Biological Gradients Help Researchers Understand More in Bioprinting

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Selective irradiation method to induce linear or radial gradients on a photosensitive substrate. Ultraviolet (UV) irradiation is selectively filtered by a mask, which is either completely opaque (a) or has a designed distribution of opacity (b). The movement of the mask or the selective irradiation induces a controlled modification of the substrate, which results in a physical or chemical gradient.

Gradients present in structures today help us understand more about their properties, and this is critical in areas like bioprinting. Through replicating gradients we can begin to make accurate high resolution parts and scaffolds. Researchers explored these needs further in a recently published paper, ‘Engineering Biological Gradients.’ Tissue engineering is growing more and more of interest to researchers around the world as they continue to strive to 3D print human organs—a subject we have followed many times over the years as progress has been made in 3D printed kidneys, 3D printed livers, and much more.

In the meantime, many different devices and implants are being created in the realm of regenerative medicine—with the researchers here categorizing structures as either scaffolds or constructs (scaffolds combined with cells prior to implantation). As researchers seek to forge ahead with even fewer limits, they have begun measurement and analysis for clinical therapy and in engineering tissue for in-vitro screening, allowing for better feedback.

“The presence of a gradient confers to each point of the substrate a specific value of the varying quantity, allowing analysis of the effect of each variable over a specific phenomenon, such as cell adhesion, spreading, morphology, or differentiation,” stated the researchers. “This offers a great advantage to both interfaced tissue engineering and drug screening, as the effect of different variables on a phenomenon is analyzed in a single experimental set-up rather than in a series of experiments at different conditions.”

Gradients are basically variables used in a range of different measurements, and they can be categorized according to variant quantities, depending on their type, to include:

  • Linear
  • Radial
  • Orthogonal
  • Exponential
  • Non-linear shapes

Classification of gradients according to their arrangements: (a) linear; (b) radial; (c) exponential; (d) orthogonal; (e) sigmoidal gradients, as representative of non-linear shape. Figure adapted from Smith Callahan

A gradient range of values over a given quality allows for a better defined more specific property at any given point. The researchers go on to explain how important gradients are in copying the anisotropy of tissue, along with acting as the basis of so many ‘phenomena’ behind cellular and bacterial results. They explore semi-immersion, diffusion, compositional topography, selective irradiation, and microfluidic devices.

“… the gradient is usually continuous, with smooth variation of the chemical or physical properties within the system. Both methods are suitable for the fabrication of scaffolds for the regeneration of a single tissue (bone, cartilage, skin, tendons, nerves, etc.) or for interface tissue engineering (cartilage-to-bone and tendon-to-bone, among others),” state the scientists.

Rapid prototyping, via 3D printing, allows for complex geometries to be created and easily modified. For studying bioprinting, the researchers used scans of target bone structure from a patient who had a scan performed. The scan was converted to 3D data, and a radial porosity gradient was formed.

“Biochemical gradients are more complex to achieve with bioprinting. Indeed, changes in the chemical composition of printed scaffolds imply the need to use different bioinks during the printing process,” stated the researchers.

Four bioinks were created and then printed to replicate both the extracellular matrix and vessel structures:

  • Polydimethylsiloxane
  • Methacryloyl-gelatin
  • Pluronic F127 (a triblock poloxamer)
  • Methacryloyl-gelatin loaded with cells

Scaffolds originating from gelatin and fibrinogen showed much better promise, but the researchers noted that printing times were ‘severely prolonged by the numerous switch times.’ They noted that inks would require crosslinking after printing, meaning that the choice of biocompatible materials is seriously restricted.

“Until now, the main application of graded structures is confined to studying the effect of each varying parameter over cell activities. Moving to bioinspired structures for regenerative medicine, some examples have been reported for graded scaffolds in bone and cartilage regeneration. However, the potential of engineering graded materials is underestimated in many applicative fields, such as in-vitro models and soft-to-hard regenerative medicine,” concluded the researchers.

“The characterization techniques should be expanded to meet the need to examine the peculiarity of graded materials and validate the productive methods. An increasing knowledge of the production and characterization of graded structures will allow new scenarios for engineering bioinspired materials to be explored.”

Find out more about other stories we have followed about scaffoldings and bioprinting, to include the creation of 3D printed devices. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at

[Source / Images: Engineering Biological Gradients]

Confined (a) or non-confined (b) plastic compression can be exploited to induce gradients in the porosity and mechanical properties of elastic polymers. The homogeneous scaffold is compressed, in a molder or after rolling, with constant or variable load to induce alterations in the polymeric network and a macroscopic plastic deformation.

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