Evaluation of microplastics sediment sampling techniques—efficiency of common methods and new approaches

Abstract

Common sediment samplers for microplastics (MP) such as grab samplers or corers are limited to certain grain sizes and known to cause disruption of sediments which results in a loss of fine and low-density particles such as MP. However, this loss has not been quantified yet and its occurrence is commonly tolerated during MP sediment sampling. In the present study we evaluate the recovery of MP of various common sediment samplers used in most recent studies. The samplers were tested on a model plant simulating a riverine environment with MP spiked sediments. Also, we investigated the feasibility of less frequently used freeze coring. The results of this study suggest that a combination of common methods is crucial in order to sufficiently evaluate a sampling site until standardized MP samplers for sediments are available. Freeze coring indicates a promising potential to monitor MP in river sediments in the future but is costly and should be optimized for regular field sampling campaigns.

Graphical Abstract

Introduction

Reports on microplastics (MP) in aquatic environments, even in remote polar regions, have increased worldwide over the last decade. Rivers are known as a major source for transportation of polymer-based waste into other freshwater systems and oceans [16]. It is estimated that only 1% of plastic which enters marine environments remains on the water surface [15]. Most MP sink to deeper water layers or accumulate in sediments due to biofilm formation or interaction with suspended particles [3, 9, 11, 24]. Especially in riverine sediments systems MP have been evaluated as emerging threats as they affect biota and may potentially alter the physicochemical and biophysical sediment composition [10, 25, 30]. Therefore, it is essential to collect representative environmental sediment samples to understand transport processes of MP in rivers and the fate of MP within sediment biota [29].

Sampling MP in sediments is challenging and sets apparent limitations to characterise MP distribution in sediment environments. Whether MP are sampled from the river bank or riverbed is determined by the object of research, the availability of equipment and staff. Moreover, MP accumulation in specific areas depends on the presence of suspected sources of contamination and is influenced by the bank-area, vegetation, sediment type and hydraulic conditions [8]. Thus, prior to sampling, the site should be carefully observed to identify potential sampling locations.

Selecting an appropriate field sampling technique depends on the sediment matrix and the MP particle size distribution targeted [29]. Traditional techniques including various coring devices or grab samplers are available to sample sediments such as clay, silt, sand and gravel with various grain size distributions [29]. In general, MP sediment samplers should not consist of plastic material to avoid sample contamination. Especially during sampling, sediments can be easily contaminated by abrasion of plastic material of the sampler caused by friction of sharp sediment grains. Preferably, the sampler should be manually operable and not require extensive training to be utilized in regular cost-efficient field campaigns rather than single elaborated field studies.

Standardized tools such as grab samplers or corers as well as simple tools such as shovels or spoons were reported in recent MP research studies [1, 20]. Most of these studies applied a grab sampling technique to collect sediment samples followed by a variety of corers and shovels. Those are either limited to certain grain sizes or known to cause sediment disruption during sampling. This results in particle loss and consequently to underestimates of fine and low-density particles such as MP. From a risk assessment perspective, thus the occurrence of MP in freshwater sediments might be underestimated.

To date, this loss has not been quantified and is tolerated to occur during MP sampling. In this study we aimed to investigate MP loss of various sediment sampling methods used in most recent MP studies. The efficiency of the techniques was investigated using a model plant containing MP spiked sediments that simulates a sample environment with defined flow conditions. Also, we assessed the feasibility of less frequently used sampling techniques including freeze coring and a suction method.

Materials and methods

Sediments and Microplastics

To investigate whether the density and size of plastic particles influence the sampling process, two polymers of various densities and sizes were used as spiking material (Table 1). For the size effect and to ensure consistent spiking, spherical particles were chosen. According to the manufacturers all spheres are produced as Grade I precision spheres with a diameter tolerance range up to ± 0.025 mm [5]. As a low-density material, representing particles floating in the water column, polypropylene (PP) was selected. This polymer type is widely used in packaging or in garments and home textiles, and thus often detected in environmental sediment samples [18]. Polyoxymethylene (POM) was selected as high-density material as representation of MP particles sinking onto the sediment layer. This polymer type is not usually found in environmental samples, because it is not as widely applied as e.g. polyethylenterephthalate (PET) [18]. POM is often a component in pharmaceutical equipment, pumps or valves due to its resistance to abrasion as well as chemicals and low moisture absorption [23]. As spherical particles of Grade I were not commercially available for PET, we decided to apply POM particles due to density with 1.41 g/cm³ which is in a similar range to PET (1.38 g/cm³) [23].

Table 1 Properties of MP that were placed into boxes

To investigate the influence of sediment composition on MP sampling, two model sediments, medium quartz sand (d₁₀ = 0.12 mm – d₉₀ = 0.55 mm) and fine gravel (d₁₀ = 2.06 mm – d₉₀ = 4 mm), were tested (Table S1).

Preparation of spiked sediments

The MP were placed into polyethylene boxes (38.5 × 20.0 × 29.5 cm) containing wet sediment at 5 cm depth below surface. Perforated metal plates (38.0 × 29.0 cm; 18.5 × 13.5 cm) containing 1 cm × 1 cm square holes and a bridge width of 0.5 cm were used to define their exact location. This process was necessary to ensure particles were spread throughout the sediment equally. The particles were placed in the middle of each square hole at a defined area depending on the sampler to be tested (Table 2). The area was defined according to the unique sampling area of each sampler. Subsequently, MP were carefully covered with wet sediment and a second layer of MP with a different particle size of the same polymer type was placed at 2 cm below the surface of the sample box. Each sampling process was conducted three times. To access the depth-based sampling efficiency for standardized sediment samplers, MP particle positions were switched: those which were placed at 5 cm were placed at 2 cm and those at 5 cm were placed at 2 cm. Above the 2 cm layer the boxes were filled to the upper edge with sediment and placed into a water filled model plant (Fig. 1). Regarding non-standardized sampling tools a single layer of MP at 2 cm was prepared as no undisturbed sample could be recovered.

Table 2 Box and grid layout of MP in sediment filled boxes, green squares: square holes with MP, red squares: square holes without MP, for coring and suction techniques box areas were divided into four equal parts and a smaller perforated metal plate was used in each section, for freeze coring, the smaller perforated metal plate was placed in the middle of the box and MP were placed into four different sediment layers

Fig. 1

Set-up of model plant to simulate sampling in a riverine environment (dimensions in cm), MP spiked sediments of medium sand or fine gravel were exchanged and the plant was cleaned after each experiment to prevent cross-contamination

Model plant operation and sampling

The model plant consists of two sections. The first section comprises a storage tank (1 m³) with a freshwater feed. The storage tank is connected to the sampling area containing a pump (M5011A, FLYGT Xylem, Germany, Langenhagen) via three suction pipes. To establish stable flow conditions the storage tank and the sampling area were filled with 0.8 m³ and 0.13 m³ tap water. Depending on the hydraulic head difference, a steady flow velocity between 0.04 and 0.01 m/s was generated by evacuating the suction pipes using a vacuum pump (Laboport N840, KNF GmbH, Freiburg, Germany) and starting the pump in the sampling section. The flow velocity in the sampling section is comparable with low flow conditions near a shore of a riverine or lake environment.

The sediment boxes were placed into the model plant at a water depth of 20 cm. After a continuous overflow period of 60 min sediments were sampled using either a Van Veen grab sampler, sediment corer, percussion corer or stainless-steel shovel. During sampling, samplers were lowered into the sediment under flow conditions ensuring the sample area fully covered the area of placed MP.

A prototype freeze corer was tested using liquid nitrogen (-196 °C) and a small-scale steel-copper lance (Figure S4). For freeze coring an MP spiked box containing medium sand was placed at a depth of 13 cm into a 0.065 m³ container containing freshwater (23.6 °C). Next, the prototype corer was inserted into the sediment and 2 l of liquid nitrogen were filled into the copper tube. After a contact time of 15 min the corer was removed from the sediment. Immediately after the freeze core was obtained, weight and diameter of the core were determined.

Laboratory processing and analysis of results

Each sampler excluding the freeze corer covers a specific sample area when lowered into the sediment (Table S2, Figure S4). Using the perforated metal plates to place the MP, the specific location and the amount of sampled MP can be determined when the sampler penetrates the sediment on a defined location (Fig. 2). This fact results into two possible scenarios:

• (1) MP particles at the edge of the sample area are captured during sampling = MP_max

• (2) MP particles at the edge of the sample area are not captured during sampling = MP_min

Fig. 2

Demonstration of MP_min (right) and MP_max (left), red circles are defined as MP retained in samples, grey circles are defined as MP not retained in samples

Immediately after sampling, samples were placed into 2 l—stainless steel bowls and covered with aluminium foil. Samples and the remaining sediment in the boxes were dried in an oven at 60 °C for 2 days. Then the dry mass of the samples was determined by a laboratory scale (M-Pact AX6202, Sartorius, Göttingen, Germany). Separation was realised by sieving (stainless steel wire mesh sieves, sieve diameter 200 mm, mesh sizes 2 mm, 1.6 mm, 0.8 mm, 0.71 mm, Retsch GmbH, Haan, Germany) and visual sorting. UV light (200 – 400 nm) was applied for visual sorting of the white coloured MP particles. After separation particles were photographed and counted using the open access image processing software ImageJ (National Institutes of Health, Wisconsin, USA) to avoid counting errors.

Results and discussion

Efficiency of standardized sediment samplers

Out of the grab samplers, the Van Veen, Ponar and Petersen are the most commonly used and effective in most types of surface sediments [27]. Van Veen grab samplers are regularly utilized in several national and international MP monitoring programs. According to Klemm [13] they can be utilized to sample most types of sediment, are less likely to block and loss of sample than Peterson or Ponar grab samplers, mostly prevent the formation of a bow wave which displaces the often light or flocculent surface layer of many sediments during descent and keep samples mostly intact.

In this study, a Van Veen grab sampler with an open surface area of 250 cm² was used. The sampler should recover at least 100 particles (MP_min = 80%), ideally 120 particles (MP_max = 100%). In medium sand MP abundances were determined in average between 76.8 ± 21.2% up to 87.3 ± 8.3%, in fine gravel values ranged between 39.2 ± 25.7% up to 83.9 ± 11.2% (Figs. 3 and 4). The variability when repeating the sampling process multiple times is due to the sampler was lowered with a rope into the sampling box (Figure S2). Because of its net weight, the grab sampler slightly oscillates when lowered into the sediment.

Fig. 3

Recovery of POM and PP particles in medium sand with a Van Veen grab sampler (n = 24), a sediment corer (n = 24) and a stainless-steel shovel (n = 12). Grey area indicates MP_min and MP_max as expected values

Fig. 4

Recovery of POM and PP particles in fine gravel with a Van Veen grab sampler (n = 24), a stainless-steel shovel (n = 12) and a percussion corer (n = 12). Grey area indicates MP_min and MP_max as expected values

Smaller particles such as POM 1 mm or PP 2 mm were lost when recovered through the water column. Abundances of large particles such as PP 4 mm show higher particle recovery rates in fine gravel. Due to the fact that the mean grain size (d₅₀) of fine gravel is 2.9 mm, PP 4 mm particles were probably retained by sediment grains. The sand with a mean grain size of 0.29 mm was finer than all MP diameters used in this study. This implies, that MP sampling is not only affected by sediment properties but is also connected to MP size and probably morphology as well. The average decrease of smaller particles in fine gravel in comparison to medium sand is due to the fact that jaws of the Van Veen grab sampler did not close sufficiently and also the sampler was not able to sink as deep into the sediment than into the medium sand. This relation becomes clear when dry masses of both sediment types recovered by the sampler are compared (Fig. 5). The median mass in medium sand is 2066.5 g while in fine gravel only 1534.5 g were obtained. Based on the number of outliers shown in the dataset of fine gravel dry masses the number of outliers underlines the limitation of the Van Veen grab sampler in fine gravel. According to U.S. EPA [27], penetration into hard sediment bottoms can be improved by adding weights to the sampler. Nevertheless, fine material including MP will be lost due to coarse sediment material will be caught in the grab samplers jaws.

Fig. 5

Dry masses of medium sand and fine gravel recovered with a Van Veen grab sampler (n = 24), sediment corer (n = 24), stainless steel shovel (n = 12) and percussion corer (n = 12)

The limitations of MP sampling using a Van Veen grab sampler arise additionally when comparing the average particles recovered at 2 cm and 5 cm sampling depths (Fig. 6). In medium sand average particle abundances ranged from 69.2 ± 46.0% to 91.9 ± 2.9% and in fine gravel from 15.6 ± 10.3% to 90.3 ± 3.9%. These data imply once more that the efficiency of grab samplers varies with different depths depending on their size, weight, the position, size of MP, and the composition of bottom substrate. Additionally, the morphology might affect recovery of polymer particles. Waldschläger and Schüttrumpf [28] investigated the infiltration behaviour of MP particles with different densities, sizes and shapes using a column filled with glass spheres. According to the authors results, spherical particles infiltrated deeper than fragments or fibres. This behaviour might be also transferred to the sampling efficiency. If the jaws of the Van Veen grab sampler do not close sufficiently, water will be flowing out generating a filtration effect in the sediment when the sample is retrieved from the water column. This might lead to a loss of spherical shaped particles, MP present in deeper sediment layers and smaller sized particles (< 1 mm) leading to an underestimation of particles present in the sample.

Fig. 6

Average number (n = 3) of MP recovered with a sediment corer at 2 cm and 5 cm depth in medium sand and fine gravel with a Van Veen grab sampler and a sediment corer

In addition to grab samplers, core samplers are less disruptive and maintain the integrity of a sampled sediment profile [2]. To compare the Van Veen grab sampler used in this study with a coring model, a sediment corer of similar size and weight of the same manufacturer was used (Table S2). The sampler can be equipped either with a Poly(methylmethacrylate) (PMMA) sampling tube or a stainless-steel tube. The stainless-steel tube was utilized for MP sampling in this study. With a diameter of 7.2 cm the stainless-steel tube has a sampling area of 41 cm². Thus, MP_min and MP_max values ranged from 12 (60%) to 20 (100%) particles (Fig. 2). Because the water pressure at the valve flap closes the tube when the sampler is lifted from the sediment, the manufacturer recommends to use the sampler in soft and sandy sediments. The vacuum holding the sample in the tube and preventing washout could not be obtained when sampling was conducted in fine gravel. Hence, solely data for medium sand samples could be generated. The average number of sampled MP in the core samples ranged from 73.3 ± 13.7% to 85.0 ± 10.0% (Fig. 3). With an exception of one datapoint (PP 2 mm, 50.0%) all MP were recovered between MP_min and MP_max.

Similar to the Van Veen grab sampler, sediment dry masses in medium sand were mostly constant (Fig. 5). Due to its smaller sampling area the median amount of sediment collected was only 629.2 g and thus more than two times lower than the samples collected with the Van Veen grab sampler. Yet, the sediment amount collected in this study was also limited by the depth of the sampling box (29.0 cm) and is expected to be higher when applied in the field.

To collect an appropriate amount of material at a special depth to analyse MP, repetitive sampling at the pilot sited might be required to obtain the desired quantity of sediment from the depth of interest.

Regarding the influence of sampling depth in medium sand no difference was observed between particles placed at 2 cm and 5 cm depth below the surface of the sample box. Average abundances of MP at 2 cm ranged from 70.0 ± 14.7% to 88 ± 10.2% and at 5 cm from 70.0 ± 10.8% to 86.7 ± 12.4%. These data confirm, that core samplers are useful for research that focuses on depth-based MP contamination but are also prone to a loss of MP as statistical variation of recovery rates and the datapoint below MP_min and MP_max in Fig. 3 indicates. These include a hydraulic shock wave at the sediment surface before the sampler is inserted into the sediment, as well as penetration disturbances, shortening, bending, smearing, tilting and liquefaction of the core sample [6]. Nevertheless, the core sampler seems to be more efficient at recovering particles from medium sand in comparison to the more varied recovery rates of the Van Veen grab sampler (15.6 ± 10.3 to 90.3 ± 3.9%).

Efficiency of non-standardized sediment samplers

At sites with shallow water conditions or coarse bottom material, sediment sampling with grab samplers or a corer might be difficult. In this case using a shovel or other simple tools might be acceptable according to U.S. EPA [27].

In this study, a cylindrical stainless-steel shovel with round bottom (11.5 × 10.0 × 19.5 cm) with a sample area of 195 cm² was used to test the efficiency of simple non-standardized sediment samplers. The results (Figs. 3 and 4) in medium sand and fine gravel indicate that MP were recovered but not in the targeted area of 91 = 77% (MP_min) to 112 = 100% (MP_max) particles. In fine gravel MP results varied from 45.2 ± 3.7% to 64.0 ± 4.3%. In medium sand samples contained 39.9 ± 4.1% to 52 ± 6.6%. A general loss of MP was observed when the shovel passed the aqueous phase. The flow velocity of the water and the angle at which the shovel was held determined the loss of fine material and low-density particles. Results also show a noticeable difference of MP abundances of PP 4 mm in medium sand and fine gravel. The particle loss was caused by the size difference of MP particles and medium sand particles (d₅₀ = 0.29 mm). Because of their low density (0.9 g/cm³) the PP 4 mm particles tended to move upwards to the water surface. Fine gravel particles (d₅₀ = 2.9 mm) prevented this loss better due to their size, weight and larger pore size.

While MP sampling was not satisfactory, sediment dry masses are relatively constant, indicating that is method is suited to collect at least coarse sediment material (Fig. 5). The median value of dry masses of medium sand was 1870.5 g and of fine gravel 2168.4 g.

Due to the reason that the sediment corer was not applicable in fine gravel, a percussion corer, which is usually utilized to sample soils and coarse material, was tested at the model plant. The probe pole (diameter 5.8 cm) and core catcher (diameter 4.6 cm) consisted of steel. The inner liner (diameter 4.6 cm) material is made of polyvinyl chloride (PVC).

The corer contains a sample area of 16.6 cm². MP_min and MP_max values ranged therefore between 7 (58%) and 12 (100%) MP particles per sample. Similar to the sediment corer (Sect. 3.1), only small amounts of sediment could be sampled because sampling was limited by sample boxes depth (Fig. 5). Thus, only a median dry mass of 92.6 g fine gravel could be achieved. MP results ranged from 31.3% ± 18.0% to 58.3 ± 15.6% in average (Fig. 4) and are mostly below MP_min. During sampling a loss of particles was caused when the sampler was lifted out of the water. The remaining water in the sampler flowed downwards through the core catcher, mobilising and transporting some MP particles. Consequently, the efficiency and limitation of this method is dependent on the core catcher integrated into the percussion corer. Smaller or twisted core catchers might improve the applicability of percussion corers in coarse sediments.

Potential of freeze coring

Obtaining undisturbed samples of sediments which contain a high amount of organic material using a gravity or piston corer is difficult [14]. Hence in-situ freeze coring techniques have been implemented, mostly for paleoclimate research [17].

As freezing agents commonly either a dry ice/alcohol mixture or liquid nitrogen are used [6]. Most freeze coring techniques developed in the last decades are either difficult to use or to control, expensive or the availability of cooling methods in remote regions is difficult [14]. To the authors knowledge, simple handheld devices for regular field and laboratory usage are not commercially available. Therefore, a freeze coring prototype was set-up to test the efficiency of freeze coring for sampling MP in sediments.

An oval shaped freeze core with a dry weight of 745.6 g was recovered from the sediment (Figure S5). Based on the diameter and area of the core at different sediment depths, MP_min and MP_max values could be calculated for the MP placed in the sediment box (Tables 2 and 3). Most MP abundances were reported between MP_min and MP_max. The highest result in a single layer was obtained with 146.2% PP 4 mm particles (Table 3). POM 3 mm particles were with 57.1% per sampling area slightly below MP_min. This loss of POM 3 mm particles as well as overestimation of PP 4 mm might be the result of vertical displacement of the sediment layer caused by radial freezing from the outside to the inside of the core [6].

Table 3 Calculation of MP_min and MP_max based on freeze core diameter

Even after decades of applying freeze coring techniques in field studies, there is still a lack of literature describing the freezing process in natural sediments. The changes which include density, pore space, shear strength, thermal changes and chemical properties are described for industrial processes such as soil freezing or sludge freeze [6]. During the freezing process the density of water decreases until below -70 °C grain material is frozen [26]. The freezing rate depends on the chosen coolant, the sediment composition, pore water velocity, and ambient conditions (e.g. water temperature, salinity) [7, 21]. Dück and Lorke [6] showed that the time until the sample is completely frozen is longer when sampling fine sediment particles (silt/sand) with a higher surface area.

Based on our results, freeze coring can be applied for MP sediment sampling, but should be developed further. The freezing process need to be optimized to keep (1) coring disturbances low and (2) avert embrittlement of polymers and thus additional fragmentation. Additional fragmentation caused by rapid freezing was not observed in this study. Weathered and porous MP in environmental samples however may break apart during sample processing and particle identification processes [4]. Thus, this sampling method might be not suitable for approaches which determine particle numbers and shape. Other disadvantages are higher costs, difficult handling and specific work safety requirements, and a comparably much longer time for sampling.

Recommendation of sediment samplers for monitoring MP in sediments

The volume of material collected by the tested sampling techniques varied by several magnitudes (Fig. 5). Samples obtained using a shovel had the highest recovered masses of 1751 – 2228 g in medium sand and fine gravel, followed by Van Veen grab samples with 250.1 – 2407 g, freeze core with 745.6 g, sediment core samples with 385.92 – 723.1 g and percussion corer samples 26.2 – 195.4 g. If sieving analysis is applied representative sample masses such as 300 g for silt, 500 g for sand and 2 kg for gravel are recommended [1]. Regarding MP analysis, we recommend to collect a sample amount which is at least exceeding the mass or volume reference unit of MP (e.g. MP/kg) to avoid unjustifiable extrapolation [19].

With regard to MP recovery efficiency the results obtained in this study show that standardized samplers are suitable for sampling MP. As those samplers are operated by hand, however, a variation across MP recovery rates was observed for the grab sampler and the sediment corer during repetitive sampling. The shovel and the percussion corer are not recommended to use for MP sampling in sediments because fine particles and low-density MP were lost during the sampling process.

The data also revealed that a successful sample collection is dependent on the sediment composition and the size of MP. Particle density did not affect the efficiency of the grab sampler, the sediment corer and freeze corer. Particle loss of low-density polymers (PP) however was observed during sampling with the stainless-steel shovel. Based on our results sediment properties should be investigated prior to MP sampling in order to select an appropriate sampling technique. Besides particle diameter also MP morphology may affect MP sampling. This influence was not investigated in this study due to the lack of appropriate material and reliable methods regarding sample processing, separation of MP from sediments and rapid techniques for MP analysis. MP found in the environment however are reported often as fragments or fibres [22]. Also, recovery rates of smaller particles need to be investigated, a majority MP found in the environment are < 1 mm [12]. Until (1) standardized sediment sampling equipment for MP is available and (2) to cover a sample area and depth as large as possible as well as (3) to yield enough material for different kinds of analysis we recommend to use a grab sampler in combination with a simple corer in coarse silt, fine sand and medium sand sediments. To avoid fine material loss in coarse sand, fine gravel and medium gravel samples a sediment corer containing an appropriate core catcher or a box corer should be applied. Sampling devices need to be free of plastic material and should tightly close in-situ, both at the top and the bottom.

Effect of water flow velocity on recovery rates during MP sampling

We generated a flow velocity in the model plant between 0.01 and 0.04 m/s. These conditions are usually expected near the shore of riverine environments and lakes. Standardized sediment sampling techniques were not affected due to their size and weight under those flow conditions. If a sample is recovered from a boat, e.g. in middle of a river, wave formation and higher flow velocities will complicate accurate sampling. Especially coring is limited under these conditions. Therefore depth-based data obtained under such conditions should be verified by at least three samples collected in the sample area.

In comparison to the standardized samplers, the shovel was affected by water flow conditions and sediment composition. We recommend to apply this method only for coarse sediment such as gravel. If standardized samplers cannot be applied, other sampling points near the area of interest should be selected.

Because freeze coring was tested without flow conditions, no estimation of the effect of the water flow velocity on the freezing process can be made. We expect that higher flow rates will affect the freezing rate due to heat transfer and friction on the surface of the core tube in the aqueous phase. This effect can be alleviated by adding an isolating material to the affected section.

Conclusions

Sampling MP in sediments is not challenging in general, as various tools such as shovels and even standardized samplers are available. The challenge lies in designing a standard sampling strategy that prevents contamination and limits the loss of MP when the sampler is recovered through the water column. Results of this study show that sampling MP in sediments is strongly affected by the sampling equipment, sediment grain size and MP particle size. When available, further work using particles of a larger set of MP morphologies such as fragments, foils or fibres of smaller sizes and also a wider set of samplers should be conducted to assess the loss of MP during sampling. Also, instead of model sediments, natural sediments should be assessed. An overall useful dataset could be also achieved if experiments are conducted with standard MP material. The growing number of recent publications regarding these topics indicate that researchers are devoting their efforts to resolve these challenges.