TECHNCAL NOTES CERDAK BIOCERAMIC WOUND HEALING DEVICES

1. THE DESIGN, MAKE-UP, ACTION AND COMPOSITION OF CERDAK
2. THE WOUND ENVIRONMENT CREATED BY CERDAK
3. PHYSICAL FACTORS THAT AFFECT THE OPERATIONAL EFFICIENCY OF THE CERDAK


1. THE DESIGN, MAKE-UP, ACTION AND COMPOSITION OF CERDAK
 
  Design and properties of the bioceramic wound healing devices

The Cerdak wound healing devices consist of sachets filled with ceramic spheres with average diameter of 0,7mm (range 0,2 - 1,0 mm). The sachets are made from highly absorbent and permeable, FDA-approved material that does not adhere to wounds. The ceramic spheres are loosely packed in the sachets, and have many interstitial voids. The ceramic phase has a high porosity and a high permeability, giving it a high liquid storage capacity and a high liquid absorptive power. Because of the fineness of the pores, the capillary suction force of the pores vary between 56 kPa for the coarsest pores and 933 kPa for the finest pores. The potency of these forces are responsible for the characteristic action of the ceramic.

Calculation of these capillary forces:

The most important functional property of the bioceramic is the capillary absorption effect created at a given pore diameter by the surface tension of the exudate. According to Kelvin's Law, the relationship between the capillary force (positive or negative) inside the ceramic, the surface tension of the exudate, the degree of wetting of the ceramic by the exudate and the size of the pores in the ceramic is as follows:

ΔP = (4 γcos Θ)/ d (N/m2)
    ΔP = hydrostatic pressure (N/m2)
    γ = surface tension of the exudate (N/m)
    θ = wetting angle (degree of spreading) of a drop of exudate on the ceramic
    d = effective pore diameter (m)
As previously stated, there are three types of pores in the ceramic, namely
  • The bulk (>95% by volume) of the pores are cellular, with an average diameter of 5 micrometers
  • The pores in the ceramic phase have an average diameter of 2 micrometers
  • The average diameter of the blow holes connecting the cellular pores is 0,3 micrometers
The range of capillary forces in the micro-porous ceramic, as calculated from Kelvin’s Law, is tabled below
 
Pores Cellular Pores in ceramic Blow holes
Surface tension of exudate (γ) 0,07 N/m 0,07 N/m 0,07 N/m
Measured wetting angle (θ) 0o 0o 0o
Cos θ 1 1 1
Average pore diameter (x 10-6m) 5 2 0.3
       
Capillary suction (ΔP) (vacuum inside ceramic)      
(N/m2 = Pa) 56 000 140 000 933 333
(kPa) 56 140 933
(Psi) 7.8 19.6 130
 
  The potency of the forces locked up in the ceramic spheres is evident when compared with body-fluid forces like the systolic blood pressure of 16 kPa and capillary filling forces of 4 kPa.

The physical actions of the Cerdak devices

The physical action created by the ceramic in the devices is extremely simple, as can be seen from the following sketch.


  The wound produces exudate at a rate V1. The exudate with associated bacteria is absorbed by the sachet, and passes through it at the same rate (V2=V1). Upon reaching the ceramic, the exudate with the associated bacteria is absorbed at a rate V3, which is much faster than the rate of supply. This creates an air gap at the point of contact with the sachet. When this happens, the absorption process stops until more moisture is supplied by the wound, and then it repeats itself. Since the ceramic spheres are in point contact with surrounding ceramic spheres with a similar high suction potential, moisture migration between the ceramic spheres takes place continuously in an attempt to equalise the hydrostatic potential of all the spheres in the sachet. There is however no driving force for the exudate to leave the ceramic spheres, so that the interstitial air gaps between the spheres remain dry and filled with air. Mal-odourous gases are adsorbed on the high surface area of the ceramic.

Ceramic microstructure

The ceramic particles are spherical in appearance, with an average diameter of 0,7mm and a porosity of 75 volume per cent. The solid portion of the sphere forms a continuous ceramic phase that encloses three types of pores with distinctly different morphology. The largest pores have an average diameter of 5 micrometers and are cellular in appearance. These cellular pores are connected to one another by means of 0,3 micrometer diameter blowholes in the thinnest portion of the ceramic phase that surrounds the cellular pores. This ceramic phase has a total porosity of 30 volume percent, and the pores have an average diameter of 2 micrometers. Because of the interconnection between the pores, the high porosity is supplemented by a high permeability.

Surface area

As is normal for micro-porous materials with very fine pores, the ceramic has a high surface area. BET nitrogen adsorption indicates a specific surface area of 6 m2/gram.

Composition.

Apart from chemical stability, the raw material used for the production of Cerdak has an ability to preserve the optimum microstructure created during the forming process. In this respect, the material selected to manufacture the micro porous spheres is alumina. The biocompatibility of alumina is well known in the medical device industry in applications like prosthesis.
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2. THE WOUND ENVIRONMENT CREATED BY CERDAK
 
  The removal and storage of excess wound exudate and associated bacteria is a property that is common to many existing state-of-the-art wound dressings. The mechanism of exudate removal and storage by the ceramic device as described in the previous section, however, results in a unique wound environment with the following typical characteristics:
  • There is no continuous liquid phase between the exudate absorbed by and trapped inside the ceramic and the fresh wound exudate. As a result of the continuous redistribution of absorbed exudate between the ceramic spheres, the oldest exudate ends up furthest away from the wound. The freshest exudate in the ceramic, remains in direct contact with the air gap against the material of the satchet. This mechanism of exudate removal makes it impossible to drain the wound of fresh exudate, and the wound will therefore always have a moist environment of fresh exudate.
  • The bacteria present in absorbed exudate, will be concentrated on the surface of the ceramic spheres since they are larger than the finer connecting blow-holes. In this position the bacteria are exposed to atmospheric oxygen which surrounds the ceramic spheres.
  • The loose stacking of the ceramic spheres and the high capillary forces that lock the exudate into the body of the ceramic and hence keep their surfaces relatively dry, means that atmospheric oxygen can circulate in the interstices between the ceramic grains, and keep the moist wound aerated.
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3. PHYSICAL FACTORS THAT AFFECT THE OPERATIONAL EFFICIENCY OF THE CERDAK
  The main physical factors that will affect the efficiency of the ceramic wound healing devices, are the degree of saturation of the ceramic, external factors that affect the wetting behaviour of the ceramic by exudate and the direct contact between the device and the wound.

Degree of saturation of the wound

For a given surface tension, wetting angle and pore diameters, the magnitude of the capillary forces in the ceramic is directly proportional to the residual unfilled volume (empty pores) of the ceramic, which decreases as the ceramic fills up with exudate. It is therefore recommended that the device must be kept away from water and must be replaced when 50% saturated with exudate.


Wetting

When brought into contact with the ceramic, the exudate will wet the ceramic completely, and therefore has a wetting angle of 0 (cos 0o = I). If the ceramic becomes contaminated with a non-wetting agent like vaseline or fat, then the exudate will not wet the ceramic properly, and will form a spherical droplet at the point of contact (like mercury on a table). In such a case (θ > 90o), i.e cos θ will be negative and in stead of possessing a capillary suction, the ceramic will repel the liquid and an external pressure will have to be applied to drive the exudate into the ceramic. Cognisance must therefore be taken that ointments and oily substances must be avoided while the ceramic devices are used.

Contact with the wound

Ceramic wound healing devices can only operate when in direct contact with the wound. Poor application where the device does not contact or cover the wound properly, or where impediments like scabs, infected or dead tissue etc prevent proper contact, must be handled in the appropriate manner.
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