Mark Waters

Analysis of the properties of silicone rubber maxillofacial prosthetic materials

OBJECTIVES Maxillofacial prosthetic materials are used to replace facial parts lost through disease or trauma. Silicone rubbers are the materials of choice, however it is widely accepted that these materials do not possess ideal properties. The objective of this study was to assess the properties of a range of commercially available silicone rubber maxillofacial materials and make recommendations for improvements. METHODS Specimens of five commonly used maxillofacial materials were prepared in dental flasks according manufacturers instructions. Tear strength, tensile strength, percentage elongation, hardness, water absorption and water contact angles were determined for each material. RESULTS The tear strength of Factor II, Cosmesil HC and Nusil were all comparable and significantly higher than Cosmesil St and Prestige (p<0.001). Nusil had a significantly higher tensile strength and elongation in comparison to the other materials (p<0.001) and Cosmesil St and Cosmesil HC were significantly harder (p<0.001). Factor II was significantly less wetted and Prestige and Cosmsesil St had a significantly higher water absorption in comparison to the other materials. CONCLUSIONS None of the commercially available silicone rubber materials possessed ideal properties for use as a maxillofacial prosthetic material. Factor II, however, showed more favourable properties due to its high tear strength, softness and ease of manipulation.

Adherence of Candida albicans to experimental denture soft lining materials

STATEMENT OF PROBLEM Colonization of denture soft lining materials by Candida albicans can result in clinical problems. Two experimental silicone soft lining materials have been developed and demonstrate good physical properties. PURPOSE OF STUDY The aim of this study was to determine the extent of candidal adherence to these materials compared with a commercially available soft lining material and an acrylic resin denture base. MATERIAL AND METHODS The experimental materials were constructed in a stainless-steel mold, and their surface energies were determined with a dynamic contact angle analyzer. The adherence of three strains of C. albicans was determined with an in vitro assay. In addition, one test strain was used to determine the effect on adherence of precoating the materials with whole saliva. RESULTS Adhesion to all materials was strain variable, with the lowest adherence recorded for the two experimental materials. Decreased adherence was also apparent after precoating the materials with saliva. Correlation between surface energy of the material and the degree of candidal adhesion was seen for one strain, but no correlation was seen for the other two strains. CONCLUSION Adherence of C. albicans to the two experimental silicone soft lining materials was significantly less than that for an acrylic resin denture base and a commercially available soft lining material.

An elastomeric material for facial prostheses: synthesis, experimental and numerical testing aspects

Current materials used for maxillofacial prostheses are far from ideal and there is a need for new improved materials which better simulate the tissues they are replacing. This study was based on a mixed experimental/analytical/numerical approach. A new polymeric material was developed to provide a better alternative to the materials currently used in maxillofacial prosthetics. A series of experimental tensile tests were performed in order to characterise the tensile properties of the material. A Mooney-Rivlin type hyperelastic formulation was chosen to describe the constitutive behaviour of the polymer which operates at the finite strain regime. The material parameters (two) of the constitutive law were identified with the experimental data. The Mooney-Rivlin material was found to be suitable to represent accurately the mechanical behaviour of the polymer up to 50% strain as shown by the excellent agreement between analytical and experimental results. An FE model reproducing all the characteristics of the experimental tensile tests was built and a series of three FE analyses were conducted and has proven the proper finite element implementation of the material model. This preliminary study will serve as a basis to introduce more complex features such as viscoelasticity and wrinkling of the soft polymeric structure in order to optimise the performances of the final prosthetic material.

Mechanical properties of an experimental denture soft lining material

OBJECTIVES The objective of the present study was to evaluate the mechanical properties of an experimental formulation (Exp. 1) in order to assess its potential as a denture soft lining material. The same properties of a popular commercially available denture soft lining material (Molloplast-B) were determined and compared with the properties of Exp. 1. METHODS Exp. 1 specimens were obtained by curing for 24 h at room temperature after the addition of the appropriate amounts of catalyst and cross-linker. Molloplast-B specimens were obtained after curing according to the manufacturers instructions. The properties measured in the study were hardness, tear resistance, tensile strength and the bond strength of the material to a heat-cured acrylic denture base material. RESULTS Exp. 1 had a significantly greater tensile strength, percent elongation, tear resistance and peel strength (p < 0.0001) than Molloplast-B. There was no significant difference in the hardness values of the two materials, although Molloplast-B had a significantly higher tensile bond and shear bond strength (p < 0.05). CONCLUSIONS It was concluded that there was no significant difference in the hardness of Exp. 1 and Molloplast-B. Exp. 1 had superior tensile and tear properties. Its peel bond strength was superior to that of Molloplast-B, although its tensile bond strength and shear bond strength were less.

Wettability of silicone rubber maxillofacial prosthetic materials

STATEMENT OF PROBLEM Maxillofacial prosthetic materials that contact skin or mucosa should have good wettability. A material that is easily wetted will form a superior lubricating layer between the supporting tissues and, thus, reduce friction and patient discomfort. The surface energy of a maxillofacial prosthetic material will give an indication of the amount of energy available for adhesion and of the susceptibility of the material to bacterial adhesion. PURPOSE This study evaluated the wettability and surface energies of a range of commercially available silicone rubber maxillofacial prosthetic materials. MATERIAL AND METHODS Contact angles and surface energies were measured by using a dynamic contact angle measuring technique. Four commonly used silicone maxillofacial materials were tested and their properties compared with those of an acrylic resin denture base material and a widely used denture soft lining material. RESULTS There were no significant differences in the wettability of the silicone rubber materials. All materials were significantly less wetted than the denture acrylic resin material. There were no significant differences in the surface energies of the silicone rubber materials, but all were significantly lower than denture acrylic resin material. CONCLUSIONS The Cahn dynamic contact angle analyzer was a quick and reproducible method for determining the contact angles and surface energies of maxillofacial materials. Further work is needed to improve the wettability of silicone rubber materials used for maxillofacial prostheses, thus, reducing their potential to produce friction with tissues.

A clinical assessment of the performance of a sensor to detect crystalline biofilm formation on indwelling bladder catheters

To test the ability of a sensor developed to signal infection by the organisms that generate the crystalline biofilms that encrust catheters, to give an early warning that encrustation was occurring on patients’ catheters, as the care of many patients undergoing long‐term bladder catheterization is complicated by the encrustation and blockage of their catheters

Effect of surface modified fillers on the water absorption of a (RTV) silicone denture soft lining material

OBJECTIVES The purpose of the present study was to modify the filler content of an experimental room-temperature vulcanizing (RTV) lining material that demonstrated high water sorption in order to produce a material with low sorption properties. METHODS Three new formulations were prepared, each containing different hydrophobic silane-treated silica fillers. Water sorption properties for specimens obtained from these formulations were determined using standard experimental techniques. RESULTS All formulations demonstrated greatly reduced water absorption and low volume change. CONCLUSIONS An experimental RTV poly(dimethylsiloxane) denture soft lining material having low water sorption properties has been produced.

A Sensor To Detect the Early Stages in the Development of Crystalline Proteus mirabilis Biofilm on Indwelling Bladder Catheters

ABSTRACT A simple sensor has been developed to detect the early stages of urinary catheter encrustation and avoid the clinical crises induced by catheter blockage. In laboratory models of colonization by Proteus mirabilis, the sensor signaled encrustation at an average time of 43 h before catheters were blocked with crystalline biofilm.

The evaluation of new multi-material human soft tissue simulants for sports impact surrogates

Previous sports impact reconstructions have highlighted the inadequacies in current measures to evaluate the effectiveness of personal protective equipment (PPE) and emphasised the need for improved impact surrogates that provide a more biofidelic representation of human impact response. The skin, muscle and subcutaneous adipose tissues were considered to constitute the structures primarily governing the mechanical behaviour of the human body segment. A preceding study by Payne et al. (in press) investigated the formulation and characterisation of muscle tissue simulants. The present study investigates the development of bespoke blends of additive cure polydimethysiloxane (PDMS) silicones to represent both skin and adipose tissues using the same processes previously reported. These simulants were characterised mechanically through a range of strain rates and a range of hyperelastic and viscoelastic constitutive models were evaluated to describe their behaviour. To explore the worth of the silicone simulants, finite element (FE) models were developed using anthropometric parameters representative of the human thigh segment, derived from the Visible Human Project. The multi-material silicone construction was validated experimentally and compared with both organic tissue data from literature and commonly used single material simulants: Dow Corning Silastic 3480 series silicones and ballistics gelatin when subject to a representative sports specific knee impact. Superior biofidelic performance is reported for the PDMS silicone formulations and surrogate predictions.

Wettability of denture soft-lining materials

Data on the wettability of long-term resilient denture base lining materials have previously been obtained by static methods. This study determined the dynamic contact angle of a range of long-term resilient lining materials and the surface free energy properties of these materials. This method was a rapid and convenient method for determining wettability parameters. The equilibrium contact angle revealed the Flexor and Novus materials to be the least wetted with the Molloplast-B material, and CoeSuper-Soft material was wetted similarly to Trevalon material. The contact angle hysteresis indicated that all the soft-lining materials tested would improve denture stability under dislodgment forces. The surface-free energies appeared to be similar for all the soft-lining materials but were significantly lower than that of Trevalon.