Extended perfused culture of cm-scale endocrine pancreatic tissues created through sacrificial embedded printing into alginate

This paper presents a novel method using sacrificial embedded 3D printing within tunable self-healing alginate to fabricate centimeter-scale, perfusable endocrine pancreatic tissues that support the long-term viability and function of high-density stem cell-derived islet clusters.

The Gist

Imagine you are trying to build a tiny, living city inside a block of Jell-O. Your goal is to house millions of tiny workers (cells) that produce a vital product (insulin) to treat diabetes. But there's a huge problem: Jell-O is solid. If you just dump the workers in, the ones in the middle will suffocate because oxygen and food can't reach them, and waste can't get out. They would die, leaving you with a dead center and a useless block.

This paper describes a clever new way to build these "living cities" so they stay alive, even when they are thick (about the size of a small coin or a centimeter).

Here is the story of how they did it, broken down into simple steps:

1. The Problem: The "Too-Solid" vs. "Too-Runny" Dilemma

The scientists wanted to use alginate, a safe, jelly-like substance made from seaweed, to hold the cells.

• The Issue: If the alginate is liquid, it's like water; you can't build a shape in it, and the "roads" you try to make will collapse. If the alginate is fully set (like a firm gel), it's too brittle. If you try to push a tool through it to make a road, it cracks and breaks.
• The Analogy: Think of trying to build a tunnel through a block of ice (too hard, it cracks) or through a swimming pool (too soft, it collapses). You need something in between.

2. The Solution: The "Magic Self-Healing Jelly"

The team figured out how to make the alginate partially gelled.

• How it works: They added just enough "glue" (calcium) to the alginate to make it thick and squishy, but not hard.
• The Superpower: This mixture acts like non-Newtonian fluid (think of "Oobleck" or a stress ball).
  • When you push it slowly, it acts like a solid and holds its shape.
  • When you push it fast (like a printing needle moving through), it acts like a liquid and lets the needle pass.
  • The best part: As soon as the needle moves away, the jelly instantly "heals" itself and snaps back to being a solid, holding the shape perfectly.

3. Building the "Roads" (Vascular Networks)

Now that they had the magic jelly, they needed to build roads for blood (or nutrient fluid) to flow through.

• The Trick: They used a technique called Sacrificial Embedded Printing.
  • Imagine you are baking a cake. You pipe a line of Pluronic (a special gel that melts when it gets cold) into the warm cake batter.
  • Once the cake is baked, you put it in the fridge. The Pluronic melts and turns into a liquid, which you can wash out, leaving behind a perfect, empty tunnel inside the cake.
• In the Lab: They 3D printed these "Pluronic roads" into the self-healing alginate jelly. Then, they fully hardened the jelly and washed out the Pluronic.
• The Result: They created complex, branching networks of hollow tubes (like a tree's root system) running through the thick block of tissue.

4. Filling the City with Workers

They filled these thick blocks with two types of "workers":

1. Beta Cells: The cells that make insulin.
2. Stem Cell-Derived Islets: Cells grown from stem cells that turn into insulin factories.

They packed them in very tightly (high density), which is necessary for a real medical treatment.

5. The Test: Do They Stay Alive?

In old methods, if you made a block of cells thicker than a few millimeters, the middle would die.

• The Breakthrough: Because they built these internal "roads," they could pump fresh, oxygen-rich fluid right through the center of the block.
• The Result: The cells stayed alive for weeks (up to 25 days in the study), even in blocks as thick as a centimeter.
• The Function: When they tested the cells with sugar (glucose), the cells responded quickly, releasing insulin just like a real pancreas should.

Why This Matters

This is a big deal for treating Type 1 Diabetes.

• Currently, doctors can transplant insulin-making cells, but they often die because they can't get enough oxygen once they are inside the body.
• This new method creates a "pre-plumbed" organ. It's like building a house with all the plumbing and electricity already installed before you move in.
• Because the material (alginate) is already used safely in medicine, and the printing method is scalable, this could lead to a future where we can grow thick, living, functional tissues in a lab and transplant them into patients to cure diabetes without needing to take insulin shots every day.

In short: They turned a squishy, self-healing jelly into a 3D printer support that allowed them to build thick, living tissues with built-in "highways" for blood, keeping the cells alive and working for a long time.

Technical Summary

1. Problem Statement

The development of bioartificial tissues for regenerative medicine, particularly for treating Type 1 Diabetes (T1D), faces a critical bottleneck: mass transport limitations in thick tissues.

• Diffusion Limits: Avacular hydrogel constructs are restricted to a thickness of approximately 200 µm. Beyond this, cells in the core suffer from hypoxia and necrosis due to insufficient oxygen and nutrient diffusion.
• Current Limitations: While alginate is the gold standard for pancreatic islet encapsulation due to its biocompatibility and immunoprotective properties, it is traditionally unsuitable for embedded 3D printing. As a liquid, it cannot support printed filaments; as a fully gelled solid, it is too brittle and lacks the self-healing (thixotropic) properties required for the nozzle to pass through and for the matrix to reform.
• Clinical Need: There is a need for centimeter-scale (cm-scale), vascularized, high-density cell constructs that can support therapeutic doses of beta cells (approx. 40 million cells/mL) for extended periods without central necrosis.

2. Methodology

The authors developed a novel sacrificial embedded 3D printing platform using a tunable, partially gelled alginate matrix.

• Material Formulation (Self-Healing Alginate):
  • They created thixotropic alginate formulations by partially crosslinking alginate with limiting concentrations of calcium ions (10.0–20.0 mM) using calcium carbonate ($CaCO_3$) and glucono-δ-lactone (GDL).
  • Rheological Tuning: The formulations were optimized to exhibit a yield stress ($\tau_0$) that allows the material to behave as a solid at rest (supporting the printed structure) but as a liquid under shear stress (allowing the nozzle to pass). The optimal formulation used 15.0–17.5 mM calcium, which demonstrated rapid recovery of storage modulus after shear.
• Sacrificial Printing Process:
  • Ink: A thermosensitive sacrificial ink, Pluronic F127 (35% w/v), was 3D printed directly into the partially gelled alginate support to create vascular templates.
  • Gelation & Removal: After printing, the alginate was fully gelled using additional crosslinkers. The Pluronic F127 was then removed by cooling (liquefaction), leaving behind a hollow, perfusable, hierarchical vascular network.
• Cell Models:
  • Cell Lines: MIN6 (mouse insulinoma) and $\beta$TC-tet (conditionally immortalized beta cells) were used for initial viability and function testing.
  • Stem Cell-Derived Islets (SC-Islets): Human pluripotent stem cells (hESCs) were differentiated into pancreatic endocrine clusters (Stage 6/7) and immobilized in the constructs for long-term culture.
• Culture Conditions: Constructs were cultured under continuous perfusion (flow rates of 3 mL/min/channel) for up to 25 days.

3. Key Contributions

1. Novel Alginate Rheology: The first demonstration of partially gelled alginate as a viable support matrix for embedded 3D printing, overcoming the historical limitation of alginate's lack of thixotropy.
2. cm-Scale Vascularization: Successful fabrication of centimeter-thick (cm-scale) constructs with complex, multi-plane, branching vascular networks (up to 10 channels) with high geometric fidelity.
3. High-Density Cell Viability: Achievement of long-term viability (up to 25 days) for therapeutic cell densities ($40 \times 10^6$ cells/mL) within these thick constructs, a feat previously unattainable with diffusion-only alginate devices.
4. Functional SC-Islet Maturation: Demonstration that immature stem cell-derived islets can mature into functional, monohormonal insulin-producing cells within a perfused alginate environment without losing endocrine identity.

4. Key Results

• Rheological Characterization:
  • The 15.0 mM calcium formulation exhibited ideal shear-thinning behavior ($n < 1$) and immediate solid-like recovery after shear, essential for maintaining channel shape during printing.
  • Fully gelled constructs showed mechanical properties (compressive and shear moduli) comparable to standard alginate, ensuring structural integrity under perfusion pressure.
• Print Fidelity:
  • $\mu$CT scanning confirmed that the printed vascular networks matched computer-assisted models with high fidelity.
  • Channel diameters could be precisely tuned (0.5 mm to 2 mm) by adjusting nozzle speed and extrusion rate.
  • Deviations from the intended geometry were minimal (mean deviation $\approx -0.014$ mm), with no cracks larger than 45 µm.
• Cell Viability and Oxygenation:
  • Short-term: MIN6 cells remained viable after the printing and gelation process.
  • Long-term: In 2-day perfusion cultures, cells remained viable up to ~600 µm from the channel wall. In 7-day cultures with complex 10-branch networks, viability extended to ~5 mm depth within the construct, significantly exceeding diffusion limits.
  • Viability correlated well with computational oxygen transport models.
• Functional Performance:
  • Glucose-Stimulated Insulin Secretion (GSIS): Constructs exhibited rapid, reversible insulin release upon glucose challenges.
  • Response Kinetics: A 10-minute delay in response was observed, largely attributed to the fluid residence time of the perfusion system (5 min) rather than cell dysfunction.
  • SC-Islet Maturation: After 25 days of perfusion, SC-islets differentiated into monohormonal $\beta$-like (insulin+) and $\alpha$-like (glucagon+) cells. Flow cytometry showed no significant difference in viability or marker expression compared to suspension controls.
  • Recovery: SC-islets recovered from the constructs retained glucose responsiveness in static assays, confirming the platform does not compromise cell function.

5. Significance

• Clinical Translation: This approach utilizes alginate, a clinically established and GMP-compatible material, making the transition from lab-scale research to clinical application more feasible than methods requiring synthetic or complex composite hydrogels.
• Scalability: The ability to create cm-scale tissues with internal vascular networks addresses the "size barrier" in tissue engineering, allowing for the implantation of therapeutic cell doses required for T1D treatment in a compact volume.
• Versatility: The platform is adaptable for various cell types (primary islets, stem cell derivatives, other organoids) and can be used for drug testing, cancer modeling, and other vascularized tissue engineering applications.
• Future Impact: By enabling the culture of thick, functional endocrine tissues, this technology offers a promising pathway toward a durable, immunoprotective cure for Type 1 Diabetes, potentially eliminating the need for lifelong immunosuppression associated with current islet transplant protocols.