The Prickle Effect Comes From Fabrics Made of South American Camelid (Alpaca and Lama) Fibers. Mechanical and/or Genetic Solutions. A Review

In this paper we intends to analyze the physical attributes that determine the comfort of fabrics made of South American Camelid fibers (Lama and Alpaca), the effect on their value and their possible mechanical and/or genetic solutions. While emphasis has always been on mean fiber diameter, the fiber frequency exceeding 30 microns has a key role in quality. This is essential for light fabrics, where the effect of prickle plays a critical part in consumer´s choice. Yet the genetic solution of the problem lies in the


INTRODUCTION
The textile fiber group to which Camelids belong is more commonly known as luxury fibers.The main attributes and characteristics that give a particular added value to these fibers have been summarized in the subjective variables of softness and brightness [1].
The diameter of the fiber is the attribute with the greatest weight when it comes to determining the price of the fiber, given its relation with softness or 'hand' [2].Nonetheless, consumers have little direct interest in the properties or attributes of the fiber, being the subjective quality of the fabric what mainly determines their opinion on the various fibers [3].The textile scientist, on the other hand, needs to understand the contribution of fiber attributes to such quality assessment [4].
Quality can be defined from a final consumer perspective by the 'hand' in order to indicate the relationship of quality to the degree of fabric acceptance.This term has been defined as the subjective assessment of a textile material obtained from the sense of touch.It is also a psychological phenomenon that involves the fingers, on the one hand, in order to be able to make a sensible and demanding evaluation and the mind, on the other, to integrate and express the results in a value judgment.It is common to define the 'hand' of a fabric as the mean of the scores of a certain number of observers or panelists, or these same differences become an important attribute to evaluate it [4].The concept of hand encompasses several attributes: comfort on the skin (itching), stiffness, bulkiness, smoothness and softness [5].The notion of prickle (itching) applies to garments used in contact with the skin (directly or indirectly), placed on the forearm or pressed with the palm of the hand and fingers when the garment is purchased [6].The fibers of domestic Camelids is seriously compromised, from a commercial standpoint, by this feature [7].
Several studies have shown that pruritus or prickle sensation comes from the coarser fibers of the right tail of the distribution of fiber diameter (´coarse edge´) [8] [9].The last author also determined that the percentage of fibers over 30 μm is a good predictor of itching sensation in knitted fabrics and still much more pronounced in weaving.
Moreover, it should be considered, that not only the diameter is a determining factor for a fiber to cause prickle, but also its stiffness, which is crucial and is influenced by the type of medulla of the fiber [10].Even though emphasis has historically been placed on the percentage of fibers over 30 microns to induce itching, a recent study reports that much finer fibers (20 microns) may trigger the response if the free length of the protruding fiber on the surface of the fabric is sufficiently short [11].Specialty animal fibers, which are obtained from angora goats, angora rabbits, cashmere goats, and alpacas, are used to test textile desirable properties, such as softness, luster, and comfort European Journal of Applied Sciences (EJAS) Services for Science and Education -United Kingdom characteristics.The results show that silk/viscose, angora/viscose, and 100% viscose fabrics are the smoothest, softest, and least prickly, while alpaca/viscose and mohair/viscose fabrics are found to be the roughest, stiffest, and most prickly surfaces according to the objective and subjective results.In the case of fabrics including 100% animal fibers, angora, cashmere, and silk fabrics are the smoothest, softest, and least prickly fabrics.However, mohair and alpaca fabrics are comparatively rougher, stiffer, and pricklier [12].
The aim of this paper is to analyze the critical physical characteristics that determine the comfort of fabrics made with Camelids fibers, the effect on its value and possible mechanical and/or genetic modification.

DIAMETER DISTRIBUTION, FIBER TYPE AND QUALITY
Fiber diameter distribution is key to determining quality, due to the effect on the appearance and comfort of the product (Holcombe, 1986) and the effect on the behavior of the fiber during textile processing [13] [14] [15].
Figure 1 illustrates the diameter distribution into the different Alpaca and Llama fiber types and the composition of the 'coarse edge' (the vertical red line shows 30 μm) almost exclusively due to the diameters of the fibers with lattice medulla and in almost half the cases to the continuous medulla [16].The diameter is very close related between Alpaca and Llama species, within the different type of fiber identified by type o medulla.--------; >95 % UL: ---------- In addition, the differentiable appearance of these fibers with respect to the others has led to the designation of objectionable fibers or observable fibers [17] [18].European Journal of Applied Sciences (EJAS) Services for Science and Education -United Kingdom Diameter distribution both in Llamas and in Alpacas follows a biphasic pattern.This can be visually noticed depending on the types of fleece, with greater or lesser intensity in Llama fiber [19] and equally in Alpaca Huacaya fiber, even in the finest one [20].A non-normal distribution of diameter is also showed in South American Camelids fibers as on wool fiber distribution [21.The relationship between mean fiber diameter (DMF) and fiber percentage >30 μm (PcF) is a curvilinear potential fitting and responds in Alpaca to: Pcf=56.38*(DMF/30)6.0,R 2 =0.98 and in Llama to: Pcf= 53.6*(DMF/30)6.13,R2=0.89 [21] (see Figure 2).
It has been confirmed that the difference of Alpaca fiber in 'hand' with respect to wool means that 27 μm Alpaca is as soft as 15 μm wool [22].A more recent research work takes reduce this value to 8.5 microns [23], which is equivalent to a 26 µm alpaca fiber diameter to 18.5 µm of wool diameter.

DEFINITION AND SOLUTION OF PRICKLE EFFECT OR ITCHING
The itching effect is directly correlated to Euler's theory of the bending or 'buckling' of a beam or wire, according to which the buckling force of a round structure is equal to the Young's modulus multiplied by the diameter to the fourth and divided by the length to the square grade,  being independent of the type of fiber used (natural or artificial) [9].Since most of the protruding fibers of the yarn/fabric do so in an angle lesser than straight, the assumption that the fiber actually bends laterally to contact the skin should be made.Then Euler's formula is slightly modified by raising the length of the fiber to the cube grade [24].It has been verified that the stiffness modulus (Young) is higher in Camelids than in wool.Therefore it is expected that the load or force to achieve the fiber should be higher or the diameter smaller [25].When Pcr=75 mgf, the results of fiber diameter calculations for different individual fiber lengths and Young's modulus values of alpaca fibers [26] are listed in Table 1.This effect is determined more by the structure of the protruding tips of the yarn or fabric than by the total yarn, although in general, this is determined with high accuracy by the mean diameter and the diameter dispersion in the total yarn [5].The average diameter of the protruding fibers is 2 -3 μm larger than that of the yarn [27] [28].
If the problem of the pruriginous effect lies in the coarse edge of the diameter distribution, two possible solutions are identified: decreasing the mean diameter (leftward shift of the normal curve with corresponding coarse-edge shift) or the range of distribution of fiber diameters (move leftwards the coarse edge leaving the mean unchanged) [29].The latter could be achieved by dehairing or genetic selection.There is anecdotal information for the former and experimental data for the latter [16].However, it seems that fineness alone would not correct the itching effect [21].
Hand-dehairing has demonstrated its feasibility [30], although the process is only efficient with double coated and intermediate coated fleece [31].However, a recent work has shown that per person and per hour yield is 9.9±1.1 g/person/h, which renders it economically unviable [32].
Dehairing or separation of different fiber types by mechanical means is the alternative to be analyzed [33].The results of South American Camelids fiber dehairing as compared to cashmere (a more studied fiber) are listed in Tables 2 [34].The reduction of the prickle factor (expressed as undesirable coarse fiber, FG) is accompanied by a reduction in fiber length.The reduction of undesirable fibers in successive passes accounts for 30 -50% reduction in both Alpaca and Llama.European Journal of Applied Sciences (EJAS) Services for Science and Education -United Kingdom After an Alpaca fiber dehairing assay conducted in Australia, it was concluded that only a relatively small amount of coarse fibers could be eliminated.In addition, dehairing considerably shortens the length of alpaca fiber.Therefore, it is unlikely that dehairing alpaca fiber is a viable practice if the only goal is to reduce the diameter of the fiber and this is only useful to reclassify dehaired fiber as a finer line.The actual benefit of dehairing should be to improve the quality of the final products of alpaca [26].A recent trial with dehairing alpaca top has yielded better results, in objectionable fiber w/w of 2.2% in 6th runs (-55.5%;Nº/weight: 0.16 (-50%) ; Fiber of > 30 μm:3.6% (a reduction of 60%) was obtained [35].

GENETIC DETERMINATION OF FIBER DIAMETER COMPONENTS
Even though there is a possibility of reducing the distribution via selection [36], this reduction would not be guaranteed by the reduction of the mean diameter [16] [21].
Table 3 shows the heritability of the diameter components of the fiber types that are identified by the type of medulla.Additionally components are correlated with average fiber diameter and coefficient of diameter variation, both vital statistical parameters to describe the quality that consumers appreciate the most.The conclusions drawn indicate that there are unfavorable correlations between the coefficient of variation and the diameter of non-medulated fibers, and fragmented and continuous fibers, being the heritability of high frequency and diameter of fibers mid to very high and null to low the heritability of diameter variation of the fiber types [16].This would indicate the possibility of selecting by fiber type, achieving a reduction of the average diameter as well as of the variation coefficient of the diameter, though not in a separated way but simultaneously by unfavorable correlations.There would also be the possibility of selecting from these parameters depending on the capacity of 'dehairablity', seeking ratios of diameter and frequency of the types of fibers more suitable for this purpose.The heritability's for percentage of medulation and medulated fiber diameter obtained in Peruvian Alpacas, were 0.225 and 0.237 in Huacaya genotype and 0.664 and 0.237 in Suri type, respectively.The genetic correlations between percentage of medulation and between mean fiber diameter and medulated fiber diameter were high and favorable in both genetic types, between 0.531 and 0.975.The genetic correlation between medulation and medulation diameter was 0.121 in Huacaya and 0.427 in Suri.The repeatabilities were 0.556 and 0.668, and 0.322 and 0.293 in Huacaya and Suri genotypes, respectively [37].
The Selection Indices methodology simulates the effects of using several variables at once in relation to production and fiber quality, including the possibility of improving fiber 'dehairablity'.This has been explored applying the Lama parameters by Frank et al., (2008) [16], yielding a possible decrease of 0.7%/year of objectionable fiber if the constant average fiber diameter [38], were maintained in Table 4.However, in a recent study (Figure 3) negative genetic correlations between the fiber diameter of the fiber of the primary/secondary follicle, with overall fiber diameter (Frank, E.N. unpublished data) were obtained.The response per generation and response per years should not be interpreted with the unit of measurement of the variable, but as response in ARpesos ($).Given the absence of rational relative economic values assigned to each variable only a relative value of the response can be given, lattice medulla diameter (LFD) would be reduced almost 2 times in relation to the nonmedulated fiber diameter (AFD) (-15.5 / -8.2 = 1.89) and so on with the other variables.A good genetic response, but only an 8.2% reduction in ten years it is not such an auspicious matter if you plan to reach the level of thick fibers identified by the panelists [10].It is clear that the relationship between expected progeny deviations (EPD´s) of primary fiber diameter/secondary fiber diameter (Pfd/Sfd) ratio on mean fiber diameter in Argentine Lamas skin sections is negative and of medium magnitude.It would imply that the reduction of the total fiber diameter would lead to an increase or maintenance of the primary/secondary diameter high ratio.This refutes the favorable response estimated previously from fiber genetic parameters by Pinares et al. (2018) [39].

CONCLUSIONS AND IMPLICATIONS
The textile fiber quality of South American Camelids appears highly promising provided the presence of undesirable or objectionable fibers, leading to a tolerable frequency by consumers of <3% is solved.This process could be explored by way of genetic selection, by applying dehairing technology or a combination of both.
Nonetheless, this implies a true paradigm shift with respect to the classic textile fiber processing of Alpaca and Llama, leading to the implementation of carding textile technology (woolen) or short fiber cotton processing.This would translate into greater softness to the touch ('hand') and other important characteristics that would turn the value of this fiber competitive with respect to other luxurious fibers better known in the market, such as cashmere.
As an implication of this Review it can be stated that the genetic change seems very slow to solve the current alpacas and llamas fiber profitability problem for both Peruvian and Argentine growers, indicating that the application of dehairing technology could be a solution in the meantime.

Figure 1 :
Histogram with Normal distribution of fiber diameter form different type of fiber defined by the type of medulla.Nm: non-medullated, Fm: fragmented medulla, Im: interrupted medulla, Cm: Continuous medulla, Lm: lattice medulla.