Automated Powder Dispensing Technology
Authored by: BioDot Inc.
Reprinted with permission by BioDot Inc.
The DisPo Powder Dispensing technology involves the volumetric delivery of dry powders and solids. This technology can be employed on several different platforms to create automated workstations for dispensing powders and solids. Because the powder dispensing approach is the same for all of the available platforms we offer, this Technical Note will focus on describing the technology and providing supporting characterization data to support its use in a wide variety of applications.
The ability to dispense dry powders and solids is a valuable tool for materials discovery, development and optimization. There are few currently available options for automated manipulation of dry powders, particularly in the microgram mass range. The DisPo Powder Dispensing Technology provides an automated means to deliver a wide variety of powders over the mass range of 100 micrograms to 20 milligrams. Powders are dispensed via a volumetric delivery from a sample probe. This volume, combined with the bulk density of the powder being sampled, determines the mass that is delivered. There are several sample probe choices to chose from, based on the target mass (and therefore volume) to be delivered. Figure 1 shows a photograph of the 5 different probes available for use with this technology.
|Figure 1: DisPo sampling probes.|
The sampling of powders can occur from many different source vessels, including microwell plates, scintillation vials, dram vials, and tube-based storage systems. Delivery can occur to the same formats as well. Figure 2 shows schematically how powders are sampled and delivered.
|Figure 2: Sampling Process.|
There is an adjustable plunger within the sampling probe that allows for the setting of a “fill height” within the probe. This fill height and the diameter of the probe determine the sampling volume of powder. The sample probe is lowered into the powder, where it extracts the determined volume of powder. This process is indicated by the first 3 steps in Figure 2. The probe is then moved out of the source location and moved over to the destination location. Note that the plunger is still retracted. Once over the destination location, the plunger is actuated and the powder is delivered into the source location. This process is indicated by the last 3 steps in Figure 2. This entire process is automated and programmable though the control software interface of the Systems.
In order to demonstrate effectiveness of powder dispensing with this technology, 20 different powders were characterized. These powders were chosen to be representative of the typical powder properties to be encountered in real world applications. Some of these properties include flowability (good and poor), granularity (course and fine), crystallinity (crystalline and amorphous), particle size (small and large) and bulk density (low and high). The ability and performance of delivering a range of powder volumes/masses, both within a sample probe and between sample probes with different diameters, was also studied.
Methods and Materials
For the characterization of the DisPo Powder Dispensing Technology, the following powders were used: cab-o-sil, Avicel PH101, garlic powder, magnesium stearate, ibuprofen, powdered sugar (10x), cocoa, cinnamon, acetaminophen, silica gel, corn starch, granulated sugar, flour, baking soda, acetylsalicylic acid, talcum powder, baking powder, grout, table salt and naproxen sodium. All of the powders used in generating the results reported here were either obtained as direct samples from manufacturers or were purchased from distributors or vendors (e.g. Sigma- Aldrich). Correlation of dispensed volume to mass was determined by dispensing discrete volumes and weighing using a calibrated analytical balance (Mettler-Toledo AG245). All measurements were made in triplicate, at a minimum, so that percent coefficient of variation (%CV) values could be determined. Five different diameter sample probes were used: 0.5mm, 0.8mm, 1.0mm, 2.0mm, and 3.0mm nominal inner diameter. For determination of exact volumes, probe diameters and fill heights were measured by vernier calipers.
Figure 3 shows the results of dispensing corn starch with each of the 5 different diameter sampling probes at the same nominal fill height. For calculations, actual fill heights and diameters as measured with vernier calipers, were used.
|Figure 3: Delivered mass vs. probe diameter.|
These results demonstrate the ability to deliver from an average of 0.14 to 5.49mg of powder using the DisPo Powder Dispensing Technology. For the 15 measurements made here the average %CV in delivered mass was 8.8%. As will be shown later, these average masses do not represent the absolute mass limits of the technology, but they do cover the practical mass range that could be delivered on a discrete dispense basis. The data in Figure 3 was regressed with a squared function to determine the goodness of fit to the theoretical dependence on crosssectional area (i.e. radius squared). The regression line is shown in Figure 4.
|Figure 4: Regression of delivered mass vs. probe surface area.|
In addition to being able to deliver different volumes/masses by changing sample probe diameters, changes in probe fill height can be made. Figure 5 shows the results of dispensing corn starch by changing fill heights with the 3mm diameter sample probe.
As can be seen, a change of fill height from 1.02mm to 2.77mm, a 2.72-fold change, yields a mass increase from 5.49mg to 15.16mg, a corresponding 2.73-fold increase. The data from Figure 3 and Figure 5 demonstrate the ability to cover two orders of magnitude of mass range, in a discrete dispense, by varying probe diameter, probe fill height, or a combination of both. Larger masses can be dispensed by simply performing multiple dispenses at a given mass. With this approach, masses in the hundreds of milligram range can easily be delivered. To further characterize this technology, a panel of 20 different powders was dispensed with the 0.8mm diameter probe, at the same nominal fill height for all powders. Each powder sample was dispensed in quadruplicate. The result of this panel of powders is shown in Figure 6.
|Figure 5: Delivered mass vs. fill height.|
|Figure 6: Delivered mass vs. powder.|
The overall variability in dispensed mass was 12% CV across all 20 powders. It is important to note that the largest source of variability in dispensed mass is the variation in powder bulk density. This can be seen more definitively in Figure 7, which shows the average delivered mass vs. powder bulk density for 19 different powders.
|Figure 7: Delivered mass vs. powder bulk density.|
The data in Figure 6 and Figure 7 indicate that a wide range of powder bulk densities can be accommodated with the DisPo Powder Dispensing Technology. While the absolute delivered mass of a powder at a fixed sample volume will vary with bulk density, it is possible to adjust the target volume to deliver a specific mass. This can be achieved by a simple calibration with the powder of interest, or can be approximated if the bulk density of the powder is known. A key consideration for dispensing powders is the minimum amount of sample required. This minimum amount is the amount of material that must be present in order to sample and dispense a given mass of powder. While this minimum sample amount is somewhat related to the target mass to be dispensed (in that this dictates the sample probe size, which in turn determines some minimal vessel geometry), the ideal scenario is to be able to sample from as small a starting mass as possible and to be as efficient in the use of this material as possible. Figure 8 shows a photograph of both an empty 384 MatritubeTM and a tube containing 8.06mg of naproxen.
|Figure 8: Comparison of empty 384 Matritube and tube containing 8.06mg of naproxen.|
Delivering low mass samples or “doses”, particularly from these small starting masses, is also a common consideration. A typical application would involve delivering 20-50 individual samples of 50-100μg from the same sample vessel. Figure 9 shows ~160μg doses of naproxen that have been sampled from the tube shown in Figure 8.
Figure 9A shows the 160μg doses as individual samples, and Figure 9B shows the samples in wells of a 384-well plate. Despite the small starting mass and the very low target mass to be delivered in these applications, this technology provides very reproducible results. Figure 10 shows a plot of delivered mass versus dispense number for delivery of a nominal 130μg dose sampled from 9.03mg of naproxen (in the same tube as shown in Figure 8).
|Figure 9: Photographs of 160μg samples of naproxen.|
|Figure 9a: individual samples.||Figure 9b: samples in wells of a 384-well plate.|
|Figure 10: Delivered mass versus dispense number for low mass delivery of naproxen.|
For the data shown in Figure 10, the average delivered mass was 140mg and this mass was delivered with an 11% CV. A final consideration in powder dispensing applications is the efficiency of use of the starting mass of powder. Ideally 100% utilization of the material provided would be possible. Due to practical considerations, of course, this is not possible. However, assuming the ability to recover and re-use sample from starting vessels, this technology is very efficient in terms of utilization of starting material. Table 1 shows results that are typically obtained.
|Starting mass (mg)||10.03|
|Total delivered mass (mg)||3.99|
|Recovered mass (mg)||5.17|
|Unrecovered mass (mg)||0.76|
|Process waste (mg)||0.11 %|
|% Sample recovered||52%|
|% Sample utilization||91%|
For the data in Table 1, “Total delivered mass” is the sum of the individual dispensed samples, “Recovered mass” is the amount of starting material that was able to be removed from the sample vessel, “Unrecovered mass” is the amount of material that was not able to be easily recovered (e.g. on the walls of the vessel), and “Process waste” is the amount of material that was truly wasted (i.e. not delivered to a target or recoverable). As can be seen, greater than 90% of the initial starting mass is typically able to be utilized. The data in Table 1 is from the previously described challenging application of working with a relatively small starting mass. Small starting masses dictate sample vessels with a large surface area to volume ratio. As a result greater than normal amounts of material are “unrecovered” due to interaction with the vessel walls. In most applications, the % utilization will exceed 95%, as sample vessels with less surface area to volume can be employed.
The DisPo Powder Dispensing Technology provides an automated means of delivering dry powders and solid materials for a variety of applications. The characterization data presented here indicates that the technology can be applied to a wide range of powders, with varying physical properties. With simple calibration, target masses can be delivered in the 100μg to 20mg range with a precision of 10-15 %CV.
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