A novel 3D-printed hydrogel platform for controlled delivery of BMP-9 coated calcium sulfate microparticles with co-delivery of preosteoblasts from a cell encapsulated coating layer

This study presents a novel 3D-printed hydrogel scaffold system that utilizes gelatin-based electrostatic interactions to achieve controlled, sustained release of BMP-9 from calcium sulfate microparticles while simultaneously encapsulating viable preosteoblasts, thereby creating an effective platform for enhanced bone regeneration.

The Gist

Imagine you are trying to heal a broken bone. The body needs two main things to do this job effectively: blueprints (growth factors that tell cells what to do) and workers (the actual bone-building cells).

The problem with current medical treatments is like trying to deliver a fragile package and a team of workers to a construction site using a standard truck. Often, the package breaks open immediately (releasing all the medicine at once), or the workers get stressed and die before they even arrive.

This paper describes a clever new "smart truck" designed by researchers at the University of Toledo to solve this problem. Here is how their invention works, broken down into simple concepts:

1. The "Smart Truck" (The 3D-Printed Scaffold)

Think of the bone defect as a pothole in the road. The researchers built a 3D-printed scaffold, which acts like a temporary bridge or scaffolding to fill that pothole. But this isn't just a static bridge; it's a delivery system with two distinct layers, like a sandwich:

• The Bottom Layer (The Base): This is the main structure. Inside it, they hid tiny particles of calcium sulfate (a type of mineral) that were coated with BMP-9.

  • What is BMP-9? Think of BMP-9 as the "Super Foreman." It is a powerful protein that screams, "Build bone here!" However, it's very sensitive. If you just dump it out, it runs away too fast (a "burst release") and gets wasted.
  • The Trick: The researchers used a special type of gelatin (Type B) in this layer. Because of the way electricity works at a molecular level (electrostatics), this gelatin acts like a magnet that gently holds onto the BMP-9, keeping it from running away immediately.

• The Top Layer (The Coating): This is a soft, clear gel that covers the base. Inside this gel, they trapped living preosteoblasts (the bone-building workers).

  • The Trick: This layer uses a different type of gelatin (Type A). This gelatin acts like a repelling force against the BMP-9. It doesn't want to hold the protein; it wants to let it go slowly over time.

2. The "Slow-Release" Mechanism

Why use two different types of gelatin?

• The Magnet and the Repeller: Imagine the BMP-9 is a ball. The bottom layer (Type B gelatin) is a sticky magnet that grabs the ball. The top layer (Type A gelatin) is a slippery slide that pushes the ball away.
• The Result: When the system is placed in the body, the BMP-9 doesn't just explode out. It has to slowly work its way from the sticky bottom, through the slippery top. This creates a controlled, slow release of the "Super Foreman" over several days, giving the body enough time to listen to the instructions and start building bone.

3. The "Worker Safety" (Cell Viability)

The researchers also needed to make sure the living cells didn't die during the manufacturing process.

• The Process: They mixed the cells into the top gel layer and then used blue light to "set" the gel (like hardening resin).
• The Test: They checked the cells and found that over 80% survived. It's like putting workers in a construction helmet and a seatbelt; the process was safe, and the workers were ready to work.
• The Escape: Over time, the top gel layer slowly breaks down (biodegrades). As it does, the "workers" (cells) naturally migrate out of the gel, attach to the bottom scaffold, and start building bone right where they are needed.

4. The Results

When they tested this system:

• Without the system: The BMP-9 would release 80% of itself in just one day (useless waste).
• With the system: The BMP-9 was released slowly over 5 days (about 50-60%), which is the "Goldilocks" zone—not too fast, not too slow.
• The Cells: The cells stayed alive, multiplied, and started differentiating into bone cells, especially when they received the "Super Foreman" (BMP-9) signals.

The Big Picture

This research is like upgrading from a firehose (which sprays water everywhere and wastes it) to a drip irrigation system (which delivers water slowly and exactly where the plant needs it).

By using 3D printing and clever chemistry (using different types of gelatin to control attraction and repulsion), the team created a platform that can deliver both the "instructions" (BMP-9) and the "workers" (cells) to a broken bone at the perfect pace. This could lead to faster healing for patients with severe bone injuries or defects in the future.

Technical Summary

1. Problem Statement

Bone tissue engineering faces significant challenges in the controlled delivery of growth factors and viable cells.

• Rapid Burst Release: Conventional delivery systems often suffer from the rapid, uncontrolled release of bioactive molecules (like Bone Morphogenetic Proteins), leading to sub-therapeutic concentrations over time and potential toxicity.
• Cell Viability: Encapsulating cells within hydrogels often compromises cell survival due to harsh processing conditions (e.g., photocuring, chemical crosslinking).
• Lack of Synergy: Few systems effectively co-deliver high-potency osteogenic growth factors (specifically BMP-9) and viable preosteoblasts in a single scaffold that mimics the complex geometry of bone defects.
• Specific Challenge with BMP-9: While BMP-9 is the most potent osteogenic cytokine in the BMP family, its signaling is not inhibited by common antagonists (like noggin), making it ideal for therapy but difficult to control in delivery systems due to its hydrophilic nature and electrostatic properties.

2. Methodology

The authors developed a dual-layer 3D-printed hydrogel scaffold system designed to regulate the release of BMP-9 via electrostatic interactions while maintaining high cell viability.

A. Material Design & Electrostatic Strategy

The core innovation relies on exploiting the Isoelectric Point (IEP) differences between two types of gelatin to control protein binding and release:

• Base Scaffold: Printed using Gelatin Type B (Bovine skin, IEP ~pH 5). At physiological pH (7.4), Gelatin B is negatively charged.
• Coating Layer: A photocurable hydrogel containing Gelatin Type A (Fish skin, IEP ~pH 9). At physiological pH, Gelatin A is positively charged.
• Mechanism: BMP-9 (IEP ~7.7–9) carries a net positive charge at pH 7.4.
  • Attraction: The positively charged BMP-9 is electrostatically attracted to the negatively charged Gelatin Type B in the base scaffold, promoting retention.
  • Repulsion: The positively charged Gelatin Type A in the coating layer repels the positively charged BMP-9, creating a barrier that slows diffusion out of the scaffold.

B. Scaffold Fabrication

1. Microparticle Preparation: Calcium Sulfate (CaS) microparticles (75–106 μm) were coated with recombinant BMP-9. CaS acts as both a delivery vehicle and an osteoconductive filler.
2. Base Scaffold Printing: The BMP-9 coated CaS particles were encapsulated in a hydrogel precursor containing Gelatin Type B, PEGDMA (750 Da), methylcellulose, and photoinitiator (LAP). This was 3D printed using an extrusion-based bioprinter (Allevi2) into a 5mm × 5mm square mesh.
3. Cell Encapsulation & Coating: A second hydrogel layer was prepared using Gelatin Type A, PEGDMA, methylcellulose, and LAP. Preosteoblasts (2 × 10⁶ cells/mL) were encapsulated in this layer.
4. Assembly: The cell-laden coating was applied over the printed base scaffold and photocured (blue light, 120s) to form a cohesive, dual-layer structure.

C. Evaluation

• Release Kinetics: BMP-9 release was monitored over 21 days using ELISA in three groups: CaS particles alone, uncoated scaffolds, and coated scaffolds.
• Cell Viability: Live/Dead assays (Calcein/ Ethidium Homodimer) were performed over 21 days.
• Proliferation & Differentiation: WST-1 assays (metabolic activity) and Alkaline Phosphatase (ALP) assays (osteogenic differentiation) were conducted.

3. Key Contributions

• Electrostatic Release Control: Demonstrated a novel strategy to regulate BMP-9 release kinetics not by degradation rates alone, but by tuning the electrostatic charge of the hydrogel matrix using specific gelatin types (Type A vs. Type B).
• Co-Delivery Platform: Successfully integrated a 3D-printed structural scaffold (containing growth factors) with a cell-encapsulating coating layer, allowing for the simultaneous delivery of BMP-9 and preosteoblasts.
• Optimized BMP-9 Delivery: Addressed the issue of burst release by combining CaS microparticles with a dual-gelatin hydrogel system, achieving sustained release profiles suitable for bone regeneration.
• High Cell Viability: Validated that the photocuring process and the specific hydrogel composition (PEGDMA/Gelatin A) are non-toxic to encapsulated preosteoblasts.

4. Key Results

• BMP-9 Release Profiles:
  • CaS Particles Alone: Exhibited a massive burst release (~80% within 24 hours).
  • Uncoated Scaffolds (Gelatin B only): Showed rapid release, with ~89% released by Day 5.
  • Coated Scaffolds (Gelatin B base + Gelatin A coating): Successfully achieved sustained release, limiting BMP-9 release to 50–60% by Day 5. The coating layer acted as a diffusion barrier.
• Cell Viability:
  • Live/Dead assays showed >80% cell viability within the coating layer throughout the culture period.
  • Cells remained healthy after encapsulation and photocuring.
  • Over time, cells migrated out of the degrading coating layer, attached to the base scaffold, and proliferated on the well plate.
• Bioactivity & Differentiation:
  • Cell Attachment: BMP-9 containing scaffolds showed significantly higher cell attachment compared to controls.
  • Proliferation (WST-1): Initial high metabolic activity was observed. While OD values decreased in the coating layer over time (due to cell migration), the overall system supported cell growth.
  • Differentiation (ALP): ALP activity was significantly higher in BMP-9 groups compared to controls, confirming the bioactivity of the released protein. The decrease in ALP activity in the coating layer over time correlated with cell migration to the base scaffold rather than a loss of function.

5. Significance

This study presents a versatile and effective platform for bone tissue engineering:

1. Precision in Drug Delivery: The use of IEP-tuned gelatin types offers a tunable mechanism to control the release of various charged proteins and peptides, moving beyond simple diffusion-based release models.
2. Clinical Relevance: The system addresses the critical need for sustained BMP-9 delivery to overcome the limitations of current clinical BMP-2 applications (which often require high doses and cause inflammation).
3. Regenerative Potential: By co-delivering viable cells and a potent growth factor, the scaffold promotes a synergistic effect where released cells can differentiate into osteoblasts under the influence of the sustained BMP-9 signal, potentially accelerating the healing of critical-sized bone defects.
4. Scalability: The use of 3D printing allows the scaffold to be customized for complex defect geometries, making it a promising candidate for personalized regenerative medicine.