Liquid marbles have presented promising avenues for treatment in tissue engineering and bone regeneration. Comprised of particle films that confine liquid droplets, they are a durable and easily reproducible material. Researchers from the University of Minho in Portugal and Jilin University in China have leveraged liquid marble fabrication to create a bioactive hydrogel marble (BHM). To do so, the authors coated a hydrogel bead with highly biocompatible hydrophobic bioactive glass nanoparticles (H-BGNPs) (Image 1).
Image 1. Selected images of BHMs with the progressive coating of H-BGNPs. From left to right: minimal coating to full coating.
First generation liquid marble technology used basic hydrophobic shells to protect the encapsulated liquid from the external environment. However, there was no additional functionality or bioactivity. Second generation research improved on this with marble shells that could respond to changes in pH or temperature. For example, a liquid marble could be programmed to release the encapsulated material upon interaction with an acidic environment. However, the shells required extreme changes in the environment to trigger a response, which can be detrimental to encapsulated materials like proteins or whole cells. This newly developed BHM solves these existing shortcomings by creating a bioactive enclosure that enables the encapsulation of cells in a biocompatible three-dimensional matrix. The BHM can essentially be used as a ready-made and portable cell culture matrix.
Image 2. Surface modification of BGNPs with fluorosilane resulted in a significant increase in contact angle, indicating increased hydrophobicity.
A glass, as defined in class, is an inorganic product of molecular fusion that is cooled to a solid without crystallization; it is thus an amorphous solid with only short-range order. The researchers started with a pure bioglass made up of both silicate and calcium phosphate components in the ratio SiO2:CaO:P2O5 (mol%) = 55:40:5. As we learned in lecture, this kind of composite glass can be tuned to increase resorbability and bioactivity. This ratio results in a highly bioactive glass, but it is naturally hydrophilic. Because the researchers wanted to coat the liquid marble with hydrophobic nanoparticles, they chemically bound fluorosilane to the surface. This resulted in a significant increase in measured contact angle—a test we have discussed in lecture that determines the hydrophobicity of a material; higher contact angles indicate increased hydrophobicity (image 2).
The researchers developed a relatively simple method to coat a liquid marble with H-BGNPs. The liquid bead was first formed from gelatin methacrylate, a photocrosslinkable polymer that studies have shown is highly biocompatible. Small volumes of gelatin methacrylate were added to a surface covered with the H-BGNPs. The droplets were then rolled over the surface, collecting NPs as they traveled. Upon completion, a cohesive layer of H-BGNPs covered the outside of the liquid sphere, with the NPs helping to maintain the marble’s semi-spherical shape. As a result, the encapsulated polymer bead was protected from the environment by the nanoparticle shell with minimal leaking. The gelatin methacrylate liquid marble was then crosslinked with UV light, generating a solid hydrogel marble (Image 3).
Image 3. Overview of fabrication of BHMs.
Because the H-BGNPs are bioactive, they have the potential to regulate osteogenesis, or the formation of bone tissue when the BHM is introduced in vivo. What is most promising about the BHM is that when introduced into a physiological microenvironment, the coating, or shell, of the bead promotes the formation of a calcium phosphate apatite layer. This apatite layer, as we learned in class, enables a cellular process called bioactive fixation. Bioactive fixation is a unique mechanism by which glasses and certain surface-reactive ceramics can chemically bind to bone cells. As a result, the BGNPs enable the encapsulated hydrogel to integrate with bone in vivo and accelerate the process of osteointegration. Osteointegration is the process by which bone cells (osteoblasts) form new growth through the bulk of the material. The bone cells can interact with the calcium phosphate apatite layer which mimics mineralized connective tissue, exposing them to the encapsulated hydrogel. The hydrogel then provides a matrix through which bone cells can migrate and proliferate.
In addition, the formulation of the BHMs is promising for a new mechanism of targeted drug delivery. The study showed that the device is effective in the carrying and delivery of ibuprofen and albumin. For example, the marbles could be delivered to the site of a bone injury or defect, then promote healing by directly delivering a drug incorporated within the hydrogel.
This technology is successful in creating an improved bioactive device that may be effective for in vivo osteogenesis. While the study showed that the H-BGNP coating creates a bone-like layer of calcium phosphate apatite, additional experimentation is necessary to determine the rate and likelihood of effective osteogenesis in vivo. The hydrophobic nature of the H-BGNPs might be detrimental to the device’s performance because it may reduce protein coating which is necessary for cellular interaction. It also might be beneficial to revisit the formulation of the bioglass nanoparticles. A glass with increased mol% of calcium oxide and phosphate molecules could increase resorbability and bioactivity, contributing to improved osteogenesis.
The BHMs are an effective, reproducible, and inexpensive system that has tremendous application in tissue engineering. They can be used to deliver sensitive agents like cells, therapeutics, and proteins in a fully protective and bioactive nanoparticle shell, without changing the effectiveness of the encapsulated material. Further, this also represents the potential for the use of polymer hydrogels in combination with ceramics and glasses, connecting the different biomaterials we have covered thus far in class.
Article:
- Leite, Á., Oliveira, N., Song, W., & Mano, J. (2018). Bioactive Hydrogel Marbles. Scientific Reports, 8(1). doi: 10.1038/s41598-018-33192-6