INTRODUCTION

P40 polyester plastination was designed to produce permanent preservation of tissue slices within a rigid, durable sheet of cured resin (von Hagens, 1994). These polyester plastinated specimens are for educational and research purposes (Barnett, 1997; Weiglein and Feigl, 1998; Henry and Weiglein, 1999; Sora et al., 1999; Latorre et al., 2002). Although designed for plastination of brain slices (Henry and Latorre, 2007), P40 plastination may also be used for plastination of other tissues (Latorre et al., 2004) as well as body slices (Latorre et al., 2007). P40 plastination relies on ultraviolet radiation to cure the resin. This study will examine the effects of different sources and delivery of UV light on the curing times and temperatures during curing of P40 resin.

MATERIALS AND METHODS

Glass casting chambers  were  constructed  as  described by Henry and Latorre (2007).  P40  resin,  without catalyst, was poured into the chambers and  the chambers were left open on one end. These chambers filled with  P40 resin were  exposed to different light sources until cured. All chambers were monitored for temperature changes and curing time during the experiment. Curing was monitored by inserting a wire through the open side of the chamber and into the resin. The three delivery methods of the light sources used for exposures included the following: uncontrolled, direct exposure; uncontrolled, indirect  exposure;  exposure under controlled conditions. Light source/light exposure combinations tested in this experiment included direct sunlight, indirect sunlight, controlled exposure to direct sunlight, controlled exposure to artificial UV-A light, fluorescent light, mercury light, controlled exposure to mercury light and exposure to combined fluorescent and mercury lighting. Temperatures  during  experiments were recorded and regulated when required using an Omega CN4400 thermo-regulator. Curing of the P40 resin was monitored  by  probing  the chamber contents with a wire and by external visual observations.

Experiment 1: Chambers exposed to direct sunlight were placed outdoors in sunlight on a non-cloudy day in June   in   Tennessee   (30  oC)   and   left   to   cure.   The temperature of the surface of the glass chamber and the progress of curing were monitored every 10 minutes.

Experiment 2: Chambers exposed to indirect  sunlight were placed outdoors in the shade on a non-cloudy day in  June  in  Tennessee  (30 oC)  and  left  to  cure.  The temperature of the surface of the glass chamber and the progress of curing were monitored every 10 minutes.

Experiment 3: Chambers under controlled exposure to direct sunlight were placed in the sun on a non-cloudy day in June in Tennessee (30 oC). The temperature of the surface of the glass chamber and the progress  of curing were monitored every 10 minutes. When the surface temperature of the casting chamber reached or exceeded 30oC, the chamber was moved into the shade and cooled with a fan to 25oC at which point it was returned to the direct sun.

Figure 1. Typical UV-A light source used to cure P40 resin.

Experiment 4: Chambers with controlled exposure to artificial UV light were placed 28.0cm  from  four  40 watt UV-A light bulbs (two above and two below) and cooled with a fan during the entire experiment (Fig. 1). The temperature of the surface of the glass chamber and the progress of curing were monitored  every  10 minutes.  When  the  surface  temperature  of  the  casting chamber  reached  at  least  30oC,  the  light  source  was turned off until the temperature of the chamber reached 25oC. At that point, the bulbs were turned on  once again.

Experiment 5: Chambers exposed to fluorescent lighting were placed on a table top and exposed to fluorescent lighting in a room with no windows.  The  fluorescent bulbs were located 150.5cm from the casting chamber. The temperature of the surface of the glass chamber and the progress of curing were monitored  every  10 minutes.

Experiment  6:  Chambers  exposed  to  direct  mercury light were placed within a cardboard shield 58.4cm beneath the mercury bulb. The shield was used to keep out fluorescent light which was also present within the room. The temperature of the surface of the glass chamber and the progress of curing were monitored every 10 minutes.

Figure 2. Mercury light source used to cure P40 resin.

Experiment 7: Chambers exposed to direct  mercury light were placed within a cardboard shield 58.4cm beneath the mercury bulb (Fig. 2) and cooled with a fan during the entire experiment. The shield  was  used  to keep out fluorescent light which was also present within the room. The temperature of the surface of the glass chamber and the progress of curing were monitored every 10 minutes. When the surface temperature of the casting chamber reached at least 30oC, the light source was turned off until the temperature of the chamber reached 25oC. At this point, the mercury light was turned on once again.

Experiment 8: Chambers exposed to combined fluorescent and mercury lighting were placed on a table top 233.7cm beneath  the mercury  bulb while also exposed to light from fluorescent bulbs within the room. The temperature of the surface of the glass chamber and the progress of curing were monitored  every  10 minutes. The chamber was 302.3cm  from  the fluorescent lights.

It should be noted  that curing of all chambers  was considered complete when all but the top 5mm of P40 resin was hard enough to resist the insertion of a metal wire. The P40 resin at the air-interface of an open chamber will not cure due to the resin being in contact with air.

RESULTS

Experiment  1:  The  P40  resin  in  chambers  exposed  to direct sunlight with no temperature regulation formed a solid gel by 20 minutes of exposure and cured completely within 40  minutes.  The  maximum temperature recorded during the curing process was 68oC and resulted in breakage of the glass used to form the casting chamber.

Experiment 2: The P40 resin in chambers exposed to indirect sunlight with no temperature regulation formed a solid gel by 40 minutes of exposure and cured completely within 80  minutes.  The  maximum temperature  recorded  during  the  curing  process  was 44oC.

Experiment 3: The P40 resin in chambers exposed  to direct sunlight under controlled conditions formed  a solid gel by 30 minutes of exposure and cured completely within 40  minutes.  The  maximum temperature  recorded  during  the  curing  process  was 40oC.

Experiment 4: The P40 resin in chambers exposed to artificial UV light under controlled conditions formed a solid gel by 30 minutes of exposure and cured completely within 40  minutes.  The  maximum temperature  recorded  during  the  curing  process  was 36oC.

Experiment 5: The P40 resin in chambers exposed to fluorescent lighting never exhibited any signs of curing or the onset of curing by 40 hours into the experiment. The resin  remained liquid  in form and  no  increase in surface temperature of the glass chamber was detected at any time. These chambers were exposed to fluorescent lighting for eight to nine hours per day for an additional month post experimentation and curing of the P40 resin was never initiated and the resin remained in liquid form.

Experiment 6: The P40 resin in chambers exposed to mercury lighting with no temperature regulation at close proximity formed a solid gel by 70 minutes of exposure and cured completely within 90 minutes. The maximum temperature recorded during the curing process was 34oC.

Experiment 7: The P40 resin in chambers exposed to mercury lighting under controlled conditions in close proximity to the light source formed a solid gel by 70 minutes of exposure and cured completely within 90 minutes.  The  maximum  temperature  recorded  during the curing process was 60oC.

Experiment 8: The P40 resin in chambers exposed to fluorescent lighting and mercury lighting, at the increased distance, formed a solid gel by 80 minutes of exposure and cured completely within 180 minutes. The maximum temperature recorded during the curing process was 24oC.

A summary of findings is listed in tables 1 and 2.

DISCUSSION

As expected, ultraviolet light sources provided the fastest cure rates of P40 resin as they are the suggested method of initiating the curing process. Cure rates were similar for both natural sunlight and artificial ultraviolet light sources when the P40 resin was placed directly within the light source. Curing P40 outdoors in  the shade doubled the time for curing when the times were compared to direct exposure to UV sources regardless of temperature regulation. P40 resin was not exposed to the artificial source of ultraviolet light in the absence of temperature regulation as it would result in the breaking of the glass of the chamber. It was discovered that placing the curing chambers in direct sunlight without temperature control would result in the breaking of the glass of those chambers as well.

Direct exposure to  mercury  lighting  produced  a curing time of P40 resin similar to that of  outdoor shade. Temperature control during curing of P40 with mercury lighting increased cure time by approximately 45% when compared to direct mercury lighting. When the distance from the mercury bulb was increased, the curing time was markedly increased.

Fluorescent lighting was found to have no effect on the curing of P40 resin. As no increase in temperature of the glass chamber was observed, no exothermic reaction as occurs in curing of P40 resin was present. The P40 resin used in this stage of the experiment was derived from the same stock used to fill all other casting chambers and should have cured if fluorescent lighting had an effect on P40 resin. After it was determined that fluorescent lighting would not cure the resin as it is not ultraviolet in nature, we set those casting chambers in the sun. The P40 resin in those chambers that did not respond to fluorescent lighting did cure in sunlight. This trial with fluorescent lighting was included within the experiment as it would be a common source of  light within a laboratory setting.

It has been suggested that P40 slices be cured a minimum of 1 hour (Henry and Latorre, 2007). While there was some relatively small variation in cure times, each of the ultraviolet light source trials (artificial or natural) reached the peak of the exothermic process within 70 minutes (range was 30 to 70 minutes). This variation can likely be explained by dissimilarities in sample positioning, time of day, atmospheric conditions and efforts to control excessive temperatures. The P40 resin was often cured to the point of resisting the insertion of metal wire prior to peak of the exothermic reaction. These trials confirm that approximately 1 hour of curing should be appropriate for ultraviolet light exposure to cure P40 resin.

The sample exposed to the combined mercury and fluorescent light sources (normal laboratory lighting in the institution hosting the experimentation) cured in approximately three hours. The increased  curing  time was due to the increased distance of the glass chamber from the source of light. Given the results of the fluorescent lighting trial, it is unlikely that the fluorescent source contributed any to the progress of the reaction. In order to expose P40 to mercury light only, the cardboard isolation shroud not only placed the glass chamber in close proximity  to the source of light but also increased the temperature in and around the glass chamber due to the shroud holding in any heat produced by the mercury bulb or the exothermic curing reaction. Thus, the maximum reported temperature for the unregulated exposure to mercury lighting is most likely artificially increased. It would be unnecessary for one to use such a shroud to cure P40 as mercury lighting will cure at a great distance. It would not be proper to use artificial UV light to cure P40 at a distance as the exposure of the light to one’s eyes is detrimental. For the combined mercury and fluorescent lighting trial, the glass chamber was simply placed on a work table in the middle of the lab in order to approximate normal conditions likely to be encountered during assembly, sample positioning, etc. Although the mercury light source should be producing the same amount of ultraviolet radiation in both trials, the waves will be spread over a larger surface area at the increased distance. As a result, the concentration of ultraviolet waves per unit area in the combined trial would be less. The inverse square law can be used to determine the influence of distance on ultraviolet ray exposure.

Mercury lights will also lose intensity as they age so if one were to use such a source, cure rates should be expected to increase as the mercury bulb ages.

While it is unlikely that anyone would  use  one  of these alternative light sources to cure P40 samples, knowledge of their respective cure rates, and that of ultraviolet sources, is of practical value. Given the absence of an effect of fluorescent lighting  on  P40 resin, it is reasonable to avoid rushing during sample preparation. Experimenters should be mindful of any mercury lighting which may be present within a laboratory or of any sunlight  entering  a  window whether it be a direct ray or merely ambient lighting. Additionally, the knowledge of expected cure time is useful when planning monitoring frequency. One may also wish to use mercury lights for curing of P40 resin should an artificial UV light apparatus be cost prohibitive or not allowed due to safety concerns.

The times recorded for the solid gel formation within the resin should be heeded as once this gel has formed, any tissue samples that may have slipped  within  the resin (i.e. to the edge of the chamber or into a second tissue sample within the chamber) may not be repositioned.