Porous Biomaterials by Hanna Naquines
Biomaterials are any materials used in biological applications, while porous biomaterials are specific materials characterized by the many holes or “voids” of empty space within. These voids leave space for other gases or liquids to enter the material. Porous biomaterials have many applications in tissue engineering, including prosthetics, drug delivery, and wound dressing.
These porous biomaterials are characterized into three groups according to sizes determined by IUPAC: microporous, macroporous, and mesoporous . Microporous biomaterials have pores that are less than 2 nm in diameter; macroporous biomaterials contain pores with widths that are larger than 50 nm; and mesoporous materials are between microporous and macroporous with pore widths between 2 nm and 50 nm. These pore sizes will affect the physical properties of the biomaterial. Properties like density, thermal conductivity, and strength are directly related to pore size and amount . A large amount and/or size of the pores will reduce the density and strength of the chosen material, which makes it easier to alter the material for the intended application. The pores themselves can be categorized by their shape and their connectivity to the surface of the solid . Closed pores are pores that are not connected to any other pore or the surface of the biomaterial (Figure 1a). These pores will have the greatest impact on the physical properties of the material discussed above but will have little to no effect on fluid flow, adsorption, diffusion, or any other process involving interaction between solid and surroundings. On the opposite end, open pores are pores that open up to the surface of the solid (Figure 1b, 1c, 1d, 1e, 1f). These can either be blind, like (b) and (f), meaning they open only on one end, or they are through pores (Figure 1c, 1d, 1e). These open pores are more active in the interactions of solid and surroundings.
Porous biomaterials can also be separated into two other groups: agglomerate and aggregate . An agglomerate solid would be a rigid solid, implying it is one single object. An aggregate would be a loosely packed structure of multiple particles. These two types of porous materials can be synthesized in different ways. One possible way is that the material is naturally porous due to the crystalline molecular structure of the material. A second pathway would be the packing of particles until they form a solid aggregate, or even further packing which consolidates the particles and creates an agglomerate. Another way would be the opposite, removing parts from a solid to form the pores.
According to a study by the American Academy of Orthopaedic Surgeons, over 1.5 million Americans underwent the most common musculoskeletal procedures in 2011 . These surgeries included total knee replacement; total and partial hip replacement; and total and partial shoulder replacement. With these numbers growing, there needs to be a development of biomaterials that are bio-compatible, strong, durable, non-toxic, and, for some, nice looking . Ideally, it must also be lightweight and affordable. The porosity of biomaterials used in prosthetics is important for many reasons. The biomaterials need sufficient porosity which will allow cells to move and grow within the prosthetic to properly integrate and vascularize . So, the size, number, and uniformity of the pores will determine the successful integration of the biomaterial, while also maintaining the strength to withstand the forces of the body.
One of the most important parts of drug delivery is transporting the drug to its target and to release said drug in a controlled manner . Such precision is needed to reduce side effects and to be at maximum efficiency. The use of porous biomaterials can deliver this because they can be highly specialized by choosing the material and changing its porosity. Porous biomaterials, by definition, are biocompatible, can hold a large volume of drug depending on porosity, have a large surface area for drug adsorption, and its surface can be functionalized to control and improve targeting and release.
Researchers at Hebei United University developed and implanted porous tantalum rods into rabbits to observe bone growth and integration with the tantalum rods . There is a lot of promise in the use of tantalum as a porous metallic bone graft because of tantalum’s high biocompatibility, high porosity, and an elastic modulus that matches bone.
A total of 24 New Zealand rabbits were used for this study. Their legs were cut right outside the femur where the femoral lateral condyle was drilled into, perpendicular to the femoral shaft. There, the tantalum rod was place and the leg was sutured back up. The tantalum rod contained pores with widths that ranged from 200-400 μm.
After 2, 4, 8, and 12 weeks, bone tissue was removed from the rabbits . Within week 2, there was bone tissue growing where the host bone (femur) and tantalum met. Mostly on the surface but also with some small blood vessels growing within the pores. This increased by week 4 and by week 8, there was new bone tissue fully covering the tantalum rods and pores. This formed a direct bond with the tantalum and bone. The rabbits also regained knee movement with no infections near the incision. The porous tantalum showed no signs of loosening and by the 12th week there a direct bond between the tantalum and the host bone.
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