Melt Electrowritten Scaffold-Reinforced Affibody-Conjugated Hydrogels for Controlled Bone Morphogenetic Protein-2 Delivery

This study presents a melt electrowritten scaffold-reinforced, affibody-conjugated hydrogel system that simultaneously enhances mechanical stability and enables controlled, affinity-based delivery of BMP-2, significantly improving bone regeneration outcomes in vivo.

The Gist

The Big Problem: The "Spilled Milk" of Bone Healing

Imagine you have a broken bone, and the doctor needs to pour a special "magic potion" (a protein called BMP-2) onto the break to help it knit back together.

Currently, doctors use a sponge to hold this potion. But there's a big problem: the sponge is like a sieve. As soon as the doctor puts it in, the potion rushes out all at once, flooding the area.

• The Result: The body gets a massive, sudden dose of the medicine. This causes swelling, inflammation, and sometimes even causes bone to grow in the wrong places (like inside your muscles). To make sure enough gets to the bone, doctors have to use huge, expensive doses, which increases the risk of side effects.

Also, the sponge is very squishy. If a surgeon tries to pick it up with tweezers, it might squish out of their hand or fall apart before they can even put it in the patient.

The Solution: A "Smart Cage" and a "Velcro Trap"

The researchers in this paper built a two-part system to fix these problems. Think of it as upgrading from a flimsy paper cup to a high-tech, reusable travel mug.

1. The "Velcro Trap" (Affibodies)

Instead of just dumping the potion into a sponge, they added tiny, invisible "Velcro hooks" (called affibodies) to the gel.

• How it works: The BMP-2 protein has a specific shape that fits perfectly into these hooks.
• The Analogy: Imagine the protein is a key, and the hydrogel is a keychain. The keys don't just fall off; they get stuck on the chain. This allows the doctor to control how fast the keys fall off. Instead of a flood, the keys drip out slowly over time, giving the bone a steady, safe dose of medicine.

2. The "Smart Cage" (MEW Scaffold)

The gel with the Velcro hooks is still very soft and squishy. To fix this, the researchers put the gel inside a tiny, 3D-printed cage made of microscopic fibers (called a Melt Electrowritten or MEW scaffold).

• How it works: This cage is like a honeycomb made of plastic threads. It's strong enough to hold the gel together but has huge holes so cells and blood can flow right through it.
• The Analogy: Think of the gel as Jell-O. If you try to move a bowl of Jell-O, it wobbles and spills. But if you put that Jell-O inside a wire mesh basket, you can carry the basket around easily without spilling a drop. The basket gives it structure, but the Jell-O can still breathe and let things pass through.

The "Freeze-Dry" Superpower

One of the coolest parts of this invention is that it can be freeze-dried (lyophilized).

• The Problem: Most medical gels need to be kept in a fridge or freezer. If they dry out, they turn into a useless, crumbly mess.
• The Fix: Because of the "Smart Cage," the researchers could freeze the gel, turn it into a dry powder, and store it on a shelf at room temperature. When a surgeon needs it, they just add water, and it instantly turns back into a perfect gel.
• The Analogy: It's like instant coffee. You can keep the dry powder in a cupboard for years. When you add hot water, it instantly becomes fresh coffee. This makes the medicine much cheaper and easier to ship to hospitals that don't have fancy freezers.

What Happened in the Lab?

The team tested this new "Cage + Velcro" system on rats with broken leg bones.

1. Handling: The surgeons could easily pick up the new system with tweezers without it falling apart.
2. Storage: They froze and dried the system, and it worked perfectly after being rehydrated.
3. Bone Growth:
   • The Cage alone helped the bone heal faster and stronger because it kept the gel exactly where it needed to be.
   • The Velcro (affibodies) was great at keeping the medicine inside the gel (preventing it from leaking out into the body), but in this specific rat model, the bone healed so well with the Cage alone that the extra Velcro didn't change the final result much.

The Bottom Line

This paper introduces a new way to deliver bone-healing medicine that is:

• Stronger: It won't squish out of the surgeon's hand.
• Smarter: It releases medicine slowly and steadily, not all at once.
• Shelf-Stable: It can be freeze-dried and stored like a powder, making it easier to use in real hospitals.

It's a step toward making bone surgery safer, cheaper, and more effective for patients everywhere.

Technical Summary

1. Problem Statement

The clinical use of recombinant Bone Morphogenetic Protein-2 (BMP-2) for bone regeneration, particularly in spinal fusion, is limited by two major challenges:

• Uncontrolled Release: Current delivery vehicles, such as absorbable collagen sponges, rely on weak electrostatic interactions with BMP-2. This leads to rapid, uncontrolled release, necessitating supraphysiological doses that cause adverse effects like ectopic ossification, inflammation, and swelling.
• Mechanical Instability: Hydrogels offer tunable release kinetics but suffer from poor mechanical integrity. They are difficult to handle surgically, prone to deformation, and often lack the stability required for long-term storage (shelf life).
• The Trade-off: Increasing hydrogel stiffness to improve handling often negatively impacts cell infiltration, protein diffusion, and the local mechanical microenvironment required for tissue remodeling.

2. Methodology

The authors engineered a dual-component delivery platform that decouples mechanical reinforcement from protein release control:

• Affinity-Controlled Release (The "Software"):

  • Affibodies: Engineered BMP-2-specific binding proteins (affibodies) with two distinct affinities were used: High-affinity ($K_D \approx 10.7$ nM) and Low-affinity ($K_D \approx 34.8$ nM).
  • Hydrogel Matrix: A polyethylene glycol maleimide (PEG-mal) hydrogel (5% w/v) was synthesized. The affibodies were conjugated to the PEG-mal via thiol-maleimide chemistry.
  • Crosslinking: Hydrogels were crosslinked using either DTT (for in vitro stability) or MMP-cleavable peptides (for in vivo degradation).

• Mechanical Reinforcement (The "Hardware"):

  • MEW Scaffolds: Tubular scaffolds were fabricated using Melt Electrowriting (MEW), a high-precision 3D printing technique using Poly-ε-caprolactone (PCL).
  • Design: The scaffolds featured a diamond pore design with ~29 µm fiber diameters and 80–95% porosity. They were designed as hollow tubes to encase the hydrogel without altering the hydrogel's bulk formulation.

• Fabrication & Processing:

  • Hydrogels were synthesized directly inside the MEW scaffolds by pipetting precursors and rotating the tube to ensure uniform filling and crosslinking.
  • Lyophilization: To test shelf stability, reinforced hydrogels were flash-frozen and lyophilized (freeze-dried), then rehydrated prior to testing.

• Evaluation Models:

  • In Vitro: Mechanical compression testing, swelling studies, and BMP-2 release kinetics (7–21 days) compared against clinical collagen sponges.
  • In Vivo (Subcutaneous): Fluorescently labeled BMP-2 retention in rat dorsal subcutaneous pockets (21 days).
  • In Vivo (Critical Bone Defect): 6-mm femoral defects in rats treated with BMP-2-loaded hydrogels. Groups included: (1) Non-reinforced hydrogel, (2) MEW-reinforced hydrogel, (3) MEW-reinforced + High-affinity affibody. Outcomes were assessed via radiography, micro-CT (6 and 12 weeks), and histology.

3. Key Contributions

• Decoupling Mechanics and Release: The study successfully demonstrated that mechanical reinforcement via MEW scaffolds improves handling and stability without altering the hydrogel's bulk stiffness or interfering with affinity-based protein release mechanisms.
• Lyophilization Compatibility: The research proved that MEW reinforcement allows soft hydrogels to undergo freeze-drying and rehydration without structural collapse, a critical step for clinical shelf stability.
• First In Vivo Affibody Validation: This is the first study to demonstrate that affibody-conjugated hydrogels can retain therapeutic proteins in vivo and that this retention is preserved even after lyophilization.

4. Key Results

Mechanical & Physical Properties:

• Handling: MEW-reinforced hydrogels could be manipulated with surgical forceps without damage, whereas non-reinforced hydrogels deformed easily.
• Stiffness: Reinforcement did not significantly change the initial bulk stiffness of the hydrogel (~159 Pa vs. ~210 Pa), but it significantly increased resistance to high-strain compression (80% strain), preventing collapse.
• Lyophilization: Non-reinforced hydrogels collapsed during freeze-drying. MEW-reinforced hydrogels retained their shape and could be rapidly rehydrated (~130 µL uptake in 1 min).
• Mesh Size: Lyophilization reduced the hydrogel mesh size (from ~140 Å to ~60 Å) due to network collapse, but the MEW scaffold maintained the overall construct integrity.

Protein Release Kinetics:

• Affinity Control: High-affinity affibodies significantly reduced BMP-2 release rates compared to low-affinity or non-conjugated hydrogels.
• Lyophilization Effect: Despite the change in mesh size, lyophilized and rehydrated hydrogels with high-affinity affibodies maintained their ability to control release. The release mechanism remained Fickian diffusion, but the effective diffusivity was lower for high-affinity groups.
• Comparison: Reinforced hydrogels with high-affinity affibodies released significantly less BMP-2 than clinical collagen sponges, which exhibited rapid, uncontrolled release.

In Vivo Performance:

• Subcutaneous Retention: After 3 weeks, hydrogels with high-affinity affibodies retained significantly more fluorescent BMP-2 than controls, confirming enhanced local retention.
• Bone Regeneration (Femoral Defect):
  • MEW Reinforcement: Significantly increased total bone volume and defect bridging compared to non-reinforced hydrogels.
  • Affibodies: While high-affinity affibodies improved BMP-2 retention, they did not significantly increase bone volume or bridging compared to the reinforced-only group.
  • Histology: Reinforced groups showed more organized lamellar bone (Type I collagen) compared to non-reinforced groups. The MEW fibers did not impede tissue infiltration.

5. Significance and Conclusion

This work presents a novel strategy for overcoming the "mechanical vs. biological" trade-off in protein delivery. By using MEW scaffolds as a structural shell, the authors created a delivery vehicle that is:

1. Surgically Robust: Easy to handle and implant without deformation.
2. Shelf-Stable: Capable of lyophilization and rehydration, reducing cold-chain logistics requirements.
3. Biochemically Tunable: Capable of affinity-controlled release via affibodies to minimize side effects.

Clinical Implication: While the specific dose of BMP-2 used (5 µg) was sufficient to heal the defect regardless of affinity control, the platform's ability to maintain protein retention and structural integrity suggests it could be optimized for lower, safer doses of BMP-2 in future studies. This system represents a significant step toward clinically viable, off-the-shelf hydrogel delivery systems for musculoskeletal repair.