Design, development, and operation of an ISO class 5 cleanroom for planetary instrumentation and planetary protection protocols

Abstract

Cleanrooms are controlled environments where the number of airborne particles is reduced to a level defined by an International Organization for Standardization (ISO) standard. These facilities have applications in different fields, such as the electronic, pharmaceutical, and healthcare sectors, and are also necessary for the assembly, testing and handling of space hardware. Cleanrooms are expensive to build and maintain and require the permanent designation of infrastructures and dedicated spaces within a building. Once built, clean rooms cannot be used for any other purpose, as contamination is a significant risk. The restricted access to these facilities limits the process of designing, testing, and calibrating instruments developed by academic institutions, small companies, and space startups. Here we present a Commercial Off-The-Shelf (COTS) procedure for building and maintaining a highly controlled ISO class 5 cleanroom, according to ISO14644 standards. We provide a detailed explanation of how to design, develop and operate a portable, modular, and cost-effective ISO class 5 cleanroom that can be used for the usual workflow of development, integration, test procedures and planetary protection associated with the design of instrumentation for planetary exploration.

1. Introduction

Cleanrooms are special, semi-closed laboratory facilities where the concentration of airborne particles is reduced with respect to the natural environment to a level defined by standards. The level of the desired reduction of ambient airborne contamination depends on the specific purpose of the application. Airborne particles are aerosols of typical sizes between 0.1 μm and 10 μm, of different origins, such as mineral dust, aerosols with chemical aggregates from industrial or atmospheric processes, ashes from natural or industrial combustion, micron-sized cells, spores, and pollen [1]. Cleanrooms are extensively used in the space sector, as all spacecraft components and payloads, including the electronic, optical elements and solar panels, need to be manipulated under strict cleanness requirements [2]. In addition, for all spacecraft and instruments launched to space to explore a solar system object, certain biomass control protocols need to be applied to prevent the forward biological contamination of planetary environments with terrestrial bioburden [3].

Cleanrooms are regulated by the International Standard Organization (ISO) 14,644 standard, irrespective of their application for one or another sector (aerospace, microelectronics, pharmaceuticals, medical devices, healthcare, food, …). The ISO 14644 standard specifies the requirements for a cleanroom's design, construction, validation, and operation and classifies cleanrooms from ISO1 to ISO9, with ISO class 1 being the cleanest and ISO class 9 being the dirtiest. The ISO 14644 standard replaced the US Federal Standard 209E, which graded cleanrooms as “Class 1/10/100/1000/10,000/100,000″, with the class number representing the maximum concentration level of airborne particles per cubic foot.

The Planetary Protection (PP) Protocols defined by the Committee on Space Research (COSPAR) aim to prevent the forward contamination of outer space by terrestrial organic constituents and biological contamination and to avoid any returned, possible harmful contamination. PP protocols are implemented to prevent scientific expeditions from jeopardising research on possible existing extraterrestrial life and to prevent backward contamination of the earth's ecosystem with extraterrestrial matter carried by returning interplanetary spacecraft missions [4]. COSPAR has categorised space missions into five categories, see Table 1, and has formulated the corresponding bioburden requirements [5].

Table 1.

Categories of missions with respect to the planetary protection total bioburden requirement, depending upon the target celestial body of interest [4].

Mission Category	Contamination Prevention	Mission type	Target planetary bodies	Bioburden level requirements
Category I	Forward	Lander/Orbiter/Flyby	Sun, Mercury, Venus, Moon, Asteroids except for C-asteroids	No bioburden requirements
Category II	Forward	Lander/Orbiter/Flyby	Jupiter, Saturn, Uranus, Neptune, Pluto, Comets, C-asteroids	contamination avoidance probability <1.0 × 10−4
Category III	Forward	Orbiter/Flyby	Mars/Europa/Enceladus, others TBD	≤5 × 105 spores at launch or <1.0 × 10−4 contamination avoidance probability 50 years after launch
Category IV	Forward	Lander	Mars/Europa/Enceladus. others TBD	≤3 × 105 spores
Category V-restricted	Backwards	Earth Sample return	Mars/Europa/Enceladus, others TBD	≤30 + (2 × 105) spores
Category V- unrestricted	Backwards	Earth Sample return	Venus, Moon	Same as Category I and Category II

The bioburden control and analysis for these missions must be performed in a controlled cleanroom environment where the concentration of airborne particulate matter is regulated to a specific limit. COSPAR recommends a minimum ISO class 8 cleanroom and better for assembly, testing and validation of space hardware. However, the parts of space hardware that may be in contact with environments of interest for life (category IV and V) must be handled in ISO class 5 or lower. The requirements are specific to each mission and may even differ for each spacecraft part. For instance, the ExoMars mission utilised ISO class 7 and ISO class 8 cleanrooms for handling and operating bioburden-controlled hardware that would not be in contact with the Martian surface, while ISO class 5 and ISO class 3 cleanrooms were used for handling and transport of hardware that were to be in contact with Mars samples [6]. Cleanrooms provide a controlled environment for space hardware development ensuring reliability of developed hardware. Once a hardware is launched into space, especially on deep space missions, there is no possibility of servicing and extreme care should be taken to avoid any possible malfunctions. Particulate matter poses a serious threat to the reliability of space hardware as they can adhere to critical surfaces such as sensing elements and electronic circuits interrupting the operation of the hardware. The need to have access to a cleanroom facility imposes operational and costs limitations to the design, testing and calibration processes of multiple instruments that academic institutions, small companies, and new space startups develop. These organisations that may have experience in electronics, optical components or fundamental research for terrestrial applications usually have no permanent access to a high ISO-level clean room. In general, cleanroom facilities are costly and designed, built, and maintained only for strategic technological sectors that can guarantee continued use. Access to a portable, modular, lower-cost clean room facility may facilitate incorporation into the space sector of other institutions such as universities, startups, and small technological companies. In addition to complying with the ISO 14644 standards, the requirements for such facilities would be that: 1) they can be installed within a pre-existing laboratory, 2) they can be dismounted and transported elsewhere if needed, 3) the cost of their construction and maintenance can be covered within the life span of a moderate project. This solution is of special interest for short-lived projects that cannot guarantee using the clean room over decades.

Cleanrooms also have various applications in other domains of industry and research and development organisations. Examples are manufacturing semiconductor and optical devices, pharmaceutical production, food industry, aerospace industry, medicine, and microbiological research. In this article, we present the design of a 2 m × 1.5 m × 2m modular, cost-effective, portable cleanroom based on a combination of hard and soft wall construction elements utilising Commercial Off-The-Shelf (COTS) components. Once built, this cleanroom is ready for planetary research and space hardware manipulation. Without loss of generality, we present an example optimised to create a small unit where two operators can work simultaneously, but the design can be adapted to other needs. The cleanroom has been designed and developed according to the requirements of the ISO14644–4:2022 [7] and ECSS-Q-ST-70-58C standard [8] and has been validated to ISO class 5; in operation; ≥0.5 μm according to the ISO 14644–1:2015 standard [9]. This paper presents a COTS-based approach to develop a highly controlled cleanroom for performing planetary investigation experiments that require a sterile background to prevent contamination by airborne particles. The cleanroom design ensures that universities and interested bodies can develop a particle-controlled environment, facilitating the development of payloads and instruments.

In this work, we provide an end-to-end description of the construction and maintenance of a clean room environment that can be used for the usual workflow of development, integration, testing and planetary protection procedures associated with the design of planetary exploration instrumentation.

The following sections cover the three critical stages associated with clean-room facilities: 1) cleanroom design and construction, 2) cleanroom validation and monitoring, and 3) cleanroom maintenance [10,11].

2. Cleanroom design and construction

Cleanrooms can be broadly classified into two categories based on their construction: hard walls and soft walls. Hard-wall cleanrooms are fabricated from acrylic, static-dissipative Polyvinyl Chloride (PVC), polycarbonate, and polypropylene with frames of powder-coated steel and aluminium soft-wall cleanrooms use a more straightforward steel frame structure to support flexible PVC curtains. Soft wall cleanrooms provide minimal positive pressure and are used in lower cleanliness ratings, while hard wall cleanrooms offer more control over air flows, temperature, and humidity control.

The proposed cleanroom is a hybrid, low-cost construction of soft and hard wall cleanrooms to achieve an ISO class 5 cleanliness level. The construction of the cleanroom is based on the guidelines of the ISO 14644–4:2022 standard [7] with requirements of cleanliness and contamination control for space hardware listed in the ECSS-Q-ST-70-01C standard [2]. The target contamination control area required for our experiments was limited to 2 m × 1.5 m.

2.1. Airflow control and particulate filters

ISO14644–4:2022 standard has undergone a significant revision since the last revision in 2001. The ISO14644–4:2022 standard emphasises the airflow concept, classifying the cleanroom airflow into three categories based on the airflow pattern. The three categories are:

• •Unidirectional Air Flow (UDAF) pattern
• •Non-Unidirectional Air Flow (non-UDAF) pattern
• •Combined Air Flow pattern

The proposed cleanroom design is based on the non-UDAF airflow pattern that relies on a dilution ventilation mechanism to control any airborne contamination by introducing clean supply air into the cleanroom through air supply units. ISO14644–4:2022 standard recommends UDAF airflow for ISO class 5 and lower cleanrooms, however, the smaller workspace area of the cleanroom (2m × 1.5m) should be sufficient with the non-UDAF airflow for particle control. Unlike the table method used to determine the airflow changes needed in the cleanroom, the latest revision provides an equation, Equation (1) to calculate the needed airflow changes per hour based on the particle source strength, desired particle concentration limit and the cleanroom's ventilation efficiency (VE). The equation is shown as follows:

$$Q=\frac{S}{\epsilon .C}$$ (1)

where,

$Q$ is supply air volume flow rate to the cleanroom (m³. s⁻¹)

$S$ is rate of particle emission in cleanroom (source strength) (number. s⁻¹)

$C$ is particle concentration limit in the cleanroom (number.m⁻³)

$\epsilon$ is ventilation effectiveness (dimensionless quantity)

One of the critical changes in the revised cleanroom construction standard was to use the air volume flow rate in the design of the cleanroom rather than the air change rate as listed in the table of the ISO14644–4:2001 standard Annex B.2. The use of air change rates does not consider the volume of the room and lead to discrepancies in sizing the air handling units needed resulting in higher capital and energy costs.

The proposed cleanroom is designed based on the occupancy of two persons and uses low-level extracts to flush out the airborne contamination of the cleanroom space. The cleanroom design assumes zero particle concentration introduced through the fan filter units, and the only source of particle emissions considered is from the operators inside the cleanroom. The source strength (S) is the total aggregate of all particles, and we have used literature references [12,13] to determine the expected source strength of the particles. With two operators wearing disposable Tyvek hood-to-boot cleanroom garments and moving through the cleanroom space, the particle emission rate is expected to be an average of 1200 particles/s of ≥0.5 μm size.

According to the ISO14644–4:2022 standard, the ventilation efficiency indexes can be obtained through literature similar building designs or can be computed using computational fluid dynamics (CFD) analysis. VE index can be either the Air Change Effectiveness (ACE) index or the Contamination Removal Effectiveness (CRE) index. According to the guidelines of the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA), CRE should be used as the VE index only when the purpose of the ventilation unit is to remove particles generated from a static source [14]. Hence, in the current design of the cleanroom, we have considered ACE as the VE index. The ACE index of non-UDAF cleanrooms has been analysed through experimental study and field testing [15,16]. The study has shown that a combination of efficient air supply and low-level extracts can provide an ACE index of 0.7. With a similar low-level extract cleanroom design, we have selected the same VE index (ε) for our cleanroom design.

With ISO class 5 as our target and the maximum particle concentration limit of 3520 particles/m³, we have used equation (1) to calculate the supplied air volume flow rate to be 0.4870 m³/s. With the supplied air volume flow rate calculated, the equivalent air changes needed in the cleanroom of 6 m³ volume is determined to be 292 changes per hour according to equation (2).

$$Airchangerate\left(h^{-1}\right)=\frac{airsupplyrate\left(m^3{.h}^{-1}\right)}{volumeofroom\left(m^3\right)}$$ (2)

The core in the modular cleanroom is the Fan Filter Unit (FFU) which is responsible for bringing in filtered air with a positive pressure inside the clean space. The positive pressure is an additional protection mechanism that minimises the uncontrolled air inflow from outside this semi-closed environment. The FFU is one of the most expensive elements of a clean room. We suggest refurbishing existing elements for our low-cost, sustainable approach when possible. Alternatively, they can, of course, be bought new. However, this paper includes a detailed description of refurbishing existing FFUs. This procedure can also extend the facility's lifetime, resetting the fan filters when they clog after long-term operation. FFUs (Model number: 69514019 Envirco® MAC 10® Original Standard) with ULPA filters were sourced from a previously operated ISO class 5 cleanroom facility. Envirco® MAC 10® Original Standard combines low sound (51dBA), low watt (310W @ 0.45 m/s air flow velocity) and low profile of 330 mm. The FFU units have an active area of 1.2 × 0.6 m with a maximum flow rate of 650 Cubic Feet per Minute (CFM). Equation (3) defines the number of FFU required to obtain the targeted airflow changes per hour in a known volume of cleanroom space.

FFU = ((Air Changes per Hour)/60) x ((Volume of Room in Cu. ft)/(Output of loaded FFU in CFM)) (3)

With one FFU providing 0.72 m² active area of air flow, we have used 2 FFUs in our cleanroom facility providing a 50 % ceiling coverage of air flow into the cleanroom. The total power consumption of the units is 620W at full capacity.

The airflow velocity of the used FFUs was tested in a standard laboratory environment to determine the blockage of the filters which had served past their lifetime. A Testo 405-V1 Mini Anemometer was used to determine the airflow velocity from the used FFU. The velocity was found to be around 0.2 m/s which was far below the rated velocity of 0.45 m/s. This type of speed test should be done during maintenance operations of the clean room. The FFU filter was dismantled for visual inspection, which revealed severe debris collection over the deep-pleated Ultra-low Penetration Air (ULPA) filter. The carbon prefilter had failed and had started disintegrating, and these small parts were accumulating over the ULPA filters clogging the pores. The ULPA filters can be replaced, and the process involves discarding the entire filter with its stainless-steel support structure and substituting it. We suggest implementing a cost-effective and sustainable approach, using commercial deep pleated High-Efficiency Particulate Air (HEPA) filters like those used in standard, portable commercial HEPA air filters. An FY2422 HEPA filter of the type compatible with Philipps 2000 air purifiers was selected as their size enabled easy integration into the existing filter frame structure of the FFU, and the 50 mm deep pleated structure offers H14 grade sufficient for ISO class 5 requirements. HEPA filters can remove 99.95 % of particles of diameter equal to 0.3 μm and find application in cleanrooms of ISO class 5 to 9. ULPA filters have a particle removal efficiency of 99.999 %, capable of removing particles of diameter equal to 0.12 μm, finding application in critical particle control environments with requirement of ISO class 1–4 cleanrooms.

Six HEPA filter cassettes of FY2422 HEPA filter type were used to refurbish the filters of each FFU. Fig. 1 illustrates the refurbishment process of the filters. The stripping of the contaminated ULPA filters from the frame structure was performed wearing a Twin HEPAC pleated respirator with a full-face mask to prevent inhaling the particles from the contaminated filter.

Fig. 1.

Refurbishment process of FFU filter assembly: (1) ULPA filter clogged by deposits from failed prefilter; (2) ULPA filter removed, and stainless-steel mount cleaned with 99.7 % IPA; (3) L-shaped plastic corners used to create a framework to hold the replacement HEPA filter cassettes; (4) HEPA filter cassettes placed inside the framework and sealed with PU40 hybrid sealant and adhesive.

Polyurethane (PU) sealant is utilised in industrial filter assembly processes to seal the deep pleated filter within the stainless-steel structure. Polyurethane offers good flowability, strength and stability needed to hold the filters in place. The existing sealant was stripped off, and the deep-pleated dirty ULPA filters were removed. PVC L-shaped angle was used to make a frame to hold the HEPA filter cassettes. Double-sided adhesive tapes held the L-shaped angles, and HEPA filter cassettes were placed into the frame slots. PU40 hybrid sealant and adhesive were used to seal the spaces around the HEPA filter cassettes, frame structure and stainless-steel structure to ensure a strong hermetic seal. PU40 is not spontaneously combustible and has a flash point greater than 200 °C [17]. The refurbished filter assembly was allowed to cure for about 24 h and covered with plastic film to avoid contamination during storage.

The damaged prefilters of the FFU were replaced with new carbon prefilters. The motors of the FFU were inspected, and the blades were cleaned with 99.7 % Iso-Propyl Alcohol (IPA). The entire fan filter unit was cleaned with WD-40 multi-purpose cleaner and 99.7 % IPA. Fig. 2 shows the FFU units before and after the refurbishment process. The refurbishment process explained in Fig. 1, Fig. 2 can be, of course, skipped if a new FFU assembly or filter assembly is acquired.

Fig. 2.

Refurbishing prefilter and FFU exterior: (1) FFU unit before refurbishment with damaged prefilter; (2) FFU Unit refurbished by degreasing and cleaning exterior with new prefilters installed.

2.2. Mechanical structure for walls and ceilings

The cleanroom structure was modelled in SolidWorks CAD, incorporating 40 mm × 40 mm aluminium strut profiles to build the framework of the cleanroom. Aluminium strut profiles offer cost-effectiveness and modularity and are easily accessible. Fig. 3 shows the CAD model of the 3 m² cleanroom designed in SolidWorks.

Fig. 3.

CAD Model of the modular cleanroom designed in SolidWorks.

The cleanroom side walls were fabricated in Polycarbonate (PC) due to their strength and toughness. With 2 m long side wall panels needed for the construction, PC offered excellent durability in transportation and handling with its high impact resistance.

In addition, 7 Clear PVC refrigeration curtain strips of 2 mm × 200 mm were overlapped to build the entrance to the cleanroom. Soft wall cleanrooms require a gap at the bottom for the filtered air to escape. A smaller gap would result in the generation of an upward draft stirring up particles settled on the floor due to the constricted area for the airflow to diffuse. A more significant gap would result in a loss of positive pressure leading to particle contamination from the outside. The required gap was determined to be 211 mm using equation (4).

Gap (m) = (Number of FFU x 0.72) / (Perimeter of room (m)) (4)

The sidewall panels, ceiling with cut-out for the FFU and lighting were designed in SolidWorks, and the 2D drawing files were generated for fabrication. Vinyl flooring was laid over the existing lab floor, over which the cleanroom was assembled. Vinyl flooring was chosen due to its low level of particle release. These floors also offer easy maintenance and cleaning and can be easily replaced if damaged. Fig. 4 shows the stages of the construction of the cleanroom.

Fig. 4.

Construction of the modular cleanroom: (1) Vinyl flooring installed and secured over existing lab floor; (2) PVC Ceiling prefabricated and fixed to aluminium profile strut frame; (3) Cleanroom frame construction using aluminium strut profiles; (4) Internal view of assembled cleanroom; (5) Exterior view of the cleanroom assembled and ready for commissioning.

The space between the aluminium strut and the panels was covered with L-shaped plastic corners to prevent particle inflow along the mating corners. Silicone sealant was used to prevent leaks and seal the space between the FFU and the ceiling PVC panel.

2.3. Electrical infrastructure

The cleanroom was fitted with an interlocked switchable socket powered by a 230V glandular industrial plug of 16A capacity. A consumer unit is fitted to the exterior of the cleanroom, which houses a 30 mA trip Sensitivity, type B Residual Current Breaker with Over-current (RCBO) rated at 16A. The RCBO is then interfaced to a dedicated type C Miniature Circuit Breaker (MCB) that feeds power to the FFU, lighting and power sockets inside the cleanroom. Type C MCBs are used considering the likely high-power surges expected during the starting of the FFU and the loads connected inside the cleanroom. The use of RCBO in the cleanroom electrical connections has been followed under the guideline of the BS EN 610091:2012 standard [18]. Fig. 5 shows the interlocked switchable socket and the consumer unit with mounted RCBO and MCBs.

Fig. 5.

Electrical connection to the cleanroom through an interlocked switchable industrial socket and a consumer unit with RCBO and dedicated MCBs.

We recommend adapting the electrical infrastructure depending on the standards and requirements according to the geographical area of installation.

3. Cleanroom validation and monitoring

The developed cleanroom has been validated for its efficiency along four different parameters as shown in Table 2. All the tests were performed in the operation state of the cleanroom with one operator in the cleanroom space.

Table 2.

Standards used in the cleanroom validation.

Test	Test Standard
ISO Classification	ISO14644–1:2015
Particle removal rate test/Recovery test	ISO14644–4:2022/ISO14644–3:2019
Airflow test	ISO14644–3:2019
Aerobic Mesophilic colony count	ECSS‐Q‐ST‐70‐55C

In the space sector, as in other fields, some manipulations have additional requirements on relative humidity and temperature [19]. If this cleanroom is installed within a pre-existing laboratory, then the temperature and relative humidity are usually actively controlled by an external system. We recommend that within the cleanroom, the users should install instrumentation to monitor the temperature and relative humidity of the semi-enclosed ambient, but as there are multiple options in the market, we do not provide any specific recommendation for this. For cleanroom validation, the 3 m² area of the cleanroom is divided into 6 cells of an equal area marked C1–C6, as shown in Fig. 6 below.

Fig. 6.

Top view of the cleanroom, divided into 6 cells of equal area. The FFU1 and FFU2 are the two fan filter units with black circle indicating the sampling locations for the validation tests.

C1–C3 are the critical zones in the cleanroom covering the workbench while C4–C6 represent the regions where the operator moves in the cleanroom. Fig. 7 shows the measuring instruments used in the cleanroom validation tests

Fig. 7.

(left) MetOne HHPC3+ particle counter and (right) Testo 405-V1 Mini Anemometer.

3.1. ISO classification

The cleanroom has been validated in operation state for ISO classification according to the ISO14644–1:2015 standards. A MetOne HHPC3+ particle counter as shown in Fig. 7 (left) was used to validate the cleanliness of the cleanroom. The MetOne HHPC3+ is a 3-channel particle counter capable of measuring particle concentration of sizes 0.3 μm; 0.5 μm; 1.0 μm. Table 3 details the air cleanliness classes by particle concentration of different sizes.

Table 3.

ISO14644–1:2015 ISO cleanroom classification in operation state based on the maximum allowable concentration of particles/m3 of considered target size, 0.5 μm.

ISO classification number	≥0.1 μm	≥0.2 μm	≥0.3 μm	≥0.5 μm	≥1 μm	≥5 μm
ISO class 1	10	–	–	–	–	–
ISO class 2	100	24	10	–	–	–
ISO class 3	1,000	237	100	35	–	–
ISO class 4	10,000	2,370	1020	352	83	–
ISO class 5	100,000	23,700	10,200	3,520	832	–
ISO class 6	1,000,000	237,000	102,000	35,200	8,320	293
ISO class 7	–	–	–	352,000	83,200	2,930
ISO class 8	–	–	–	3,520,000	832,000	29,300
ISO class 9	–	–		35,200,000	8,320,000	293,000

Considering the cleanroom application for planetary research and bioburden control, 0.5 μm was chosen as the single target size to sample. Aerobic Mesophilic Bacteria are of interest in planetary protection and bioburden control experiments as these microbes can survive under extreme conditions and form spores, which, when reintroduced to optimum conditions, begin to proliferate. These spores have a mean size of 0.5 μm [20] and were chosen as the reference target particle size. The ISO14644–1:2015 standard stipulates that the minimum sample volume is the volume whereby a minimum of 20 particles would be detected if the particle concentration for the largest particle size were at the class limit for the designated ISO class. The volume and the number of places to sample were calculated based on equation (5).

Minimum Sample Volume (L) = (20 / (Limit on number of particles (particles/m³))) x 1000 (5)

The 3 m² area of the constructed cleanroom requires sampling at a minimum of 2 locations. However, we have sampled at 6 locations inside the cleanroom as shown in Fig. 6 with three samples at each location. The sampling volume was calculated to be 5.68 L. The MetOne HHPC3+ has a sampling rate of 2.83 L per minute, and hence the sampling was done for 2 min at each location. Before the particle measurements, the operation of the particle counter was verified for zero counts using the zero-count purge filter. Table 4 lists the particle counts measured at each location with the grand location average. The need for computing 95 % Upper Confidence Limit (UCL) has been removed in the ISO14644–1:2015 standard. The 2015 revision states that 90 % of the cleanroom space should be within the target particle limit and in our analysis, we have found that the 6 sampling locations are within the target limit of 3520 particles of 0.5 μm size.

Table 4.

ISO Class 5 validation of the developed modular cleanroom based on 0.5 μm target size particle, in operation state with one operator inside the cleanroom.

Cleanroom area - 3 sq.m	Number of locations = 6						Grand mean of Location averages
Target particle size - 0.5 μm
Counts/m3	C1	C2	C3	C4	C5	C6	1835.33
Sample 1	1785.70	1071.40	1785.70	1785.70	2142.90	1785.70
Sample 2	1428.60	2500.00	2500.00	2500.00	1785.87	1071.40
Sample 3	2500.00	1428.60	1428.60	2500.00	1785.70	2142.90
Location average	1607.15	1666.67	1904.77	2261.90	1904.82	1666.67
ISO class limit of 0.5 μm particles							3520

The cleanroom described in this work was built within a standard university research laboratory. The laboratory space in which the cleanroom was deployed and the corridor to the lab were tested for particle concentration using the MetOne HHPC3+ particle counter. The lab and the corridor space are prone to higher particle contamination and the minimum sample volume for these locations would be under the minimum threshold set by the ISO14644–1:2015 standard which places a lower limit on the sampling time and volume of a minimum of 2 L sampled for at least 1 min. Fig. 8 compares the average 0.5 μm particle concentration in a logarithmic base 10 scale in the three locations.

Fig. 8.

Comparison of 0.5 μm average particle concentration inside, outside the cleanroom and the lab corridor.

We can observe three orders of magnitude differences in the particle concentration levels with the developed modular cleanroom capable of yielding an ISO class 5 cleanliness level with less than 3520 particles of 0.5 μm size in a cubic meter of air in operation state.

3.2. Particle removal rate test/recovery test

The ISO14644–4:2022 recommends calculating the particle removal rate to determine the ventilation performance of the cleanroom and its ability to reduce the particle concentration when there is an increase in the particle concentration from the source. The particle removal rate test is performed in accordance with the ISO14644–3:2019 guidelines. A commercial fog generator machine was used to introduce the increased particle concentration. A mixture of 30 % EP/USP grade vegetable glycerine and 70 % deionized water was used to generate the particles. The particle concentration of 0.5 μm was increased to 100 orders of magnitude higher than the target base particle concentration and the time taken to reduce to 10 orders of magnitude higher than the target particle concentration was determined. The FFU’s were switched off during the particle generation and were allowed to mix before turning on the FFU’s. The particle concentration was monitored with the HHPC3+ particle counter during the entire test, taking measurements every minute. Fig. 9 shows the particle generation process during the particle recovery test. The semi-log plot shown in Fig. 10 represents the decay of particle concentration with time on the linear scale.

Fig. 9.

Particles generated inside the cleanroom during the particle recovery test.

Fig. 10.

Decay curve of the particle concentration with time.

According to the ISO14644–3:2019 standards the recovery test can be evaluated by recovery time or recovery rate. The recovery time ($t_{0.1}$) taken for 10:1 was determined to be 25 min.

3.3. Airflow test

The airflow velocity is a critical parameter in the cleanroom, which governs the particle motion and impacts how the particles are swept away from the cleanroom space. Poor airflow uniformity leads to turbulent flow and can form vortices that can stir up deposited particles from surfaces back into the air. In order to measure the airflow velocity in the cleanroom, the number of measuring points should be sufficient to determine the supply airflow rate from the FFU. ISO 14644–3:2019 mentions the number of sampling locations according to equation (6).

Number of Sampling locations = √ (10 x Area of cleanroom (m²)) (6)

Testo 405-V1 Mini Anemometer was used to sample the airflow velocity at 6 locations (Fig. 6) based on equation (3). Table 5 shows the average air flow velocity measured at 3 locations within each cell of area 0.5 m². The measurements were made at a distance of 300 mm from the filter face. The mean airflow velocity in the room was measured to be 0.46 m/s.

Table 5.

Airflow velocity measured at the centre of each cell in the cleanroom area.

Cell	Airflow Velocity (m/s)
C1	0.6
C2	0.15
C3	0.5
C4	0.7
C5	0.15
C6	0.65

3.4. Aerobic mesophilic assay

In order to guarantee an adequate planetary protection cleanness level for microbiological experiments and bioburden analysis, the cleanroom was also validated according to the ECSS‐Q‐ST‐70‐55C guidelines [19].

A Sartorius MD8 Airport portable air sampler was used to sample 300 L of air at 30 L/min over a sterile gelatine membrane filter to collect any aerobic mesophiles if present in the cleanroom space. The gelatine membrane filters were transferred to a petri dish with R2A Agar media, and the assay was incubated at 32 °C. The assay was observed for microbial colony counts at intervals of 24, 48 and 72 h, respectively. A comparative assay was also made by sampling the air outside the cleanroom space to detect aerobic mesophiles. A negative control with no filter over the R2A agar was used in the assay. Fig. 11 shows the results of the assay at 24, 48 and 72 h.

Fig. 11.

Aerobic mesophile colony counts at 24, 48 and 72 h in a 300 L air sample.

4. Cleanroom operation and maintenance

The most significant contamination risk to any cleanroom is the personnel operating in it. Over 75–80 % of the contamination in a cleanroom is attributed to poor operating procedures inside the cleanroom. To mitigate the contamination, we have adopted a strict cleanroom operation procedure as per the cleanroom requirements detailed in ECSS-Q-70-01C standard.

The cleanroom space must be subjected to rigorous cleaning progress from the ceiling to the walls, exposed aluminium profile strut frame, table and floor. ISO class 5-rated cleanroom garment was worn for the cleaning process to prevent introducing any contaminants during the cleaning progress. 70 % Ethanol was used with sterile wipes to clean all the surfaces inside the cleanroom, followed by Chemgene HLD4L, which was used to clean the table inside the cleanroom to ensure high-level surface sterility for critical bioburden experiments. With the interior of the cleanroom cleaned, the FFUs were switched on and the cleanroom space was allowed to prime with positive pressure for 48 h.

ISO class 5-rated single-use cleanroom garments are mandated to operate anytime inside the cleanroom. The regulations of cleanroom garmenting are followed in strict accordance with the ISO14644-5 standards. As an additional contamination mitigation measurement, a commercial 2 m × 2 m gazebo tent, shown in Fig. 12, was deployed outside the cleanroom entrance to provide an enclosed space for cleanroom garmenting, reducing the risk of particle deposition over the cleanroom garments during the downing and doffing procedure. Sticky mats were used to ensure particles stuck to the cleanroom boot covers were prevented from entering the cleanroom.

Fig. 12.

Commercial gazebo tent used as a cleanroom garment space before entry into cleanroom space. An operator applies regular cleaning procedures.

A solution of 70 % ethanol must be applied with sterile wipes to disinfect any instrument or equipment brought into the cleanroom to prevent contamination of the cleanroom space. Papers and carboards are restricted into the cleanroom space as they can generate particles when subjected to motion and were allowed in only after laminating. ISO 14644-2 standard [21] recommends doing a risk-assessment to determine the supporting test needed depending on the application and the minimum interval between each test to formulate a regular monitoring strategy, ensuring evidence of optimal cleanroom performance. ISO classification test according to ISO14644–1:2015 standards, airflow velocity and particle recovery test according to the ISO14644–3:2019 standards should be performed bi-annually to ensure the cleanroom facility can regulate the concentration of particles in the workspace [22]. All cleanroom users should be trained to have access to this space, and regular monitoring of the bioburden contamination within the workspace, walls, and instruments should be applied. With this minimal, regular housekeeping program both inside the cleanroom and at the cleanroom access pre-camera, the ISO class 5 clean level can be maintained with minimum effort.

5. Conclusions

A modular, cost-effective, portable cleanroom has been designed and constructed using a COTS-based approach and has been validated to ISO class 5 levels according to the ISO14644 standards. The article has validated the cleanroom for application to bioburden reduction and planetary protection procedures on space hardware [[23], [24], [25]]. This work illustrates that cleanrooms which are considered to be a costly high-technology investment can be designed and constructed with a cost-effective approach with adherence to the quality standards laid out by the ISO organisation.

The developed cleanroom can be used in earth and planetary research, hardware development and planetary protection protocol, aerospace applications, and domains that require tight control of airborne particles in a process area. For example, the use of cleanrooms has increased in recent years, with growth in advances and applications in the field of semiconductor devices, optics, pharmaceuticals, microbiology, and medicine. The reduction in the size of the dies in semiconductor manufacturing has enabled the development of System-On-Chip (SOC) architecture, which has significantly improved performance with low power consumption compared to the traditional circuitry involving discrete components. This development has enabled the electronics consumer industry to introduce faster smartphones, computers, and televisions. Also, the communication industry has evolved radically from analogue technologies to digital technologies utilising fibre optics, enabling high bandwidth, immunity to electromagnetic interference and communication capability over long distances. Optical sensors are now replacing electronic sensors owing to their higher sensitivity, accuracy, and robustness. All these technological advancements require a highly controlled particle environment and mandate the need for cleanrooms. In the health industry, cleanrooms are necessitated for pharmaceutical development or to maintain a sterile environment for surgeries and intensive care units in hospitals.

The developed modular cleanroom can be adapted to meet the requirements of the end users. The current design of the cleanroom has specifically targeted cleanroom application for space hardware development. However, the design of the modular cleanroom can be scaled to satisfy the ISO requirements of the application needed and sized accordingly to accommodate the cleanroom volume required. Finally, to promote sustainability and accessibility for industry and, in particular, for the space sector, we encourage the community to use a COTS-based approach and reuse refurbished filter units when possible.

Data availability

The data used in this research is publicly available in the Mendeley Data Repository at http://doi.org/10.17632/5bvnysv5rv.1 [26].

Funding

T.M. and J.M.-T were supported by the UK Space Agency projects ST/W00190X/1 and ST/V00610X/1. M.-P.Z. was supported by project PID2019-104205 GB-C21 funded by MCIN/AEI/ 10.13039/501100011033.

CRediT authorship contribution statement

Thasshwin Mathanlal: Writing – original draft, Validation, Methodology, Investigation. Maria-Paz Zorzano: Writing – review & editing, Conceptualization. Javier Martin-Torres: Writing – review & editing, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Thasshwin Mathanlal reports financial support was provided by UK Space Agency. Javier Martin-Torres reports financial support was provided by UK Space Agency. Maria-Paz Zorzano reports financial support was provided by Spanish Ministry of Science and Innovation.