Trabecular structure with thin walls.

From manufacturing customized prosthetics and implants to surgical planning and bioprinting organs and tissues, 3D printing has many medical applications. In terms of bone tissue engineering and 3D bioprinting, 3D printed scaffolds are used as templates to help with tissue formation and initial cell attachment, as well as to fix prostheses through osseointegration. We’ve seen scaffolds made with all sorts of materials, like a compound in turmeric, sugar, and plastic, but the best are those with good porosity that can simulate tissue.

It’s not always easy to fabricate porous structures with specific pore sizes using FDM technology. A trio of researchers from the Polytechnic University of Catalonia (UPC) in Barcelona published a paper, titled “3D Printing of Porous Scaffolds with Controlled Porosity and Pore Size Values,” explaining how they developed a new method of designing porous scaffolds for FDM 3D printing.

The abstract reads, “The present paper provides a methodology to design porous structures to be printed. First, a model is defined with some theoretical parallel planes, which are bounded within a geometrical figure, for example a disk. Each plane has randomly distributed points on it. Then, the points are joined with lines. Finally, the lines are given a certain volume and the structure is obtained. The porosity of the structure depends on three geometrical variables: the distance between parallel layers, the number of columns on each layer and the radius of the columns. In order to obtain mathematical models to relate the variables with three responses, the porosity, the mean of pore diameter and the variance of pore diameter of the structures, design of experiments with three-level factorial analysis was used. Finally, multiobjective optimization was carried out by means of the desirability function method. In order to favour fixation of the structures by osseointegration, porosity range between 0.5 and 0.75, mean of pore size between 0.1 and 0.3 mm, and variance of pore size between 0.000 and 0.010 mm2 were selected. Results showed that the optimal solution consists of a structure with a height between layers of 0.72 mm, 3.65 points per mm2 and a radius of 0.15 mm. It was observed that, given fixed height and radius values, the three responses decrease with the number of points per surface unit. The increase of the radius of the columns implies the decrease of the porosity and of the mean of pore size. The decrease of the height between layers leads to a sharper decrease of both the porosity and the mean of pore size. In order to compare calculated and experimental values, scaffolds were printed in polylactic acid (PLA) with FDM technology. Porosity and pore size were measured with X-ray tomography. Average value of measured porosity was 0.594, while calculated porosity was 0.537. Average value of measured mean of pore size was 0.372 mm, while calculated value was 0.434 mm. Average value of variance of pore size was 0.048 mm2, higher than the calculated one of 0.008 mm2. In addition, both round and elongated pores were observed in the printed structures. The current methodology allows designing structures with different requirements for porosity and pore size. In addition, it can be applied to other responses. It will be very useful in medical applications such as the simulation of body tissues or the manufacture of prostheses.”

Cross-section of specimen 1: (a) 3D view, and (b) 2D view.

In order to design 3D printed porous scaffolds that can simulate tissues, they need mechanical strength, which helps with protection and support; permeability, which can direct the transport of nutrients; and surface area and interconnectivity, both of which relate to good cell growth. Other researchers have tried to achieve the necessary porosity in scaffolds by using hierarchical design and topology optimization. But the UPC team went a different way.

“Unlike other methods that are based on truss structures, the present model allows obtaining irregular porous structures from random location of columns in the space, which leave voids among them. Specifically, the structure was modelled with parallel planes joined by columns, with a certain number of columns on each plane,” the researchers wrote.

They applied their model to a disc shape and defined three different variables:

  1. Distance between parallel planes
  2. Number of base points for columns on each plane
  3. Radius of each column

Then, the team used dimensional analysis to lower the number of process variables to just two, and so defined their requirements “for a specific application case: the use of a porous structure in external layers of hemispherical hip prostheses.”

Printed porous structure (rescaling of the designed scaffold by scaling factor of five).

To compare the results of their experiment with computationally calculated results, the researchers used a dual-extruder Sigma 3D printer from BCN3D to fabricate three sample scaffolds out of PLA, then measured their pore size and total porosity. The researchers found that the measured results were not dissimilar to the calculated results.

“In future work, other requirements for structures, related to either mechanical strength or mass transport, will be addressed. In addition, improvement of the FDM printing process is required in order to obtain more accurate and smooth parts,” the researchers concluded.

Co-authors of the paper are Irene Buj-CorralAli Bagheri, and Oriol Petit-Rojo.

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