Bioactive glass scaffolds for bone regeneration

Natural materials are renowned for their unique combination of outstanding mechanical properties and exquisite microstructure. For example, bone, cork, and wood are porous biological materials with high specific stiffness (stiffness per unit weight) and specific strength. The outstanding mechanical properties of these materials are attributed to their anisotropic structures, which have optimized strength-to-density and stiffness-to-density ratios. Now, researchers from Berkeley Lab and the Imperial College London have created bioactive glass scaffolds that mirror nature’s efficient materials. The three-dimensional glass scaffold is as porous as trabecular bone, has a compressive strength comparable to that of cortical bone, and a strength-to-porosity ratio higher than any previously reported scaffolds.

 

Scientists are developing new materials and structures inspired by biological materials with properties to satisfy a variety of applications. The ability to develop porous constructs with high mechanical strength, for example, is important for a broad range of emerging applications, including filters, catalyst support, and tissue engineering scaffolds. Particularly for orthopedic surgery, the regeneration of large bone defects in load-bearing limbs remains a challenging problem. The compressive strength of cortical bone, primarily in the shaft of long bones and as the outer shell around trabecular bone, has been reported to be in the range of 100 – 150 MPa in the direction parallel to the axis of orientation (long axis). It is difficult to design and fabricate a construct that will combine the large pores necessary to promote bone regeneration while substituting for, at least temporarily, the tissue by maintaining these loads in vivo.

 

Porous metallic implants used for replacement in fractures have well-documented fixation problems, and unlike natural bone, cannot self-repair or adapt to changing physiological conditions. As a consequence, the implant becomes loose over time. Bioactive glass and ceramic alternatives have shown excellent potential in repair and regeneration of bone defects due to their ability to support bone cell growth, form strong bonds to both hard and soft tissues, and adjust their degradation rate while newly formed bone and tissue are being remodeled.

 

Recently, researchers from Berkeley Lab and the Imperial College London worked at ALS Beamline 8.3.2 to emulate nature’s design by robocasting bioactive glass scaffolds. Robocasting, or direct-ink write assembly, is a layer-by-layer assembly technique used to build scaffolds with structures following computer designs, creating periodic patterning and controlled fabrication of the filaments into constructs with qualities similar to those of biological materials. The scaffold architecture can be optimized to achieve the desired mechanical response, accelerate the bone-regeneration process, and guide the formation of bone with the anatomic cortical-trabecular structure.

 

The final product is a three-dimensional glass scaffold whose compressive strength (136 MPa) is comparable to that of cortical bone, the compact, dense bone material that forms the outer shell of most bones. The scaffold’s porosity, or void fraction, is 60%, comparable to that of trabecular bone, the spongy material that comprises the inside of vertebra and the ends of longer bones and is the site of metabolic activity such as ion exchange and red blood cell production. The strength of this porous glass scaffold is ~100 times that of polymer scaffolds and 4 – 5 times that of ceramic and glass scaffolds with comparable porosities previously reported in the literature. The glass scaffold’s biological performance in both small animals (mice) and big (miniature pigs) is currently under systematic evaluation at the University of California, San Francisco.

 

The ability to create structures that are both strong and porous could make scaffold fabrication applicable in a wide array of applications, including tissue engineering, filtration, and catalyst support. The use of glass also opens new possibilities in the field of bone regeneration, utilizing the easily tailored bioactivity and biodegradation rates, as well as the release kinetics of different ions, to achieve materials with the desired properties.