Design of next generation sports helmets for the asian market. (Material Science)
gbomsom
dehydration_detector_for_protective_gear.docx
DEHYDRATION DETECTOR FOR PROTECTIVE GEAR
Abstract
Recent studies have shown that many of the fatal cases of sport activities are caused by dehydration and over-hydration, which are due to the deficiency of water or sodium in blood plasma. In South East Asian countries, the sun is available throughout the year. Therefore, people stand a higher chance from suffering heat injury during strenuous exercises compared to those in the cooler regions.
The current methods for measuring dehydration and over-hydration are complicated, inconvenient and unable to give direct feedback during exercise. The objective of this project is to innovate a new product that will measure the hydration level of an individual. The main idea of the design is to apply the concept of color change when the stability of colloidal gold nanoparticles is affected by various sodium concentration levels in the sweat.
The design will work in a way where three distinction colors will be revealed on a wristband in different status of hydration level. For the design, our project team is working hard on the research about colloidal gold nanoparticle and its capping agents and how to dope the gold nanoparticles onto a wristband, which will hold a matrix of polymers absorbent material to contain sweat.
Table of Contents Abstract ii List of Figures vi 1 Introduction 7 1.1 Background 7 1.2 The Need 8 1.3 Objectives 9 1.4 Scope 9 1.5 Schematics of Final Products 9 2 Literature Review 10 2.1 Human Sweat 10 2.2 Conventional way of measuring hydration level 11 2.2.1 Body mass changes 12 2.2.2 Urinary Indices 12 2.2.2.1 Urine Specific Gravity 12 2.2.2.2 Urine osmolality 12 2.2.2.3 Urine Color 13 2.2.2.4 Urine Volume 13 2.2.3 Hematological indices 13 2.2.4 Salivary parameter 13 2.2.5 Sweat composition assessment 14 2.3 Hydration Sensor Integrated on Fabric 14 2.4 Gold nanoparticles 15 2.5 Preparation Methods for AuNPs as Sensors 16 2.5.1 Turkevich method 16 2.5.1.1 Citrate-capped Gold Nanoparticles 16 2.5.1.2 AuNPs stabilized by 3-thiophenacetic acid 16 2.5.2 Peptide Capping Ligands for Gold Nanoparticles 17 2.5.3 Brust-Schiffrin method 18 2.5.4 Seed-growth method 18 3 Comparison & Analysis 19 3.1 Wristband 19 3.2 Indicator 20 3.3 Superabsorbent pad 21 3.4 Absorbent pad with indicator 22 3.5 Top Cover 23 3.6 Temporary Bottom Cover 25 4 Final Design 26 4.1 Components of wristband 26 4.2 Indication of Hydration Levels 27 4.2.1 Sweat sodium concentration 27 4.2.2 Device Requirements 27 4.2.3 Sample synthesis of AuNP 28 4.2.3.1 Turkevich method 28 4.2.3.2 Theory 28 4.2.3.3 Addition of Na+ ions 28 4.2.4 Application of the concept 28 4.3 Problem encountered 29 4.4 Development of AuNPs for Sweat Na+ Concentration 29 4.5 Processing 31 4.5.1 Fabrication of AuNPs treated polypropylene 31 4.5.2 Producing components of the wristband 31 4.6 Assembling the components 32 5 Environmental Impact 33 6 Discussion 34 6.1 Advantages 34 6.2 Limitations 35 6.3 Recommendations 35 7 Conclusion 35 References 37 Appendix 41
List of Tables
Table 21: Sweat sodium concentrations at different hydration levels 10
Table 31: Criteria for the wristband 19
Table 32: Constraints for wristband 19
Table 33: Objectives for wristband design 13
Table 34: Criteria for the Indicator 13
Table 35: Constraints for the Indicator 14
Table 36: Criteria for Superabsorbent pad 14
Table 37: Constraints for Superabsorbent pad 15
Table 38: Objectives for Superabsorbent pad 15
Table 39: Criteria for Absorbent pad with indicator 15
Table 310: Screening for Absorbent pad with Indicator 16
Table 311: Ranking for Absorbent pad with Indicator 16
Table 312: Criteria for Top Cover 17
Table 313: Screening for Top Cover 17
Table 314: Ranking for Top Cover 17
Table 315: Criteria for Temporary Bottom Cover 18
Table 316: Screening for Temporary Bottom Cover 18
Table 317: Objectives for Temporary Bottom Cover 19
Table 51: The embodied energy and carbon footprint for each of the components 27
List of Figures
Figure 1: Schematic of final product 10
Figure 2: Electrochemical sensor network (left) and control printed circuit board (right) 15
Figure 3: TEM images of AuNPs synthesized with different concentration of TA, concentration decreasing from (A) to (D) 17
Figure 4: Final design of the Wristband 26
Figure 5: Schematic of Transparent Indicator Pad 20
Figure 6: Regional distribution of physiologically active eccrine sweat glands, skin surface areas, gland counts and glandular dimensions 27
Figure 7: Color change upon addition of NaCl into stabilized citrate capped gold solution 28
Figure 8: Illustration of stabilized AuNP 23
Figure 9: Ionic strength vs distance between colloidal gold nanoparticles 24
Figure 10: Carbon dioxide footprint for PVC and Silicone production 26
Figure 11: Embodied energy for PVC and Silicone 26
Figure 12: Surface charge 34
Figure 13: Effect of ionic strength 35
Introduction
Background
Water is a vital nutrient for life as it plays a crucial role in regulating our body temperature, lubricating our joints and transporting nutrients throughout the body. About 60% of our body weight is made up of water and it is important to maintain that balance. Staying hydrated is important for everyone and yet people who are active in sport need to maintain the balance of body fluid more. Therefore, adequate fluid intake is essential for people who do sport before, during, and after exercise in order to stay hydrated.
Dehydration is most commonly seen after exercise in which heavy sweating has occurred. A side effect of sweating is the loss of valuable fluids from the finite reservoir within the body, the rate being related to exercise intensity, environment conditions and so on. Dehydration of 1% to 2% of body weight begins to compromise physiologic function and negatively influences performance. Dehydration of greater than 3% of body weight further disturbs physiologic function and increase an athlete’s risk of developing an exertional heat illness (e.g. heat cramps, heat exhaustion, heat stroke).
A major consequence of dehydration is a noted increase in core temperature during physical activity. Core temperature raises an additional 0.15 to 0.20 degrees Celsius for every 1 percent of body weight lost due to sweating during activity. This thermal strain also influences a greater cardiovascular strain. One example of the changes in the cardiovascular system is a rise in heart rate by an additional 3 to 5 beats per minute for every 1 percent of body weight lost. Further injury to the musculoskeletal system is also present. These changes include elevated muscle temperature and increased lactate levels. Studies investigating the role of dehydration on muscle strength have generally shown decrements in performance at 5 percent or more hydration.
Fluids in the body are either inside the cell or outside of the cell. When we become dehydrated, the fluid outside of the cells decreases. Reductions in fluids cause nerve endings to be squished together, overexcited and spontaneously discharge. That spontaneous discharge is a muscle twitch, which can lead to a muscle cramp. By maintaining proper hydration, you can prevent dramatic shifts in fluids that contribute to abnormal muscle contractions.
Although rare, over-hydration can occur during long bouts of exercise when electrolytes lost through sweat are not replaced, yet excessive amounts of water are consumed. Over-hydration can lead to potentially dangerous imbalances of electrolytes, including hyponatremia, a serious condition in which the sodium level in the blood becomes too low. Hyponatremia can be a problem for athletes who experience excessive sodium loss through perspiration as part of prolonged exercise or heat exposure, such as running a marathon.
The Need
The effects of dehydration are greater in hot environments like Singapore. Steinberg (2013) wrote that in dozens of cases in the last thirty years athletes have died of dehydration. On the other hand, drinking too much water can also be hazardous to your health, leading to a condition called hyponatremia. Hyponatremia means having low level sodium concentration in your body fluids and it can even lead to fatal cases. Some enthusiastic marathon participants sometimes take fluid more than what their body have lost and encountered fatal cases. According to an article of Biomechanics Fitness and performance, the cases of over hydration are as serious as the following.
· During the Chicago marathon in 1998, Kelly Barrett collapsed and later died from hyponatremia.
· After the Houston marathon in 1999, 4 runners were hospitalized with comas due to hyponatremia.
· During the Boston marathon in 2002, Cynthia Lucero died from hyponatremia. In the same marathon, 13% of 488 runners tested were hyponatremic.
· At the 2002 Marine Corps marathon in Washington, DC, Hilary Bellamy died from hyponatremia.
· At the London marathon in 2007, David Rogers died from hyponatremia.
It is difficult to know what your levels of hydration are. Thirst can be a poor indicator of hydration because exercise blunts the thirst mechanism. Body is already dehydrated when thirst becomes detectable. Other indicators such as urine colors and body weight lost during exercise are also not detectable while exercising. The same principle applies for hyponatremia as well. In addition, older people can have severe affects from dehydration and hyponatremia. The thirst mechanism diminishes as people age, and thus increases the risk for dehydration. Maximal urinary concentrating ability also decreases as people age and this can also increase risk for dehydration.
Reviewing the number of cases dehydration and over hydration, it is obvious that there is a need to indicate the hydration level while we are exercising. A portable and user friendly device that can tell a person’s hydration level will become handy for athletes or people who are active in sports. As the effects of dehydration and hyponatremia are more severe for older people, the device will become even more essential for older adults in hot and humid Singapore and other South East Asia countries.
Objectives
The purpose of this project is to produce a device that can indicate the hydration level of a person by manipulating the stability of the gold nanoparticles in various sodium concentration levels. In this way, a person can check his hydration level readily while exercising or doing sports. This in turn will protect them from cramps; heat strokes and others negative effects of dehydration and over hydration.
Scope
Due to the limitation of time and resource, the dehydration level of sweat at only forearm ventral will be measured. In addition, the design mainly targets to marathon runners as dehydration occurs more in the marathon runners as stated in background of this report.
Indicator Pad
Adjustable Wristband
Schematics of Final Products
Figure 1: Schematic of final product
The schematic of the final design will be a wristband with an indicator pad, which will change colour according to the hydration level of the users. For more ergonomic design, the wristband is made to be adjustable in order to fit the users forearm ventral.
Literature Review
Human Sweat
Dehydration is often accompanied by losing body’s mineral salt or electrolyte balance. Human sweat mostly contains mainly water. Besides, it also contains minerals, lactate and urea. The mineral consists of sodium (0.9 gram/liter), potassium (0.2 gram/liter), calcium (0.015 gram/liter), magnesium (0.0013 gram/liter). Other elements such as zinc (0.4 milligrams/liter), copper (0.3–0.8 milligram/liter), iron (1 milligram/liter), chromium (0.1 milligram/liter), nickel (0.05 milligram/liter) and lead (0.05 milligram/liter) are also present in human sweat. The pH level of sweat typically ranges from 4.5 to 7.0. Losing of slightly high concentration of sodium in sweat indicates one’s under dehydration condition.
Nevertheless, the sodium concentration in plasma is finite in the human body. Besides, sodium is considered a vital nutrient as it is limitedly available. Athletes should consume sodium in order to replace sodium losses over time. A long-term shortage of sodium will be harmful to the body. On the other hand, over consumption of sodium can lead to over hydration. This normally happens during marathon races which have plenty of stations that give out electrolytes to the runner. For instance, the athlete replaces 100% of the fluid loss (1 L/h) with the addition of 400 mg sodium/h. Across whole-body sweat rates from 0.72 to 3.65 mg.cm-12.min-1, sodium losses of 26.5–49.7 mmol.L-1 could be expected.
Table 21: Sweat sodium concentrations at different hydration levels
|
Condition |
Na+ Concentration |
|
Over hydrate |
26.5 mM |
|
Normal |
40 mM |
|
Dehydrate |
47.9 mM |
(Taylor, 2013)
Conventional way of measuring hydration level
Common techniques used to monitor hydration status include body mass changes, urinary and hematological indices, salivary parameters, and total body water assessment. As dehydration progresses, there are negative effects on heart rate, stroke volume, cardiac output, fatigue, skin blood flow, plasma volume, and rate of perceived exertion. Body mass changes and total body water assessment need several procedures to go through to get the accurate result while measurements of salivary parameters and urinary properties are less reliable since they vary with the food consumed.
With acute hypo hydration, urine demonstrates acute changes in volume, color, specific gravity, osmolality, and conductance, creating an opportunity to assess hydration status via relatively noninvasive urinary indices.
A refractometer requires only a small volume of urine, is temperature compensated, and can be used as a general guide to an athlete’s hydration status. Furthermore, as refractometry is portable, noninvasive, inexpensive, objective, and simple to use by clinicians, it has become the preferred method for hydration assessment by many investigators. Manual hand-held units and digital refractometers are commercially available, with the manual unit being both cheaper and more robust. Both techniques provide almost identical results.
The methods, which are currently available like what has been stated above, are not very feasible for the project implications. Preferred tool should be noninvasive, economical with minimal consumable requirements, technically simple, portable, valid, precise, and not influenced by factors unrelated to hydration status. Therefore, further researches need to be done on more reliable, accurate and fast methods.
There are several methods to assess the hydration level of the human body. Body mass changes, urinary and hematological indices, salivary parameters and total body water assessment.
Body mass changes
Rapid body mass changes are due to the gain or loss of body water or the intake of food and fluids, as no other body component is able to gain rapidly. Random variations of body mass have been proven to be within ±1%. Therefore, the change in excess of this amount is caused by the hydration level of the body. Body mass measurement should be done at the same time of the day such as in the morning before breakfast and training with an empty bladder and bowel. In such condition, a baseline body mass is measured and the value is advised to be use not more than two weeks. To achieve the accuracy of the body mass monitoring, the person should be wearing minimal clothing, using the same scales throughout the assessment, consideration of menstrual cycle in female and chronic energy imbalance. Body mass assessment is a simple, noninvasive, inexpensive method but it requires consistency and the person needs pay close attention to the amount of water intake and urine output.
Urinary Indices
Urine Specific Gravity
This is the measurement of urinary density in relation with pure water density. This can be used to determine the urine osmolality and hence the hydration level. Common methods used for measuring urine specific gravity are dipstick, hydrometer and refractometer. However, urinary composition also depends on the concentration of protein, urea and glucose. Therefore, urine specific gravity becomes unreliable when there are heavy molecules, which can cause increase in urine osmolality despite of normal hydration level.
Urine osmolality
Urine osmolality determined the total urine solute concentration. This is a laboratory-based technique in which the concentration is measured through a phenomenon called freezing point depression in an osmometer. The method is independent of big molecules such as protein, glucose and urea. Therefore, it is considered the most accurate method to find out the concentration of urine. Despite its accuracy, the method is not useful as it is not portable.
Urine Color
Using an eight-color strip, the hydration level is obtained as a reasonable index. The strip consists of a color spectrum ranging from pale yellow to greenish brown. This method is a rather simple, expensive and portable method to determine the hydration status. Urine color is assessed in a well-lit room by comparing to the color chart with a white-colored background. However, certain food and drugs ingested can affected the color of the urine regardless of the hydration level.
Urine Volume
Urine Volume monitoring is another technique which can be used by the athletes to independently measure their hydration level. Athletes are advised to take note of the fluid intake and frequency of urination. However, urine volume is quite inconvenient to collect and assess. It also reflects the fluid intake and provides too little information about hydration status.
Hematological indices
This is a blood-borne measurement to determine the plasma osmolality by analyzing concentration changes in hemoglobin and hemocrit. To obtain accurate result, the standardized posture must be maintained for 15-20 min. This method is capable of detecting the body water deficit of 1-5%. Despite all the advantage, taking blood sample in this field is impractical. This can introduce risk of getting infection, vein damage and bring discomfort to the athletes. In addition, it needs qualified specialist and complex laboratory equipment. Therefore, such test is suitable only when reliability, precision and accurate results are needed.
Salivary parameter
Salivary parameter is a viable method to assess hydration level, as it is noninvasive, simple and inexpensive. Salivary plasma osmolality has a strong correlation with the body mass loss. But the changes in saliva index are less sensitive to hydration level compared to plasma osmolality and body mass assessment. In addition, the temporary effect of food consumed can bring minus point to this method as well.
Sweat composition assessment
Sodium and chloride are the primary components in the sweat. Other mineral comprises calcium, magnesium, and potassium in small quantity. During endurance, exercise, the imbalance in water and sodium concentration can lead to hyponatremia and hypernatremia. There are two ways to assess sweat composition. They are whole body wash down and regional skin surface collection. The former one seems to be more reliable while the latter is more viable to use because of its simplicity. It is almost impossible to collect all the sweat from the whole body during exercise. However, the average sweat concentration can be collected with carefully selection of skin area such as forearm, which represents about the average concentration value of whole body.
Hydration Sensor Integrated on Fabric
The electrochemical sensor has host molecules in conducting polymer to selectively measure sodium ions concentration in the sweat. This device is made of fabric network deposited with organic-metallic fluid and patterned copper and gold ions electrodes as shown in figure 2. These networks act as electrodes, called ionic selective electrodes, for measuring the electrochemical reaction by applying a voltage of 1.2V across them. The sensor network is electrically connected to a portable electronic board as shown in figure. This electronic circuit board is equipped with analog parts, control block (with microprocessor), and connection to electrochemical electrodes. The device is able to measure a sodium concentration sensitivity of 2mV/mM from 1.25-62.5mM. The paper has not mentioned how user will be able to realize the hydration levels. The complexity and bulkiness of electronic circuit becomes a barrier for its commercial value.
Figure 2: Electrochemical sensor network (left) and control printed circuit board (right)
Gold nanoparticles
Determination of analytes using aggregation of gold nanoparticles has received much attention in recent years. It is because determination of analytes using aggregation of gold nanoparticles is much simpler if compared to some other processes and in some cases using only own eyes can even be the way to determine because the color change from red to blue or vice versa is easily detectable. The two major groups of analytes that could be determined from this process are inorganic ions and biologically analytes such as amino acids, peptides and DNA fragments.
Gold nanoparticles are produced by using citrate to reduce gold salt (HAuCl4) at a temperature about 100ᵒC. The reaction is the reduction process of Au3+ to Au0 and can be described by the following equation.
Citrate + HAuCl4 Au0 + by-product
A certain amount of energy is required and thus the reaction does not occur at room temperature. When HAuCl4 solution is boiling the addition of citrate will induce instant formation of gold atoms in the solution. Moreover, the gold atoms concentration will rapidly increase until a super-saturation stage is reached. Thus, a nucleation process will occur until the aggregation of the gold atoms. The remaining gold atoms will bind to the nuclei which gradually grow towards the final gold particles.
Preparation Methods for AuNPs as Sensors
Turkevich method
Citrate-capped Gold Nanoparticles
Citrate acts as both stabilizing and reducing agent. Citrate stabilized AuNPs has been considered to be the most popular ones. After the HAuCl4 solution is boiled, the trisodium citrate dihydrate is then added quickly by stirring it vigorously. The wine-red colloidal suspension will be obtained after a few minutes, and the average size of AuNP is approximately 20nm. Furthermore, it was investigated that there is a broad size range of AuNPs (from 15 to 150nm) was obtained by manipulating the ratio of trisodium citrate to Au. Nevertheless, particles larger than 20nm were always polydispersed.
This mechanism of AuNPs formation has been examined further in order to improve the Turkevich Frens method. Kimling et al. found that AuNPs of smaller sizes will be stabilized under high concentration of citrate more rapidly. However, a low concentration of citrate brings instability instead. It leads to large-size AuNPs formation and even causing the aggregation of AuNPs. Citrate-stabilized AuNPs were also used for further usage such as intermediates in further preparations or functionalizations for ligand substitution reaction and seed-growth-mediated syntheses.
AuNPs stabilized by 3-thiophenacetic acid
Amongst the increasingly interested field, Nano world of metals, gold has the most attention. Gold nanoparticles have shown big contrast in chemical and physical properties from the bulk since its introduction by Turkevich in 1951. Researches in this field have expanded rapidly and many methods are now available to improve the quality and control the physical properties of gold nanoparticles. By using thiols, polymers, surfactants and ligands as capping agents not only can control the particle size, shape and prevent agglomeration, but also can equip surface of gold nanoparticles with functional groups.
3-thiophenacetic acid acts as a reducing agent for preparation of gold nanoparticles from HAuCl4. This research has proven that different concentration of TA can affect the shape and size of gold nanoparticles. With high concentration of TA, number of small spherical AuNP is increased and the ionic solution is more isotropic. When the concentration is eventually decreased, more polygonal particles are formed as shown in the TEM images in figure.
Figure 3: TEM images of AuNPs synthesized with different concentration of TA, concentration decreasing from (A) to (D)
Peptide Capping Ligands for Gold Nanoparticles
The stabilities depend on these peptide ligands in the aspect of their length, hydrophobicity and charge and in some cases resulted in further improved stability compared with CALNN, yielding detailed design criteria for peptide capping ligands.
The presence of charged amino acids substituted into the hydrophobic core, resulting in peptide sequences that generally provide poor protection against aggregation. For instances, CDDNN-, CKLNN-, and CDLNN-capped nanoparticles will aggregate at relatively low NaCl concentration, although the presence of a negative charge in the third position (CADNN) provide better stability than at the second position. The concentration of NaCl that induces aggregation of the peptide-capped nanoparticles will be reduced further by introducing a second terminal negative charge (CALND, CALLD, CALSD, CALKD).
The stability of gold nanoparticles against NaCl-induced aggregation is strongly dependent on the amino acid sequence. The presence of charged amino acids in the peptide core decreases its stability against NaCl-induced aggregation.
Brust-Schiffrin method
Brust-Schiffrin method uses alkylthiols of different chain lengths to stabilize AuNPs. The two-phase Brust-Schiffrin method was the method which able to prepare the thiolate-stabilized AuNPs. Its high impact is due to relative high thermal and air stability of the AuNPs prepared. Besides, it has repeated isolation and re-dissolution without aggregation or decomposition. This method is able to control of the small size (less than 5nm) with narrow dispersity and relatively easily functionalization and modification by ligand substitution.
The AuNPs are stabilized by relatively strong Au-Sbonds, and their diameters are in the 2–5nm range. Fast NaBH4 addition and cooled solutions will be producing smaller, more monodispersed AuNPs.
The reducing agent used in Brust-Schiffrin method has larger strength than the citrate used in the Turkevich method. Therefore, the size of the AuNPs synthesized using the citrate reductant in Turkevich method is much larger than that of Brust-Schiffrin method using the NaBH4 reductant.
Other capping agents for this method are polymers, dendrimers (surfactants and reverse micelles)
Seed-growth method
This method is another popular technique for AuNP synthesis that has been used. The particles size is enlarged by carrying out this method step by step. It will be easier to control over the sizes and shapes of AuNPs formed. Thus, this procedure is widely used in the size-and shape-controlled AuNPs syntheses.
This method produces AuNPs in different shape. E.g. Spherical or quasi-spherical AuNPs, gold nanorods (AuNPs).
Comparison & Analysis
Wristband
The function of the wristband is to mount the indicator pad onto forearm. Therefore, the wristband must be adjustable in order to fit different users. On the other hand, the wristband will mainly be used by runners and therefore it must be water resistant and able to function at human body temperature. It is also required to be flexible, robust and light-weight at the same time so that it will be ergonomic and user friendly. Therefore, these statements can be translated into the constraints and objectives, which are listed below:
Table 31: Criteria for the wristband
|
FUNCTIONS
|
Hold the indicator pad |
|
CONSTRAINTS
|
· Flexible · Water resistant · Operating temperature 35ᵒC - 40ᵒC · Robust · Light weight |
|
OBJECTIVES
|
· Adjustable Length · Minimize cost |
|
FREE VARIABLES
|
Choice of materials
|
Screening through the constraints
Table 32: Constraints for wristband
|
MATERIALS
|
Silicone Elastomers |
Polyvinylchloride |
Polyurethane |
|
Flexible |
Yes |
Yes |
Yes |
|
Water Resistant |
Yes |
Yes |
Yes |
|
Operating Temperature |
-60.67oC to 256.67oC |
-98.25oC to 65oC |
-48.16oC to 76.67oC
|
|
Robust (Fracture Toughness)
|
0.26 MPa.sqrt1/2 |
3.3 MPa.sqrt1/2 |
0.3 MPa. sqrt1/2 |
|
Density |
1.55 g/cm-3 |
1.42 g/cm-3 |
1.13 g/cm-3 |
The above table clearly shows that those three materials fulfill all the constraints. Thus, the ranking will of the materials will be based on the objectives.
Ranking through the Objectives
Table 33: Objectives for wristband design
|
MATERIALS |
Silicone |
Polyvinylchloride |
Polyurethane |
|
Adjustable length |
Yes |
Yes |
Yes |
|
Cost |
11.6 USD/kg |
1.49 USD/kg |
5.88 USD/kg |
According to the above table, the first objective, which is “Adjustable length” can be obtained by altering the design of the wristband. Therefore, the ranking will be based on cost as minimizing cost is the second objective.
All the data are obtained from CES Edupack software. Polyvinyl chloride, the cheapest among three materials, is chosen as it fulfills the second objective which is “Minimizing cost”.
Indicator
Table 34: Criteria for the Indicator
|
FUNCTION
|
Change colour when ions concentration change |
|
CONSTRAINTS
|
· Able to change colour · Respond to human sweat · Non-toxic |
|
OBJECTIVES
|
· Responsive · Minimize cost |
|
FREE VARIABLE
|
Choice of Technology |
Screening through the constraints
The ability to reflect the changes of ion concentration by changing colour is crucial for deciding which technology to be used for indicators.
Table 35: Constraints for the Indicator
|
MATERIALS |
Colloidal Technology (Gold nanoparticles) |
pH Technology (Litmus paper) |
|
Change Colour |
Yes
|
Yes |
|
Response to hydration level |
Yes
|
No |
|
Non-toxic |
Yes
|
Yes |
The above table clearly indicates that only colloidal technology using gold nanoparticles is able to pass the required constraints as it will response to ions changed in human sweat and hence to hydration level.
Superabsorbent pad
Table 36: Criteria for Superabsorbent pad
|
FUNCTION
|
Absorb and store sweat |
|
CONSTRAINTS
|
· High absorbency · Non-toxic
|
|
OBJECTIVES
|
· Minimize cost
|
|
FREE VARIABLES |
Choice of material |
Screening through the constraints
Table 37: Constraints for Superabsorbent pad
|
MATERIALS |
Sodium Polyacrylate
|
Potassium Polyacrylate |
|
Absorbency |
800 times its weight
|
200-1000 times its weight |
|
Non-toxic |
Yes
|
Yes |
After screening through both constrains both Sodium Polyacrylate and Potassium Polyacrylate, it is found that both candidates are able to meet the constraints. Therefore, final decision should be based on the objective.
Ranking through the objectives
Table 38: Objectives for Superabsorbent pad
|
MATERIALS |
Sodium Polyacrylate
|
Potassium Polyacrylate |
|
Price |
10 USD/kg
|
18 USD/kg |
Ranking through the objective which is minimizing cost, sodium polyacrylate is chosen due to cheaper price compared to potassium polyacrylate. During our research, it is also found that sodium polyarcylate is widely used in baby diapers and feminine hygiene products whereas potassium polyarcylate is mostly used for fertilizers.
Absorbent pad with indicator
Table 39: Criteria for Absorbent pad with indicator
|
FUNCTION
|
Absorb sweat and accommodate indicator |
|
CONSTRAINTS
|
· Lower absorbency compared to super absorbent pad · Good durability when in contact with sodium(weak alkalis) |
|
OBJECTIVES
|
· Minimize cost · Material compatibility
|
|
FREE VARIABLES
|
Choice of materials |
Screening through the constraints
Table 310: Screening for Absorbent pad with Indicator
|
MATERIALS |
Polyester(cotton) |
Polypropylene |
Polychloroprene(sponge) |
|
Durability with weak alkalis
|
Acceptable |
Excellent |
Excellent |
|
Water absorption @24hrs
|
1.25% |
2.2% |
0.7% |
Screening through the constraints, it is observed that all the materials are durable and have low water absorption rate. Therefore, ranking needs to be done by objectives.
Ranking through the objectives
Table 311: Ranking for Absorbent pad with Indicator
|
MATERIALS |
Polyester(cotton) |
Polypropylene |
Polychloroprene(sponge) |
|
Materials Compatibility |
Excellent |
Excellent |
Poor |
|
Price
|
10.97 USD/kg |
1 USD/kg |
6.68 USD/kg |
http://www.skydrol.com/pages/materials_chart.asp
Ranking through the objectives, it is found that polycholoroprene has very poor materials compatibility. On the other hand, polyester and polypropylene have excellent materials compatibility. Nevertheless, polypropylene is chosen as it fulfills both of the objectives which are minimizing cost and materials compatibility.
Top Cover
Table 312: Criteria for Top Cover
|
FUNCTION
|
Protect the indicator pad and reduce evaporation
|
|
CONSTRAINTS
|
· Transparent · Able to react with epoxy(glue)
|
|
OBJECTIVES
|
· Good in blocking UV radiation · Minimize cost
|
|
FREE VARIABLES |
Choice of materials |
Screening through the constraints
Table 313: Screening for Top Cover
|
MATERIALS |
Polycarbonate |
Polyethylene terephthalate |
Polystyrene |
|
Transparency |
Transparent |
Transparent |
Transparent |
|
React with Epoxy |
Yes |
Yes |
Yes |
Screening through the constraints, it is observed that all the materials can meet the constraints. Therefore, it is needed to rank the materials through the objectives.
Ranking through the objectives
Table 314: Ranking for Top Cover
|
MATERIALS |
Polycarbonate |
Polyethylene terephthalate |
Polystyrene |
|
Good in blocking UV radiation |
Fair |
Good |
Fair |
|
Cost |
4.34 USD/kg |
2.8 USD/kg |
2.27 USD/kg |
From the above table, polyethylene terephthalate is better in blocking UV radiation compared to the other two. However, it is not a high prerequisite to have so much high in both transparency and blocking UV radiation. Since, polystyrene is cheaper than polyethylene terephthalate; it is more rational to choose polystyrene to save the overall production cost.
Temporary Bottom Cover
Table 315: Criteria for Temporary Bottom Cover
|
FUNCTION
|
Protect the indicator pad from contamination |
|
CONSTRAINTS |
· Prevent particles from passing through |
|
OBJECTIVES
|
· Minimize thickness · Minimize cost
|
|
FREE VARIABLES
|
Choice of materials |
Screening through the objectives
Table 316: Screening for Temporary Bottom Cover
|
MATERIALS |
High Density Polyethylene |
Polyurethane |
Polycarbonate |
|
Permeability |
59.6 cm3.mm.m-2.day-1.atm-1
|
70.75 cm3.mm.m-2.day-1.atm-1 |
98.3 cm3.mm.m-2.day-1.atm-1 |
Screening through the objectives, it can be seen that all the materials can prevent particles such as dust from passing through. There, it is again needed to rank the materials through the objective.
Ranking through the objectives
Table 317: Objectives for Temporary Bottom Cover
|
MATERIALS |
High Density Polyethylene |
Polyurethane |
Polycarbonate |
|
Price |
1.86 USD/kg
|
5.89 USD/kg |
4.34 USD/kg |
|
Density |
0.96 g/cm3
|
1.10 g/cm3
|
1.20 g/cm3
|
Ranking through the objectives, it is observed that the densities of the materials are close enough and thus the only objective that will help us to choose is the pricing. High Density polyethylene, being the lowest price is therefore chosen as the temporary bottom cover.
Final Design
After considering different aspects of the components, the final design is to be made into a colour changing wristband. This section will illustrate the details of the wristband and the technology used to produce it.
Components of wristband
2cm
30 cm
1.5cm
Figure 4: Final design of the Wristband
The final design of the wristband will have the following dimensions.
· Length of wristband = 30cm
· Width of wristband = 1.5cm
· The transparent indicator pad = 3 cm x 2cm
The transparent indicator pad consists of transparent polycarbonate, nonwoven polyester with AuNPs, pulp with sodium polyacrylate (superabsorbent) and temporary polystyrene sheet. The diagram below shows the schematic of the transparent indicator pad.
Transparent polystyrene
Nonwoven polypropylene with AuNPs
Nonwoven polypropylene with AuNPs
Pulp + Sodium Polyacrylate
Temporary polyethylene sheet (Optional)
Figure 5: Schematic of Transparent Indicator Pad
Indication of Hydration Levels
Sweat sodium concentration
Sweat rate is highly depends on the density of eccrine glands. These glands are found within the first 3 mm of the skin and their density range from 37 to 518 glands.cm-2. Forearm have been chosen for wearing the device as it has the average gland density of 104 glands.cm-2 and hence, it represents the average sweat rate of the whole body.
Figure 6: Regional distribution of physiologically active eccrine sweat glands, skin surface areas, gland counts and glandular dimensions
During exercise, sweat sodium ions concentration will vary from 26.5 to 49.7 mM. The average value for the normal condition is 40mM according to Jonathan Toker, 2009. Therefore, 26.5 mM corresponds as one extreme condition for over hydration, where the sodium concentration of the body hit the dangerous level leading to hyponatremia. On the other hand, 49.7 mM represents the dehydration extreme, hypernatremia.
Device Requirements
Colloidal property of gold nanoparticles (AuNP) is the main concept of the device. The citrate capped gold particles will aggregate and show color change upon adding different concentration of sodium ions. The idea has been applied in the device. When the sweat gets absorbed into the materials, the stability of the nanoparticles will be affected.
Sample synthesis of AuNP
Turkevich method
This method was developed in 1951 and it is the most established way to synthesize gold nanoparticles. Following this method, several methods were developed with different types of capping. In Turkevich method, sodium citrate acts as both stabilizing and reducing agent.
Theory
AuNPs are produced by reducing gold salt (HAuCl4) with sodium citrate at a temperature of ~ 100˚C. The solution will be stabilized with 15 nm to 150 nm of AuNPs equally dispersed throughout the solution. The particles are usually described as colloids and they show Tyndall effect of light scattering. Therefore, upon synthesis, the sizes of the colloids are too small when compared to the wavelength of the visible light (400-800nm). Hence, the solution is observed as colorless.
Addition of Na+ ions
In the presence of positive salt ions, Na+ (> 0.1M) in this case, the colloid will start to aggregate and the solution color will be changed from colorless all the way to dark grey. The step of color change spectrum is shown in Figure 7.
Figure 7: Color change upon addition of NaCl into stabilized citrate capped gold solution
Three drops of NaCl solution
One drop of NaCl solution
Application of the concept
The color change of the gold nanoparticles in visible range is utilized to identify the hydration level of the athlete. Under normal condition, the start condition will be yellowish color. As the exercise proceed, the change in color will correspond the hydration level of the athlete. For example, while an athlete is exercise without any water compensation, the perspiration will eventually lead to dehydration (hypernatremia) in which the concentration of the sweat will be getting higher and hit the extreme condition of 49.7mM (red wine). On the other hand, when the athlete is well aware of dehydration treat and intakes too much water while exercising. In this case, the concentration of Na+ in the body will greatly reduced and leads to overhydration (hyponatremia). So, the device will respond as the other extreme case, becoming colorless at about 26.5mM.
Problem encountered
The stability of AuNPs is only altered at above 0.1M of NaCl concentration. However, the concentration of Na+ ions in the human sweat concentration is way too low when compared to the laboratory trigger. Therefore, the stability of colloidal gold nanoparticle will have to be developed in a way that the particle will be less stable and perform color change at low Na+ concentration level.
Development of AuNPs for Sweat Na+ Concentration
In citrate capped gold solution, the gold nanoparticles are surrounded by negatively charged citrate ions as shown in Figure 8. Hence, gold-citrate interaction leads to a protection layer rich in negative charges, which create repulsive forces between gold nanoparticles, in turn, preventing the aggregation the nanoparticles. Upon adding Na+ ions, those ions will react with the negative citrate ion. So, the protection layer is destroyed and the AuNPs will start to aggregate.
Figure 8: Illustration of stabilized AuNP
For the project application, the stability of the gold nanoparticles has to be tuned to be stable at 40 mM of Na+ concentration with yellow color. This is where the concept of DLVO theory comes into importance. The theory states that the stability of the colloids in the ions solution is achieved by the balance between the electrostatic repulsion force and Van der Waals attraction force. Van der Waals force is determined by the size of the colloid and the electrostatic repulsion depends on the amount of citrate ions on the surface of the gold nanoparticles. According to the previous research, when the zeta potential of the colloid is above 20mV of magnitude, the particles are stable. Therefore, the objective is to tune the stabilization of zeta potential. The one which starts to aggregate at 0.1 M of sodium chloride has the size of 20 nm and zeta potential of 20 mV. Researches has proven that size, shape and capped ion concentration adjustments are done by varying reaction temperature, pressure, and concentration ratio of the gold salt and capping agent.
The shaded area will be the project scope of nanoparticles reaction region. Point A is the initial position for AuNP. The color of the solution will be yellow and the citrate shell of two AuNPs will be slightly overlapping. During dehydration, the position on the graph will move to point B where the solution becomes more overlapped and color changes to red wine. On the other hand, when overhydration condition, the particles will split far apart again which is Point C.
Figure 9: Ionic strength vs distance between colloidal gold nanoparticles
A = normal condition
B = dehydration condition
C= over hydration
Processing
Fabrication of AuNPs treated polypropylene
AuNPs were synthesized by using sodium citrate and aqueous HAuCl4 solution. Non-woven polypropylene sheets were dipped into container containing AuNPs until the AuNPs are diffused homogenously into polypropylene sheet. After dipping, the polypropylene sheets were rinsed thoroughly with distilled water to remove any loosely bound AuNPs, and the papers were air-dried and stored at 50% relative humidity and 23 ᵒC.
Producing components of the wristband
Absorbent pad is produced on a movable conveyor belt that crossed through a forming chamber. At different point of the chamber, there will be pressurized nozzles that spray either polymer particles or fibrous materials onto the conveyor surface. The conveyor is holed so while the material is sprayed onto the belt, a vacuum is applied from below so the fibers are pulled down to form a flat pad. The mixture of polymer and fibrous material is important since it controls the consistency of the pad. Multiple spray dispensers are applied to form several layers of polymers and fibers. As the vacuum pulled down the fibers to the bottom pad, polymer is added to form a layer of combined polymer and fiber, and then more fibers will be added on top. Moreover, PVC wristbands are produced using injection molding. On the other hand, transparent polycarbonate sheets are produced by thermoforming process. Polystyrene sheets are produced by expandable blow molding.
Assembling the components
The internal pad (the pad containing AuNPs) is proposed process in continuous process for example conveyor belt.
1. The pre-produced absorbent pad which has the dimension of 1cm x 2cm are put onto conveyer belt of another machine once they have been produced.
2. A layer of non-woven polypropylene with AuNPs particles which has the dimension of is 2cm x 3cm are placed on the top and bottom the absorbent pad.
3. Once three layer pads are formed, the pads are then compressed using roller.
4. A transparent polyethylene film is added at the bottom of the pad and transparent polystyrene is added at the top.
5. Transparent polyethylene side is then glued (using epoxy) to the pre-manufactured PVC wristband.
Environmental Impact
Eco Audit
Figure 10: Carbon dioxide footprint for PVC and Silicone production
Figure 11: Embodied energy for PVC and Silicone
After going through the eco audit using CES software, it is apparent that PVC is more environmentally friendly due to its low carbon emission and embodied energy in the material life cycle. However, the carbon emission and embodied energy for manufacturing is slightly higher for PVC rather than Silicone, yet it is still compensated as a whole.
Details of the materials
The embodied energy and carbon footprint for each of the components are tabulated in the table below.
Table 51: The embodied energy and carbon footprint for each of the components
|
Material
|
Function |
Embodied Energy |
Carbon Footprint |
|
Polyvinyl Chloride
|
Wristband |
58.3 MJ/kg |
2.495 kg/kg |
|
Polypropylene
|
Absorbent pad with indicator |
24.2 MJ/kg |
0.674 kg/kg |
|
Polystyrene |
Top Cover
|
29.1 MJ/kg |
1.11 kg/kg |
|
High Density Polyethylene |
Bottom Temporary Cover |
27.5 MJ/kg |
0.944 kg/kg |
|
Total |
139.1 MJ/kg |
5.223 kg/kg |
From the above table, it can be seen that the embodied energy and the carbon footprint of the design is not very high and actually most of the materials can be recycled. In conclusion, it can be said that our design is environmental friendly.
Discussion
Advantages
The common way for individuals to measure their hydration level is by testing their body’s liquid such as blood and urine or by measuring their body’s weight. Unlike those common assessment methods, this design will be able to give direct feedback throughout the exercise. The color change method makes the design in a way that an athlete can monitor his/her hydration status without the help of an expert. In addition, the method simply is a strap to be worn on the forearm. It is non-invasive like assessing hematological index. The athlete can exercise anytime of the day without having to be worried about the amount of fluid intake and output like in body weight assessment. No color strip is needed to dip into the urine, which can be very inconvenient during exercise. The design will detect the change without the help of any fancy laboratory equipment and procedure. It reduces all the intricate method of monitoring hydration condition by just a strap on the wrist. The design can easily be adjusted to fit different individuals. Moreover, the overall manufacturing processes and materials chosen are safe and eco-friendly.
Limitations
The prototype will be developed based on the figures available for the average non-athlete person. Since the sweat sodium concentration will vary according to the type and class of athlete such as from that of runners to swimmers, different classes of product are needed to respond change in sweat concentration range. In addition, the design is disposable and it can only be used for one section of competition for not more than five hours. The specifications mentioned in this project are hypothetical and have to be adjusted through research and development in order to achieve optimum performance.
Recommendations
The current material used for detecting hydration levels is gold-nanoparticles-based. The hypothesis method to cap stable gold nanoparticles at desired range is citrate capping. For further development of different ranges of sweat concentration, other capping agents such as 3-thiophenacetic acid (TA), peptides, polymer ligands and chitosan are available. Each and individual will affect the stability and size of the nanoparticles and hence, their colloidal behaviors.
Conclusion
The purpose of the designed device was to enable individuals to detect their hydration level while exercising. Apart from the existing technology to assess hydration status, the nanotechnology has brought a breakthrough in this industry. The application of gold nanoparticles has a huge potential in biomedical field such as drug delivery. Hence, we have brought this advanced technology to monitoring the hydration status effortlessly.
The purpose was achieved through manipulating the stability of gold nanoparticles by changing the zeta potential. It was found that the average sodium concentration level in normal condition for a person is 40mM, while overhydration and dehydration is 26.5mM and 47.9mM respectively. The indicator pad will change color from yellow to colorless when the person overhydrates whilst yellow to red-wine when the person dehydrates.
In conclusion, although the design is theoretically viable to develop the idea of the project, the actually experiments are yet to be carried out to prove that the concept is indeed working. External factor such as individual sweat rate variations and heat acclimation can be major drawbacks in producing a standard device for all classes of people.
Despite the limitations and weaknesses of the design, our team strongly believes that the barriers will surely be overcome through consistency and hard work in further research and development.
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Appendix
DLVO Theory
The DLVO theory is proposed by Deryagin and Landau and in the other, were called Verway and Overbeek. They were curious on how colloidal particles managed to stay dispersed in solution, on the other hands, they tried to figure out the factors that causing colloid to precipitate.They worked out the mathematics which described the forces acting on colloidal particles. The DLVO theory named after the initials of their names.
The DLVO theory states that there are two main forces acting on charged colloidal particles in a solution. The two forces are electrostatic repulsion and van der Waals attractive forces. Colloidal particles carry a surface charge with a certain magnitude in aqueous solution. The surface charge will attract ions of opposite charge to form a Stern layer where the ions are strongly bound. Van der Waals attraction happens when one molecule of the colloid has attractive force to each molecule in the other colloid. However, adsorbed layer on each particle prevents the particles from coming close enough for van der Waals attraction to cause aggregation.
Figure 12: Surface charge
The location of slipping plane varies with surface morphology. It is purely for the purpose of zeta potential determination. Zeta potential is the potential at the slipping plane. It shows that colloid particles start to aggregate at a zeta potential of around 10mV, whereas it is stable above this. Zeta potential determines the electrostatic repulsion between particles. The repulsion would be stronger and system becomes more stable, if the zeta potential is high enough. The most common method to measure zeta potential is Optical Methods by measuring electrophoretic mobility of particles under an applied parallel electric field.
The overall force which acting on a colloidal particle by adding these two opposite forces together. It can be done when it approaches another particle or a charged surface.
The overall shape of this resultant force which shown on the figure below depends on the ionic strength of the fluid. It is important as this affects the depth of the electric double layer and, therefore, the electrostatic repulsion. Energy can be thought to be a energy barrier in the repulsion regime. We can increase or decrease the energy barrier by changing the ionic or pH environment or adding surfactants to affect the surface charge of the colloid. In this case, zeta potential can be measured to know the stability of dispersion.
The repulsion force is sensitive to the ionic strength of the surrounding fluid. The force due to the overlapping electric double layers as the salt concentration increasing. This is because the depth of the electric double layer shrinks as the ionic strength increases by increasing salt concentration.
|
The green line shows the force acting on the cells as they approach each other. The particles feel a small attraction over a distance of about 10nm. When the particles approach each other, but electrostatic repulsion start to increase meanwhile van der Waals force does not change over a shot range. However, there is an increasingly stronger repulsion when the particles get closer to each other. The van der Waal force kicks in at a distance of approximately 2nm, and becomes dominant at 1nm. The particles undergo strong attraction to the other particles or surface at a distance lesser than 1nm. |
|
Figure 13 |
The graph above shows what is happening at physiological ionic strength of the particles. At higher ionic strength however, the electric double layer shrinks. This affects the strength of the repulsive force but not the van der Waals force.
The electrostatic repulsion reduces as ionic strength increasing, which means that the particles can get closer. If the ionic strength is high enough the cells feel no repulsion at all and slam together at high speed. So, adding salt (sodium ions) causes the cells to aggregate and fall out of suspension. It would also cause them to stick tightly to a surface.
