Aug 2025).
• Cryopreservation and biobanking: Modular tissue "bricks" facilitate storage, deployment, and on-demand assembly, in contrast to bespoke, cast slab fabrication (Hosseinabadi, 27 Aug 2025).
• Minimally invasive delivery and conformal tissue engineering: The flat-to-tube or planar-to-curved transformation, enabled by DLP's spatial selectivity, supports deployment through small access routes or conformation to complex anatomical surfaces (Bonetti et al., 13 Jan 2025).
• Multifunctional and bio-integrated constructs: The planar state facilitates integration of electronics, bioactive coatings, or surface topographies prior to folding—a principle borrowed from the mechanical metamaterials literature (Zadpoor, 2021).
• Translational limitations and future directions: Current challenges include restricted mass transfer in thick constructs, low reproducibility, difficulty in large-number handling, and the need for controlled scaffold degradability. Miniaturization via two-photon polymerization and advances in bioresin chemistry are explicitly noted as future priorities (Hosseinabadi, 27 Aug 2025, Bonetti et al., 13 Jan 2025).

7. Representative Evidence: Cardiac, Airway, and Vascular Tissue Engineering

Direct evidence for the application of 4D-DLP-printed foldable scaffolds is provided by:

• Cardiac bricks (PUPEGDA hydrogel, DLP, moisture-triggered folding): Anisotropic, contractile, modular cardiac units assembled from initially planar, grooved, biocompatible films (Hosseinabadi, 27 Aug 2025).
• Tracheal substitutes (Sil-MA DLP-bioprinting): Flat bilayer hydrogel sheets (Sil-MA) fold to form tubular airway scaffolds upon exposure to culture medium, supporting chondrocyte integration and in vivo repair (Bonetti et al., 13 Jan 2025).
• Cardiac patches (GelMA/PEGDA, SLA): Curvature-matching cardiac patches with enhanced contractility and engraftment.
• Hydrogel tubes and programmable folding bilayers (non-DLP paradigms): Extrapolations from extrusion-based and photo-degradable hydrogel systems inform the broader design rules for DLP-printed foldable scaffolds (Bonetti et al., 13 Jan 2025).

While LCE-based DLP systems can achieve reversible, large-strain actuation and spatial programming (via mosaic director fields and magnetic alignment (Wang et al., 2024)), current chemical systems do not align with cytocompatibility and physiological actuation constraints for tissue engineering.

8. Quantitative and Conceptual Limits

Explicit quantitative metrics reported for tissue-engineered 4D-DLP foldable scaffolds include:

• DLP parameters: 7 µm theoretical, >40 µm practical resolution, 20 mW/cm² light intensity
• Material composition: PEG (Mn = 3000 Da), 80 °C synthesis, 0.1% LAP, 3:30 mM Ru:SPS, 1-week dialysis
• Cellular context: 70:30 CM:fibroblast, 1:2000 ROCK inhibitor, 3 days cytotoxicity window, 3-week maturation
• Assembly scale: "hundreds-of-micrometers" brick diameter, thousands of bricks for large constructs
• Comparative contractility: aligned tissues yield 4–10× random-tissue force (Hosseinabadi, 27 Aug 2025)

Absent are detailed mechanical or folding-time metrics, explicit actuation curvatures, swelling kinetics, or force amplitudes.

9. Open Challenges and Critical Perspectives

Critical unresolved issues for 4D-DLP-printed foldable scaffolds include:

• Development of stimuli-responsive, biocompatible, and biodegradable DLP-ready hydrogel resins with robust actuation and printability (Bonetti et al., 13 Jan 2025)
• Balancing shape-morphing efficiency with mechanical performance and biological integration (Manen et al., 2021)
• Achieving reliable, reproducible, and scalable folding in practice; current protocols report low reproducibility and lack detailed process optimization or constitutive folding models (Hosseinabadi, 27 Aug 2025)
• Mass transport limitations in thick constructs, necessitating advanced architectural and porosity designs
• Decoupling and controlling in-plane mechanical programming from out-of-plane folding paths, as well as managing edge effects, self-contact, and locking mechanisms (Zadpoor, 2021)
• Transitioning from proof-of-concept "temporary" scaffolds to scaffold-free, clinically translatable engineered muscle, requiring controlled degradability and removal strategies (Hosseinabadi, 27 Aug 2025)

Table: Key Features of 4D-DLP-Printed Foldable Scaffold Paradigms

Aspect	Typical Choices / Metrics	Importance for TE Context
Material	PUPEGDA, Sil-MA, GelMA, PEGDA, SMPs, SMHs	Biocompatibility, foldability, longevity
Geometry	Planar sheet, grooved/porous, micro-meshed	Resolution, cell orientation
Folding Stimulus	Aqueous swelling, moisture, temperature	Cytocompatibility, processability
Actuation Mechanism	Swelling mismatch, gradient, anisotropy	Precision of shape transformation
Fabrication Platform	High-res DLP, fluidic chip, robotic integration	Sterility, scalability
Functional Output	Aligned EHM bricks, tubes, tracheal scaffolds	Thick tissue engineering, modularity
Reported Metrics	7–40 µm resolution, 0.1% LAP/3:30 mM Ru:SPS	Scaffold fine structure, cytotoxicity
Limitations	Folding reproducibility, handling, biobankability	Translation to clinical platform

In summary, 4D-DLP-printed foldable scaffolds represent a highly promising, yet evolving, strategy for the realization of complex, morphogenetically controlled, modular tissue templates. Their success in translational contexts will require further foundational advances in biomaterial photochemistry, actuation mechanism integration, mechanical and architectural optimization, and process reproducibility (Hosseinabadi, 27 Aug 2025, Bonetti et al., 13 Jan 2025).