Novel preparation of graded porous structures for medical engineering

Abstract

The gradation of porosity in a biomaterial can be very useful for a variety of medical engineering applications such as filtration, bone replacement and implant development. However, the preparation of such structures is not a technologically trivial task and replication methods do not offer an easy solution. In this work, we elucidate the preparation of structures having a graded porosity by electrohydrodynamic spraying, using zirconia (ZrO2), which is widely used in biomedical and other applications. The processes are generic and can be achieved using other bioactive ceramics with similar particle characteristics. The pores on the sprayed surface, the innermost surface and lengthwise cross sections have been analysed in addition to the change in depth of penetration as a function of spraying time. Control of porosity, pore size and depth of penetration has been obtained by varying parameters such as the spraying time, sintering temperature and the sacrificial template. It has been possible to obtain structures with interconnected pore networks of pore size greater than 100microm as well as scattered pores smaller than 10microm in size.

Figure 1

Experimental set-up used for electrohydrodynamic spraying of the zirconia suspension.

Figure 2

The depth of ceramic suspension penetration as a function of spraying time for the structures sintered at 1200°C (60 ppi templates).

Figure 3

Comparisons of penetration depth obtained by (a) using different sacrificial templates (open triangles, 60 ppi; filled triangles, 45 ppi) and (b) varying the height of the sacrificial template (45 ppi; open triangles, 5 mm depth; filled triangles, 10 mm depth).

Figure 4

Microstructural features. (a) The electrosprayed surface (the 60 ppi template was electrosprayed for 7 min and sintered at a maximum temperature of 1400°C). (b) An example of inverted pyramid shape observed, gradually narrowing towards the innermost surface (the 60 ppi template was electrosprayed for 3 min and sintered at a maximum temperature of 1400°C). (c) The thick ceramic coating forms a pyramidal shape on top of the foam with increased spraying time (60 ppi template). ESED, environmental secondary electron detector.

Figure 5

Microstructural features. (a) Micropores smaller than 10 μm observed on the electrosprayed surface (45 ppi template, electrosprayed for 5 min and sintered at 1400°C). (b) Interconnected pore network on the innermost surface with pores greater than 100 μm (60 ppi template, electrosprayed for 3 min and sintered at 1400°C). (c) Lengthwise cross section showing the variation of pores from the sprayed to the inner surface. Interconnected porosity is present throughout the structure (45 ppi template, electrosprayed for 3 min and sintered at 1400°C). ESED, environmental secondary electron detector.

Figure 6

Variation of pore sizes observed on the (a) sprayed surface and the (b) innermost surface as a function of spray time at different sintering temperatures (60 ppi templates). Open triangles, 1200°C; filled triangles, 1400°C.

Figure 7

Variation of pore sizes observed on the (a) sprayed and (b) innermost surfaces as a function of spray time using different templates (open triangles, 60 ppi; filled triangles, 45 ppi).

Figure 8

Bone-like structure replication (45 ppi templates). (a) Low magnification image of the sprayed surface, (b) larger and scattered pores of diameter 130–280 μm, (c) the coating exhibiting a rough surface consisting of smaller pores less than 20 μm and (d) entire cross section showing the thicker outer layer of zirconia and the more porous interior. SE, secondary electron.