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Communication
Aled D. Roberts and Nigel S. Scrutton*
StarCrete: A starch-based biocomposite for off-
world construction
https://doi.org/10.1515/eng-2022-0390
received September 23, 2022; accepted November 30, 2022
Abstract: Robust and affordable technology capabilities
are needed before a sustained human presence on the
lunar and Martian surfaces can be established. A key
challenge is the production of high-strength structural
materials from in situ resources to provide spacious
habitats with adequate radiation shielding. Ideally, the
production of such materials will be achieved through
relatively simple, low-energy processes that support
other critical systems. Here, we demonstrate the use of
ordinary starch as a binder for simulated extraterrestrial
regolith to produce a high-strength biocomposite mate-
rial, termed StarCrete. With this technique, surplus starch
produced as food for inhabitants could be used for con-
struction, integrating two critical systems and significantly
simplifying the architecture needed to sustain early extra-
terrestrial colonies. After optimisation, lunar and Martian
StarCrete achieved compressive strengths of 91.7 and
72.0 MPa, respectively, which is well within the domain
of high-strength concrete (>42 MPa) and surpasses most
other proposed technology solutions despite being a
relatively low-energy process. The flexural strength of
the lunar and Martian StarCrete, at 2.1 and 8.4 MPa,
respectively, was also comparable to ordinary concrete
(2.5–4.5 MPa).
Keywords: starch, biocomposite, in situ resource utilisa-
tion, space, biopolymer–bound soil composites, design
of experiments
List of abbreviations
ERB extraterrestrial regolith biocomposite
DoE design of experiments
DSD definitive screening design
UCS ultimate compressive strength
LHS-1 lunar highlands simulant 1
MGS-1 martian global simulant 1
FE-SEM field-emission scanning electron microscopy
1 Introduction
A sustained human presence on the lunar and Martian
surfaces will require habitats with thick walls and ceil-
ings for protection against radiation exposure and meteor
strikes [1–3]. Due to the high cost of transporting mass
from Earth to space, bulk construction materials will be
produced from locally available resources – a concept
known as in situ resource utilisation (ISRU) [4–7]. The
stabilisation of loose, unconsolidated regolith (i.e., dust
and soil) into a solid concrete-like material would not
only provide radiation- and micrometeoroid-shielding,
but could also allow the deployment of relatively light-
weight, inflatable habitats by countering the extreme
thermal and pressure differences between indoor and
outdoor environments [7,8]. Although there have been
several proposed solutions to the stabilisation of regolith
for extraterrestrial construction, most have major draw-
backs such as extremely high energy or water use, or the
need for additional high-mass mining, transportation,
Aled D. Roberts: Department of Chemistry, Future Biomanufacturing
Research Hub, Manchester Institute of Biotechnology, The
University of Manchester, M1 7DN, Manchester, UK

* Corresponding author: Nigel S. Scrutton, Department of
Chemistry, Future Biomanufacturing Research Hub, Manchester
Institute of Biotechnology, The University of Manchester, M1 7DN,
Manchester, UK, e-mail: Nigel.Scrutton@manchester.ac.uk
Graphical abstract
Open Engineering 2023; 13: 20220390
Open Access. © 2023 the author(s), published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International
License.
processing or fabrication equipment which would add to
the cost and complexity of any mission (Figure 1) [9–13].
One potential solution is the use of naturally occur-
ring biopolymers as regolith binding agents to produce
extraterrestrial regolith biocomposites (ERBs) [14–17]. Since
biopolymers are produced under relatively mild, low-energy
conditions, they could potentially overcome many of the
shortcomings faced by other techniques. Recently, Shiwei
et al. proposed a technique to stabilise Martian regolith
using a chitosan-based biopolymer binder derived from
arthropod cuticle [14]. This ERB, termed Martian Biolith,
achieved an UCS of up to 3.6 MPa. In another series of
reports [15,16], D. Loftus and co-workers demonstrated
that a protein obtained from cow blood plasma (bovine
serum albumin, BSA) could also act as an effective binder
to produce ERBs with UCSs as high as 22.2 MPa – which is
about as strong as ordinary brick. Since it is not convenient
to transport cows into space, we expanded on this concept
by investigating the human equivalent of BSA (human
serum albumin, HSA) as a binder to produce ERBs [17].
Here, HSA obtained from human blood plasma could be
combined with urea (abundant in human urine) and rego-
lith to produce ERBs with compressive strengths as high as
39.7 MPa. Although the notion of considering humans as an
in situ resource has some advantages, the fact that the tech-
nique could compromise the health of the crew is a signifi-
cant drawback.
Starch (amylum) is an abundant plant-based carbo-
hydrate and is the main source of calories in the human
diet [18]. In addition to food, starch is also employed
industrially as an adhesive/binder for various applica-
tions – including paper, cardboard, and textile manufac-
ture [19,20]. Starch has been extensively investigated as a
binder for plant fibre-based biocomposite materials [21–28];
however, relatively poor mechanical properties (compres-
sive strengths <2.5 MPa) and moisture sensitivity limit their
applicability. Recently, corn starch was employed as a
binder for inorganic aggregates such as sand and limestone
powder [29,30]. Termed CoRncrete, these materials dis-
played impressive compressive strengths as high as 30 MPa;
however, moisture sensitivity remained a key weakness for
practical Earth-based applications [31].
Having extremely limited amounts of water, the issue
of moisture sensitivity is irrelevant for the Lunar and
Martian environments – meaning a CoRncrete-like mate-
rial could be well-suited for extraterrestrial construction.
Furthermore, since starch is the primary constituent of
staple foods such as rice, potatoes, and maize (corn), any
sustained off-world habitat will likely have the capability
to produce starch as food for inhabitants. To mitigate
risks such as crop failure or poor yields, a surplus of
starch will likely be produced under ordinary conditions:
the use of surplus starch as a binder for regolith would
therefore avoid the need for additional construction
Figure 2: Scheme depicting the steps taken to produce StarCrete.
Figure 1: Comparison of proposed ISRU technologies for the stabi-
lisation of extraterrestrial regolith into solid materials. Ultimate
compressive strength (UCS) range of materials plotted against the
proportion of material required beyond unprocessed regolith.
Purple, yellow and green colours indicate high-, medium- and low-
energy processes, respectively. Figure adapted from Karl et al. [9].
2  Aled D. Roberts and Nigel S. Scrutton
material fabrication equipment and supporting infrastruc-
ture. This integration of the food- and construction-mate-
rial-production systems would therefore reduce launch
mass, energy use, and technology development costs,
whilst also improving system robustness and flexibility.
In this work, we further developed and investigated
the CoRncrete concept for use as an extraterrestrial con-
struction material (Figure 2). Since our materials use
potato starch rather than corn starch, we renamed this
adaption StarCrete (starch-concrete). By employing a sta-
tistical Design of Experiments (DoE) methodology to
develop and optimise the formulation and process para-
meters, UCS’s as high as 72.0 and 91.7 MPa were obtained
for Martian and lunar regolith, respectively. This is within
the domain of high-strength concrete (>42 MPa), despite
being a relatively simple, low-energy approach.
2 Results and discussion
Starch gelatinisation occurs through a complex multi-
phase transition, which is influenced by factors such as
starch source, concentration, temperature profile, pH,
and the presence of metal salts, enzymes and other addi-
tives [32,33]. Moreover, the size, shape, and crystallinity
of the starch granules, as well as the molecular weights
and ratios of amylose to amylopectin, have a strong effect
on starch gelatinisation and vary significantly between
plant species. Selective breeding has also resulted in sig-
nificant differences within species, such as high-amylo-
pectin (waxy) varieties of maize, rice, and potatoes [19,32].
Due to the high complexity of this process and the likelihood
of complex multi-factor interactions, a statistical DoE meth-
odology was employed in this study.
For time efficiency, a single Martian regolith simulant
(Mars Global Simulant, MGS-1) was employed to optimise
the system before translation of the optimised conditions
to a lunar regolith simulant [34]. After developing a basic
procedure for the fabrication of starch-based ERBs adapted
from published methods (see SI for details) [29,30], several
starch sources were screened to identify the most promising
type (Table 1). The results from this screening experiment
indicated that potato starch was by far the most effective
source, having an UCS of 17.7 MPa. Potato starch differs
from most other grain-derived starches in that it has rela-
tively large starch granules (up to 100 µm), a relatively low
gelatinisation temperature (60–65°C), minimal fat and
protein content, and relatively high phosphate content
[19]. Potato starch also produces a relatively viscous paste
upon gelatinisation, which may be the reason it acted as a
Table
1:
Properties
of
native
starch
from
various
sources
(data
reproduced
from
refs.
[19,20,41,42])
along
with
UCS
values
of
resulting
ERBs
Starch
source
Granule
diameter
(µm)
Lipid
(wt%)
Protein
(wt%)
Phosphorous
(wt%)
Amylopectin
(%)
Amylose
DP
Gel.
temp
(°C)
Paste
viscosity
UCS
(MPa)
Maize
3–26
0.6
0.35
0.02
28
800
75–80
Medium
2.28
±
0.80
Waxy
maize
3–26
0.15
0.25
0.01
>99
—
65–70
Medium-high
3.69
±
0.33
Tapioca
4–35
0.1
0.1
0.01
17
3,000
65–70
High
1.43
±
0.07
Potato
5–100
0.05
0.06
0.08
21
3,000
60–65
Very
high
17.7
±
2.1
Wheat
1–40
0.8
0.4
0.06
28
800
80–85
Medium-low
11.5
±
1.6
Rice
3–8
0.01
0.06
—
19
—
70–80
Medium
1.36
±
0.13
Waxy
rice
3–8
0.01
0.06
—
>99
—
70–80
Medium
8.35
±
0.59
Samples
are
tested
in
triplicate.
DP
denotes
degree
of
polymerisation.
StarCrete: A starch-based regolith biocomposite  3
relatively strong binder. Due to its clear superiority over
other starch sources, potato starch was carried forward for
subsequent optimisation experiments.
The next experiment involved screening a range of
additives that could feasibly be obtained from the Martian
surface and were deemed to potentially have a beneficial
effect on the properties of the resulting ERBs. The selected
additives were as follows: MgCl2, acetic acid, Na2CO3,
FeSO4, urea, and human saliva. Metal chloride salt deposits
have been detected on the surface of Mars [35] and are
known to affect the gelatinisation of starch [36]. Acetic
acid can also be produced from starch via anaerobic fer-
mentation (e.g., rice vinegar or malted grain vinegar) and
can also affect starch gelatinisation [37]. Na2CO3 has been
employed as an additive in starch adhesives [20], and iron
and sulphate salts (e.g., FeSO4) could be obtained from the
Martian surface and could promote ionic bridging. Urea can
form strong hydrogen bonding interactions and is available
in abundance from human urine, and human saliva con-
tains amylose – a starch-active enzyme that has been used
to produce starch-based adhesives [38]. These additives
were incorporated into the ERBs by replacing the addition
of DI water with high-concentration aqueous solutions of
these substances. The resulting ERBs were then evaluated
for UCS with results presented in Table 2.
The results found that urea and acetic acid had a
strong positive effect on the resulting UCS of the ERBs
(about 50% stronger), whereas human saliva had little
effect and the other additives were detrimental to UCS.
However, it was observed that MgCl2 substantially altered
the viscoelastic properties of the mixture after gelatinisa-
tion (i.e., the mixture was “stickier” than others) and was
therefore included out of curiosity – along with urea and
acetic acid – for subsequent investigation.
Having accumulated a substantial number of process
variables and formulation parameters, a statistical
Definitive Screening Design (DSD) DoE experiment
was next conducted. A DSD experiment is a highly
efficient way to rapidly screen variables in a complex
system to determine which factors and factor interac-
tions are significant and should therefore be the focus
of subsequent optimisation [39]. In this experiment,
ten input variables were screened, namely, starch-
regolith ratio, effective starch concentration, urea con-
centration, MgCl2 concentration, acetic acid concentration,
gelatinisation temperature, gelatinisation time, compres-
sion force, drying temperature, and drying time. The mea-
sured output variable (or response) was UCS.
Details of the experiment are given in the SI, but to
summarise, 25 experimental runs were conducted which
revealed that gelatinisation temperature and gelatinisa-
tion time were highly significant factors whose ranges
(70–90°C and 10–60 min, respectively) had been set too
low. This meant that the effects of other factors were
eclipsed by these dominating effects, but that significantly
better performance could be obtained by simply increasing
gelatinisation temperature and gelatinisation time.
In order to find more optimal conditions for gelatini-
sation temperature and time, another statistical DoE
experiment was conducted. This time, a two-factor cen-
tral composite design (CCD) with one centre point was
employed, the details of which are given in the Supporting
Information. This experiment found a higher gelatinisation
temperature and time did improve the UCS as indicated by
the previous DSD experiment, with a UCS as high as
53.5 MPa being achieved. Moreover, the results suggested
that even higher gelatinisation temperatures and times
would continue to improve the compressive strength of
the materials.
A further CCD experiment was then conduced, pushing
the gelatinisation time and temperature even higher
(120–180 min and 120–180°C, respectively). However,
these higher temperatures resulted in the thermal decom-
position of urea with the liberation of ammonia and iso-
cyanic acid – the latter being a poisonous gas. Since the
generation of poisonous gasses should ideally be avoided
in confined environments such as off-world habitats, the
gelatinisation temperature was limited to 120°C while urea
was included as an additive.
While conducting the above experiment, a serendipi-
tous finding revealed that – after starch gelatinisation –
the materials could be fully dehydrated and rehydrated
without a detrimental effect on the UCS. This allowed the
decoupling of the extent of hydration needed for the gela-
tinisation step – where a relatively high amount of water
seemed to be beneficial – with the extent of hydration
needed for the final forming/compression step – where
a relatively low about of water seemed to be beneficial.
Table 2: Effect of additive incorporation on UCS of ERBs. UCS pre-
sented as a % relative to no additive
Additive Concentration Relative UCS (%)
None n.a. 100
Urea Saturated 152.5 ± 19.6
MgCl2 Saturated 69.1 ± 24.4
Acetic acid 24 vol% 150.2 ± 42.6
FeSO4 Saturated 10.6 ± 0.9
Na2CO3 Saturated 59.4 ± 3.5
Human saliva Pure 97.2 ± 6.6
Samples tested in triplicate.
4  Aled D. Roberts and Nigel S. Scrutton
This modification to the process (i.e., full drying after
gelatinisation before controlled rehydration) was incor-
porated into subsequent experiments.
Having now established a clearer idea of the relevant
process parameters and suitable ranges, another DoE
experiment was conducted with the aim of mapping the
experimental space through a Response Surface Model
(RSM). This custom DoE design, the details of which are
given in the SI, consisted of 54 runs grouped into six
blocks. To summarise the results, the following conclu-
sions were drawn: 1) a starch-regolith ratio of about 4.5%
appeared to be optimal, 2) a lower effective binder con-
centration (i.e., more water during the gelatinisation
step) increased UCS, 3) a higher compression force
increased UCS slightly, 4) a longer gelatinisation time
increased UCS, 5) a lower rehydration extent increased
UCS, and 6) both urea and acetic acid were detrimental
to UCS, whereas MgCl2 was beneficial. The latter point
was both surprising and interesting, since urea and
acetic acid had a strong positive effect from the initial
additive screening experiment, whereas MgCl2 initially
had a detrimental effect (Table 2). This highlights the
importance of pursuing interesting observations – other-
wise the beneficial effect of MgCl2 incorporation could have
been missed. The highest compressive strength achieved in
this experiment was 71.10 MPa.
Having further refined our understanding of signifi-
cant process factors and factor ranges, a subsequent
custom DoE experiment was conducted to explore the
experimental space that the abovementioned RSM was
Figure 3: Stress–strain profiles for Martian (MGS-1) and lunar (LHS-1) Starcrete undergoing (a) uniaxial compression tests and (b) three-
point flexural tests. (c) and (d) Camera images Martian and lunar Starcrete, respectively. (e) and (f) SEM images of Martian and lunar
Starcrete, respectively. Scale bars = 20 µm.
StarCrete: A starch-based regolith biocomposite  5
indicating as being more optimal. Urea and acetic acid
were dropped from the formulation since their incorpora-
tion was found to be detrimental. This allowed higher
gelatinisation temperatures to be employed without
the risk of producing poisonous isocyanic acid gas
from urea decomposition. In addition to higher gelati-
nisation temperatures – higher gelatinisation times,
lower effective starch concentrations and higher MgCl2
concentrations were investigated in this design. Rehydration
extent was fixed at 5% because a lower value of 4% was
found to be insufficient and resulted in materials with poor
mechanical properties. The results from this experiment are
again detailed in the Supplementary Information, but to
summarise – it was found that lower effective starch con-
centrations, higher MgCl2 concentrations and higher gelati-
nisation temperatures all decreased the UCS, which was
the opposite of the prediction of the prior experiment. This
suggested that the optimal conditions had already been
identified, and pushing the variables to further extremes
was detrimental. The conditions that resulted in a UCS of
71.10 MPa from the previous DoE experiment were there-
fore taken as optimal, with specific conditions presented in
the experimental details section of the Supplementary
Information.
Having optimised the fabrication procedure and for-
mulation, five further replicates were produced and tested
to evaluate the reproducibility of the system (Figure 3,
Table 3). The average UCS of these replicates was 71.95 ±
1.45 MPa, which was remarkably similar to the previous
71.10 MPa figure obtained from the DoE experiment. This
low variance between samples suggested that there were
no significant hidden variables influencing the results. The
compressive elastic modulus also displayed low variance,
with an average value of 4.12 ± 0.27 GPa.
The optimised process was then translated to a lunar
regolith simulant (Lunar Highlands Simulant, LHS-1),
which gave a remarkably high UCS of 91.68 ± 2.69 MPa.
Given that the system had specifically been optimised for
MGS-1, such a high value for LHS-1 was surprising. This
increased UCS was attributed to the particle size, shape, dis-
tribution, and chemical composition of LHS-1 being better
suited than MGS-1 for ERBs – supporting the observations
made in our previous study [17]. The compressive elastic
modulus was also remarkably high at 5.66 ± 0.09 GPa.
Three-point flexural tests were also conducted on
Martian and lunar StarCrete to determine flexural strength
and modulus (Figure 3b, Table 3.). Here, tile-like speci-
mens (55 mm × 55 mm × 12 mm) were prepared following
the optimised procedure and tested. The results found that
Martian (MGS-1) samples had a flexural strength of 8.41 ±
0.60 MPa and flexural modulus of 658.4 ± 43.7 MPa,
whereas Lunar (LHS-1) samples were weaker with a flex-
ural strength of 2.14 ± 0.22 MPa and flexural modulus of
137.3 ± 37.7 MPa. For comparison, ordinary concrete typi-
cally has a flexural strength between 2.5 and 4.5 MPa [40].
Finally, scanning electron microscopy (SEM) images
were taken to probe the structure and morphology of
StarCrete. This revealed some evidence of ligament-like
bonding between particles as has previously been observed
for protein-based binders (Figure 3e and f) [15,17].
3 Conclusions and outlook
Future habitats on the lunar and Martian surfaces will
need robust and affordable technology capabilities to
produce substantial quantities of construction materials
in situ. In this work, we demonstrate that ordinary plant-
derived starch can serve as an effective binder for extra-
terrestrial regolith to produce ERBs with compressive
strengths within the domain of high-strength concrete
(>42 MPa). The advantages of StarCrete over other pro-
posed technology options include the following: 1) risk
reduction: having an edible binder means it could be
consumed in the event of an ‘Apollo 13’ type emergency
where the ship or habitat enters ‘lifeboat mode’, 2) prac-
ticality: unlike many other proposed technology options,
StarCrete is a relatively simple solution with a high tech-
nology readiness, 3) system integration: the production
of starch could be integrated with food and oxygen pro-
duction systems (i.e., plant growth), simplifying mission
architecture and lowering costs, 4) resourcefulness: unlike
many other technology options, starch production doesn’t
require high energy processing, and most water can be
recovered since the mechanism is driven by dehydration,
5) resource locality: starch will be produced on-site, which
is an advantage over some other technology options that
Table 3: Summary of the mechanical property data of MGS-1- and LHS-1-based Starcrete following optimisation
Regolith UCS (MPa) Compressive modulus (GPa) Flexural strength (MPa) Flexural modulus (MPa)
MGS-1 71.95 ± 1.45 (5) 4.12 ± 0.27 (5) 8.41 ± 0.60 (3) 658.4 ± 43.7 (3)
LHS-1 91.68 ± 2.69 (3) 5.66 ± 0.09 (3) 2.14 ± 0.22 (3) 137.3 ± 37.7 (3)
6  Aled D. Roberts and Nigel S. Scrutton
would require mining and transportation of sparse mineral
deposits, and 6) architectural flexibility: being an excep-
tionally high-strength material, habitats can be designed
with fewer architectural constraints.
Although StarCrete displays significant potential as
an extraterrestrial construction material, further studies
will be needed to evaluate its full potential and limita-
tions. We suggest the following studies as avenues for
future work: 1) screening a broader range of starch sources
and additives, 2) further investigation into the bonding
mechanism and adhesion strength of the starch binder
with regolith, 3) further testing of StarCrete under simu-
lated off-world conditions (i.e., repeated thermal swings,
high radiation, and low pressure) with a focus on dur-
ability and longevity of the materials 4) hypervelocity
impact testing to evaluate resistance to meteor strikes, 5)
regolith particle size optimisation, 6) tailoring the bio-
synthesis of starch for further optimisation (e.g., directed
evolution of the gene corresponding to starch synthase),
and 7) investigating the potential of StarCrete for additive
manufacturing (3D-printing). Also, since starch granule
formation in plants is dependent on various environ-
mental conditions, such as sunlight exposure and circa-
dian rhythms, [32] plants grown under reduced gravity
and controlled lighting could form differently from those
grown on Earth and hence produce StarCrete with differing
properties – therefore, validation of the results under
expected operating conditions would be needed before
its practical application.
Finally, it is worth noting that since cement and con-
crete account for about 8% of global CO2 emissions,
further development of StarCrete could result in a rela-
tively sustainable alternative for Earth-based construc-
tion. For this to be achieved, the moisture-sensitivity of
starch binder needs to be overcome. This could be
achieved through the incorporation of covalent cross-
linking agents, heat-induced crosslinking, or other
biopolymer additives such as proteins, waxes, or terpene-
based resins.
Acknowledgements: This work was supported by the
Future Biomanufacturing Research Hub (grant EP/
S01778X/1), funded by the Engineering and Physical
Sciences Research Council (EPSRC) and Biotechnology
and Biological Sciences Research Council (BBSRC) as
part of UK Research and Innovation (UKRI). We acknowl-
edge the University of Manchester Electron Microscope
Centre and Mechanical Testing facility. The authors would
like to thank Mr Stuart Morse for assistance with mechan-
ical testing and Dr Will Finnigan for his helpful discus-
sions regarding Design of Experiments. We would also
like to thank Annie Carpenter and Andrew Wilson as the
organisers of the art-science collaboration group Para-lab,
and the artists Michelle Harrison and Hannah Leighton-
Boyce for the insightful discussions that resulted in the
conceptualisation of this concept. We would also like to
thank the Year 5 pupils from St Margaret’s CoE Primary
School (Whalley Range, Manchester) for their contribution
to material development and enthusiastic questions.
Conflict of interest: Authors state no conflict of interest.
Data availability statement: The authors confirm that the
data supporting the findings of this study are available
within the article [and/or] its supplementary materials,
and any additional data is available on request.
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8  Aled D. Roberts and Nigel S. Scrutton

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StarCrete: A starch-based biocomposite for offworld construction

  • 1. Communication Aled D. Roberts and Nigel S. Scrutton* StarCrete: A starch-based biocomposite for off- world construction https://doi.org/10.1515/eng-2022-0390 received September 23, 2022; accepted November 30, 2022 Abstract: Robust and affordable technology capabilities are needed before a sustained human presence on the lunar and Martian surfaces can be established. A key challenge is the production of high-strength structural materials from in situ resources to provide spacious habitats with adequate radiation shielding. Ideally, the production of such materials will be achieved through relatively simple, low-energy processes that support other critical systems. Here, we demonstrate the use of ordinary starch as a binder for simulated extraterrestrial regolith to produce a high-strength biocomposite mate- rial, termed StarCrete. With this technique, surplus starch produced as food for inhabitants could be used for con- struction, integrating two critical systems and significantly simplifying the architecture needed to sustain early extra- terrestrial colonies. After optimisation, lunar and Martian StarCrete achieved compressive strengths of 91.7 and 72.0 MPa, respectively, which is well within the domain of high-strength concrete (>42 MPa) and surpasses most other proposed technology solutions despite being a relatively low-energy process. The flexural strength of the lunar and Martian StarCrete, at 2.1 and 8.4 MPa, respectively, was also comparable to ordinary concrete (2.5–4.5 MPa). Keywords: starch, biocomposite, in situ resource utilisa- tion, space, biopolymer–bound soil composites, design of experiments List of abbreviations ERB extraterrestrial regolith biocomposite DoE design of experiments DSD definitive screening design UCS ultimate compressive strength LHS-1 lunar highlands simulant 1 MGS-1 martian global simulant 1 FE-SEM field-emission scanning electron microscopy 1 Introduction A sustained human presence on the lunar and Martian surfaces will require habitats with thick walls and ceil- ings for protection against radiation exposure and meteor strikes [1–3]. Due to the high cost of transporting mass from Earth to space, bulk construction materials will be produced from locally available resources – a concept known as in situ resource utilisation (ISRU) [4–7]. The stabilisation of loose, unconsolidated regolith (i.e., dust and soil) into a solid concrete-like material would not only provide radiation- and micrometeoroid-shielding, but could also allow the deployment of relatively light- weight, inflatable habitats by countering the extreme thermal and pressure differences between indoor and outdoor environments [7,8]. Although there have been several proposed solutions to the stabilisation of regolith for extraterrestrial construction, most have major draw- backs such as extremely high energy or water use, or the need for additional high-mass mining, transportation, Aled D. Roberts: Department of Chemistry, Future Biomanufacturing Research Hub, Manchester Institute of Biotechnology, The University of Manchester, M1 7DN, Manchester, UK  * Corresponding author: Nigel S. Scrutton, Department of Chemistry, Future Biomanufacturing Research Hub, Manchester Institute of Biotechnology, The University of Manchester, M1 7DN, Manchester, UK, e-mail: Nigel.Scrutton@manchester.ac.uk Graphical abstract Open Engineering 2023; 13: 20220390 Open Access. © 2023 the author(s), published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License.
  • 2. processing or fabrication equipment which would add to the cost and complexity of any mission (Figure 1) [9–13]. One potential solution is the use of naturally occur- ring biopolymers as regolith binding agents to produce extraterrestrial regolith biocomposites (ERBs) [14–17]. Since biopolymers are produced under relatively mild, low-energy conditions, they could potentially overcome many of the shortcomings faced by other techniques. Recently, Shiwei et al. proposed a technique to stabilise Martian regolith using a chitosan-based biopolymer binder derived from arthropod cuticle [14]. This ERB, termed Martian Biolith, achieved an UCS of up to 3.6 MPa. In another series of reports [15,16], D. Loftus and co-workers demonstrated that a protein obtained from cow blood plasma (bovine serum albumin, BSA) could also act as an effective binder to produce ERBs with UCSs as high as 22.2 MPa – which is about as strong as ordinary brick. Since it is not convenient to transport cows into space, we expanded on this concept by investigating the human equivalent of BSA (human serum albumin, HSA) as a binder to produce ERBs [17]. Here, HSA obtained from human blood plasma could be combined with urea (abundant in human urine) and rego- lith to produce ERBs with compressive strengths as high as 39.7 MPa. Although the notion of considering humans as an in situ resource has some advantages, the fact that the tech- nique could compromise the health of the crew is a signifi- cant drawback. Starch (amylum) is an abundant plant-based carbo- hydrate and is the main source of calories in the human diet [18]. In addition to food, starch is also employed industrially as an adhesive/binder for various applica- tions – including paper, cardboard, and textile manufac- ture [19,20]. Starch has been extensively investigated as a binder for plant fibre-based biocomposite materials [21–28]; however, relatively poor mechanical properties (compres- sive strengths <2.5 MPa) and moisture sensitivity limit their applicability. Recently, corn starch was employed as a binder for inorganic aggregates such as sand and limestone powder [29,30]. Termed CoRncrete, these materials dis- played impressive compressive strengths as high as 30 MPa; however, moisture sensitivity remained a key weakness for practical Earth-based applications [31]. Having extremely limited amounts of water, the issue of moisture sensitivity is irrelevant for the Lunar and Martian environments – meaning a CoRncrete-like mate- rial could be well-suited for extraterrestrial construction. Furthermore, since starch is the primary constituent of staple foods such as rice, potatoes, and maize (corn), any sustained off-world habitat will likely have the capability to produce starch as food for inhabitants. To mitigate risks such as crop failure or poor yields, a surplus of starch will likely be produced under ordinary conditions: the use of surplus starch as a binder for regolith would therefore avoid the need for additional construction Figure 2: Scheme depicting the steps taken to produce StarCrete. Figure 1: Comparison of proposed ISRU technologies for the stabi- lisation of extraterrestrial regolith into solid materials. Ultimate compressive strength (UCS) range of materials plotted against the proportion of material required beyond unprocessed regolith. Purple, yellow and green colours indicate high-, medium- and low- energy processes, respectively. Figure adapted from Karl et al. [9]. 2  Aled D. Roberts and Nigel S. Scrutton
  • 3. material fabrication equipment and supporting infrastruc- ture. This integration of the food- and construction-mate- rial-production systems would therefore reduce launch mass, energy use, and technology development costs, whilst also improving system robustness and flexibility. In this work, we further developed and investigated the CoRncrete concept for use as an extraterrestrial con- struction material (Figure 2). Since our materials use potato starch rather than corn starch, we renamed this adaption StarCrete (starch-concrete). By employing a sta- tistical Design of Experiments (DoE) methodology to develop and optimise the formulation and process para- meters, UCS’s as high as 72.0 and 91.7 MPa were obtained for Martian and lunar regolith, respectively. This is within the domain of high-strength concrete (>42 MPa), despite being a relatively simple, low-energy approach. 2 Results and discussion Starch gelatinisation occurs through a complex multi- phase transition, which is influenced by factors such as starch source, concentration, temperature profile, pH, and the presence of metal salts, enzymes and other addi- tives [32,33]. Moreover, the size, shape, and crystallinity of the starch granules, as well as the molecular weights and ratios of amylose to amylopectin, have a strong effect on starch gelatinisation and vary significantly between plant species. Selective breeding has also resulted in sig- nificant differences within species, such as high-amylo- pectin (waxy) varieties of maize, rice, and potatoes [19,32]. Due to the high complexity of this process and the likelihood of complex multi-factor interactions, a statistical DoE meth- odology was employed in this study. For time efficiency, a single Martian regolith simulant (Mars Global Simulant, MGS-1) was employed to optimise the system before translation of the optimised conditions to a lunar regolith simulant [34]. After developing a basic procedure for the fabrication of starch-based ERBs adapted from published methods (see SI for details) [29,30], several starch sources were screened to identify the most promising type (Table 1). The results from this screening experiment indicated that potato starch was by far the most effective source, having an UCS of 17.7 MPa. Potato starch differs from most other grain-derived starches in that it has rela- tively large starch granules (up to 100 µm), a relatively low gelatinisation temperature (60–65°C), minimal fat and protein content, and relatively high phosphate content [19]. Potato starch also produces a relatively viscous paste upon gelatinisation, which may be the reason it acted as a Table 1: Properties of native starch from various sources (data reproduced from refs. [19,20,41,42]) along with UCS values of resulting ERBs Starch source Granule diameter (µm) Lipid (wt%) Protein (wt%) Phosphorous (wt%) Amylopectin (%) Amylose DP Gel. temp (°C) Paste viscosity UCS (MPa) Maize 3–26 0.6 0.35 0.02 28 800 75–80 Medium 2.28 ± 0.80 Waxy maize 3–26 0.15 0.25 0.01 >99 — 65–70 Medium-high 3.69 ± 0.33 Tapioca 4–35 0.1 0.1 0.01 17 3,000 65–70 High 1.43 ± 0.07 Potato 5–100 0.05 0.06 0.08 21 3,000 60–65 Very high 17.7 ± 2.1 Wheat 1–40 0.8 0.4 0.06 28 800 80–85 Medium-low 11.5 ± 1.6 Rice 3–8 0.01 0.06 — 19 — 70–80 Medium 1.36 ± 0.13 Waxy rice 3–8 0.01 0.06 — >99 — 70–80 Medium 8.35 ± 0.59 Samples are tested in triplicate. DP denotes degree of polymerisation. StarCrete: A starch-based regolith biocomposite  3
  • 4. relatively strong binder. Due to its clear superiority over other starch sources, potato starch was carried forward for subsequent optimisation experiments. The next experiment involved screening a range of additives that could feasibly be obtained from the Martian surface and were deemed to potentially have a beneficial effect on the properties of the resulting ERBs. The selected additives were as follows: MgCl2, acetic acid, Na2CO3, FeSO4, urea, and human saliva. Metal chloride salt deposits have been detected on the surface of Mars [35] and are known to affect the gelatinisation of starch [36]. Acetic acid can also be produced from starch via anaerobic fer- mentation (e.g., rice vinegar or malted grain vinegar) and can also affect starch gelatinisation [37]. Na2CO3 has been employed as an additive in starch adhesives [20], and iron and sulphate salts (e.g., FeSO4) could be obtained from the Martian surface and could promote ionic bridging. Urea can form strong hydrogen bonding interactions and is available in abundance from human urine, and human saliva con- tains amylose – a starch-active enzyme that has been used to produce starch-based adhesives [38]. These additives were incorporated into the ERBs by replacing the addition of DI water with high-concentration aqueous solutions of these substances. The resulting ERBs were then evaluated for UCS with results presented in Table 2. The results found that urea and acetic acid had a strong positive effect on the resulting UCS of the ERBs (about 50% stronger), whereas human saliva had little effect and the other additives were detrimental to UCS. However, it was observed that MgCl2 substantially altered the viscoelastic properties of the mixture after gelatinisa- tion (i.e., the mixture was “stickier” than others) and was therefore included out of curiosity – along with urea and acetic acid – for subsequent investigation. Having accumulated a substantial number of process variables and formulation parameters, a statistical Definitive Screening Design (DSD) DoE experiment was next conducted. A DSD experiment is a highly efficient way to rapidly screen variables in a complex system to determine which factors and factor interac- tions are significant and should therefore be the focus of subsequent optimisation [39]. In this experiment, ten input variables were screened, namely, starch- regolith ratio, effective starch concentration, urea con- centration, MgCl2 concentration, acetic acid concentration, gelatinisation temperature, gelatinisation time, compres- sion force, drying temperature, and drying time. The mea- sured output variable (or response) was UCS. Details of the experiment are given in the SI, but to summarise, 25 experimental runs were conducted which revealed that gelatinisation temperature and gelatinisa- tion time were highly significant factors whose ranges (70–90°C and 10–60 min, respectively) had been set too low. This meant that the effects of other factors were eclipsed by these dominating effects, but that significantly better performance could be obtained by simply increasing gelatinisation temperature and gelatinisation time. In order to find more optimal conditions for gelatini- sation temperature and time, another statistical DoE experiment was conducted. This time, a two-factor cen- tral composite design (CCD) with one centre point was employed, the details of which are given in the Supporting Information. This experiment found a higher gelatinisation temperature and time did improve the UCS as indicated by the previous DSD experiment, with a UCS as high as 53.5 MPa being achieved. Moreover, the results suggested that even higher gelatinisation temperatures and times would continue to improve the compressive strength of the materials. A further CCD experiment was then conduced, pushing the gelatinisation time and temperature even higher (120–180 min and 120–180°C, respectively). However, these higher temperatures resulted in the thermal decom- position of urea with the liberation of ammonia and iso- cyanic acid – the latter being a poisonous gas. Since the generation of poisonous gasses should ideally be avoided in confined environments such as off-world habitats, the gelatinisation temperature was limited to 120°C while urea was included as an additive. While conducting the above experiment, a serendipi- tous finding revealed that – after starch gelatinisation – the materials could be fully dehydrated and rehydrated without a detrimental effect on the UCS. This allowed the decoupling of the extent of hydration needed for the gela- tinisation step – where a relatively high amount of water seemed to be beneficial – with the extent of hydration needed for the final forming/compression step – where a relatively low about of water seemed to be beneficial. Table 2: Effect of additive incorporation on UCS of ERBs. UCS pre- sented as a % relative to no additive Additive Concentration Relative UCS (%) None n.a. 100 Urea Saturated 152.5 ± 19.6 MgCl2 Saturated 69.1 ± 24.4 Acetic acid 24 vol% 150.2 ± 42.6 FeSO4 Saturated 10.6 ± 0.9 Na2CO3 Saturated 59.4 ± 3.5 Human saliva Pure 97.2 ± 6.6 Samples tested in triplicate. 4  Aled D. Roberts and Nigel S. Scrutton
  • 5. This modification to the process (i.e., full drying after gelatinisation before controlled rehydration) was incor- porated into subsequent experiments. Having now established a clearer idea of the relevant process parameters and suitable ranges, another DoE experiment was conducted with the aim of mapping the experimental space through a Response Surface Model (RSM). This custom DoE design, the details of which are given in the SI, consisted of 54 runs grouped into six blocks. To summarise the results, the following conclu- sions were drawn: 1) a starch-regolith ratio of about 4.5% appeared to be optimal, 2) a lower effective binder con- centration (i.e., more water during the gelatinisation step) increased UCS, 3) a higher compression force increased UCS slightly, 4) a longer gelatinisation time increased UCS, 5) a lower rehydration extent increased UCS, and 6) both urea and acetic acid were detrimental to UCS, whereas MgCl2 was beneficial. The latter point was both surprising and interesting, since urea and acetic acid had a strong positive effect from the initial additive screening experiment, whereas MgCl2 initially had a detrimental effect (Table 2). This highlights the importance of pursuing interesting observations – other- wise the beneficial effect of MgCl2 incorporation could have been missed. The highest compressive strength achieved in this experiment was 71.10 MPa. Having further refined our understanding of signifi- cant process factors and factor ranges, a subsequent custom DoE experiment was conducted to explore the experimental space that the abovementioned RSM was Figure 3: Stress–strain profiles for Martian (MGS-1) and lunar (LHS-1) Starcrete undergoing (a) uniaxial compression tests and (b) three- point flexural tests. (c) and (d) Camera images Martian and lunar Starcrete, respectively. (e) and (f) SEM images of Martian and lunar Starcrete, respectively. Scale bars = 20 µm. StarCrete: A starch-based regolith biocomposite  5
  • 6. indicating as being more optimal. Urea and acetic acid were dropped from the formulation since their incorpora- tion was found to be detrimental. This allowed higher gelatinisation temperatures to be employed without the risk of producing poisonous isocyanic acid gas from urea decomposition. In addition to higher gelati- nisation temperatures – higher gelatinisation times, lower effective starch concentrations and higher MgCl2 concentrations were investigated in this design. Rehydration extent was fixed at 5% because a lower value of 4% was found to be insufficient and resulted in materials with poor mechanical properties. The results from this experiment are again detailed in the Supplementary Information, but to summarise – it was found that lower effective starch con- centrations, higher MgCl2 concentrations and higher gelati- nisation temperatures all decreased the UCS, which was the opposite of the prediction of the prior experiment. This suggested that the optimal conditions had already been identified, and pushing the variables to further extremes was detrimental. The conditions that resulted in a UCS of 71.10 MPa from the previous DoE experiment were there- fore taken as optimal, with specific conditions presented in the experimental details section of the Supplementary Information. Having optimised the fabrication procedure and for- mulation, five further replicates were produced and tested to evaluate the reproducibility of the system (Figure 3, Table 3). The average UCS of these replicates was 71.95 ± 1.45 MPa, which was remarkably similar to the previous 71.10 MPa figure obtained from the DoE experiment. This low variance between samples suggested that there were no significant hidden variables influencing the results. The compressive elastic modulus also displayed low variance, with an average value of 4.12 ± 0.27 GPa. The optimised process was then translated to a lunar regolith simulant (Lunar Highlands Simulant, LHS-1), which gave a remarkably high UCS of 91.68 ± 2.69 MPa. Given that the system had specifically been optimised for MGS-1, such a high value for LHS-1 was surprising. This increased UCS was attributed to the particle size, shape, dis- tribution, and chemical composition of LHS-1 being better suited than MGS-1 for ERBs – supporting the observations made in our previous study [17]. The compressive elastic modulus was also remarkably high at 5.66 ± 0.09 GPa. Three-point flexural tests were also conducted on Martian and lunar StarCrete to determine flexural strength and modulus (Figure 3b, Table 3.). Here, tile-like speci- mens (55 mm × 55 mm × 12 mm) were prepared following the optimised procedure and tested. The results found that Martian (MGS-1) samples had a flexural strength of 8.41 ± 0.60 MPa and flexural modulus of 658.4 ± 43.7 MPa, whereas Lunar (LHS-1) samples were weaker with a flex- ural strength of 2.14 ± 0.22 MPa and flexural modulus of 137.3 ± 37.7 MPa. For comparison, ordinary concrete typi- cally has a flexural strength between 2.5 and 4.5 MPa [40]. Finally, scanning electron microscopy (SEM) images were taken to probe the structure and morphology of StarCrete. This revealed some evidence of ligament-like bonding between particles as has previously been observed for protein-based binders (Figure 3e and f) [15,17]. 3 Conclusions and outlook Future habitats on the lunar and Martian surfaces will need robust and affordable technology capabilities to produce substantial quantities of construction materials in situ. In this work, we demonstrate that ordinary plant- derived starch can serve as an effective binder for extra- terrestrial regolith to produce ERBs with compressive strengths within the domain of high-strength concrete (>42 MPa). The advantages of StarCrete over other pro- posed technology options include the following: 1) risk reduction: having an edible binder means it could be consumed in the event of an ‘Apollo 13’ type emergency where the ship or habitat enters ‘lifeboat mode’, 2) prac- ticality: unlike many other proposed technology options, StarCrete is a relatively simple solution with a high tech- nology readiness, 3) system integration: the production of starch could be integrated with food and oxygen pro- duction systems (i.e., plant growth), simplifying mission architecture and lowering costs, 4) resourcefulness: unlike many other technology options, starch production doesn’t require high energy processing, and most water can be recovered since the mechanism is driven by dehydration, 5) resource locality: starch will be produced on-site, which is an advantage over some other technology options that Table 3: Summary of the mechanical property data of MGS-1- and LHS-1-based Starcrete following optimisation Regolith UCS (MPa) Compressive modulus (GPa) Flexural strength (MPa) Flexural modulus (MPa) MGS-1 71.95 ± 1.45 (5) 4.12 ± 0.27 (5) 8.41 ± 0.60 (3) 658.4 ± 43.7 (3) LHS-1 91.68 ± 2.69 (3) 5.66 ± 0.09 (3) 2.14 ± 0.22 (3) 137.3 ± 37.7 (3) 6  Aled D. Roberts and Nigel S. Scrutton
  • 7. would require mining and transportation of sparse mineral deposits, and 6) architectural flexibility: being an excep- tionally high-strength material, habitats can be designed with fewer architectural constraints. Although StarCrete displays significant potential as an extraterrestrial construction material, further studies will be needed to evaluate its full potential and limita- tions. We suggest the following studies as avenues for future work: 1) screening a broader range of starch sources and additives, 2) further investigation into the bonding mechanism and adhesion strength of the starch binder with regolith, 3) further testing of StarCrete under simu- lated off-world conditions (i.e., repeated thermal swings, high radiation, and low pressure) with a focus on dur- ability and longevity of the materials 4) hypervelocity impact testing to evaluate resistance to meteor strikes, 5) regolith particle size optimisation, 6) tailoring the bio- synthesis of starch for further optimisation (e.g., directed evolution of the gene corresponding to starch synthase), and 7) investigating the potential of StarCrete for additive manufacturing (3D-printing). Also, since starch granule formation in plants is dependent on various environ- mental conditions, such as sunlight exposure and circa- dian rhythms, [32] plants grown under reduced gravity and controlled lighting could form differently from those grown on Earth and hence produce StarCrete with differing properties – therefore, validation of the results under expected operating conditions would be needed before its practical application. Finally, it is worth noting that since cement and con- crete account for about 8% of global CO2 emissions, further development of StarCrete could result in a rela- tively sustainable alternative for Earth-based construc- tion. For this to be achieved, the moisture-sensitivity of starch binder needs to be overcome. This could be achieved through the incorporation of covalent cross- linking agents, heat-induced crosslinking, or other biopolymer additives such as proteins, waxes, or terpene- based resins. Acknowledgements: This work was supported by the Future Biomanufacturing Research Hub (grant EP/ S01778X/1), funded by the Engineering and Physical Sciences Research Council (EPSRC) and Biotechnology and Biological Sciences Research Council (BBSRC) as part of UK Research and Innovation (UKRI). We acknowl- edge the University of Manchester Electron Microscope Centre and Mechanical Testing facility. The authors would like to thank Mr Stuart Morse for assistance with mechan- ical testing and Dr Will Finnigan for his helpful discus- sions regarding Design of Experiments. We would also like to thank Annie Carpenter and Andrew Wilson as the organisers of the art-science collaboration group Para-lab, and the artists Michelle Harrison and Hannah Leighton- Boyce for the insightful discussions that resulted in the conceptualisation of this concept. We would also like to thank the Year 5 pupils from St Margaret’s CoE Primary School (Whalley Range, Manchester) for their contribution to material development and enthusiastic questions. Conflict of interest: Authors state no conflict of interest. Data availability statement: The authors confirm that the data supporting the findings of this study are available within the article [and/or] its supplementary materials, and any additional data is available on request. References [1] Simonsen LC, Nealy JE. Radiation Protection for Human Missions to the Moon and Mars. NASA Tech Rep Serv; 1991. https://ntrs. nasa.gov/citations/19910008686. (accessed June 12, 2022). [2] Drake BG, Hoffman SJ, Beaty DW. Human exploration of mars, design reference architecture 5.0. IEEE Aerosp. Conf. Proc.; 2010. p. 1–24. [3] Durante M. Space radiation protection: Destination Mars. Life Sci Sp Res. 2014;1:2–9. [4] Ellery A. Sustainable in-situ resource utilization on the moon. Planet Space Sci. 2020;184:104870. 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