Higher Optimum Temperature Research Articles

Agarase cocktail from agarolytic Alteromonas sp. Aga1552 converts homogenized Gelidium amansii into monosaccharide

Marine algae biomass utilization has attracted considerable attention, however, the preparation of monosaccharides from raw algae is still hindered by many technical barriers. In this study, three genes, aga1365, aga1364, and aga1360, encoding key enzymes constituting a complete agar decomposition pathway were expressed and characterized. Recombinant Aga1365, Aga1364, and Aga1360 exhibited high optimal reaction temperatures and excellent thermal stability. Moreover, enzyme cocktail was proved to have higher synergistic effect to prepare monosaccharide from raw seaweed. The enzyme cocktail of Aga1360 (GH117) with Aga1365 (GH16) and enzyme cocktail of Aga1360 with both Aga1365 and 1364 (GH50) were used to synergistically degrade homogenized Gelidium amansii, maximum monosaccharide production of 21.47 mg/g and 39.28 mg/g could be achieved, respectively. This study presents an environment-friendly, time saving and efficient way to prepare monosaccharides from raw seaweed, which also provide a potential strategy to effectively convert algae biomass for biofuel and biochemical production by utilizing the synergistic effects of enzyme cocktail.

Effect of the Digestibility of Cassava Flour (Manihot esculenta Crantz) by Enzymes Extracted from Corn Malt.

The aim of this study is to determine the effect of the digestibility of cassava starch by the enzymes extracted from corn malt, which will constitute one of the answers to the problem of integrating local products into the process in a modern brewery. Cassava starch solutions of different concentrations (E0: 0g/L; E1: 1g/L; E2: 1.1g/L; E3: 1.2g/L; E4: 1.3g/L; E5: 1.4g/L and E6: 1.5g/L) were prepared and subjected to two treatments (gelatinized and non-gelatinized) and 5mL of each were placed in a test tube. Three millilitres (3mL) of the solution containing amylases extracted from malt corn was then added to each of the test tubes containing the cassava flour solutions. All the treatments were subjected to three temperature stages (50°C for 15min, 90°C for 20min, and 100°C for 75min). Twenty-eight (28) objects (two duplicates) were experimented in a complete factorial design (2 treatments × 2 temperature levels). The results obtained showed that gelatinization had no effect, which could be due to the high optimum temperatures of corn enzyme activity. The concentrations also did not have significant differences which shows that these concentrations can well be used on an industrial scale to digest cassava starch by corn malt enzymes.

Purification and immobilization of β-glucosidase using surface modified mesoporous silica Santa Barbara Amorphous 15 for eco-friendly preparation of sagittatoside A

Functionalized mesoporous materials have become a promising carrier for enzyme immobilization. In this study, Santa Barbara Amorphous 15 (SBA-15) was modified by N-aminoethyl-γ-aminopropyl trimethoxy (R). R-SBA-15 was employed to purify and immobilize recombinant β-glucosidase from Terrabacter ginsenosidimutans (BgpA) in one step for the first time. Optimum pH of the constructed R-SBA-15@BgpA were 7.0, and it has 20 ℃ higher optimal temperature than free enzyme. Relative activity of R-SBA-15@BgpA still retained > 70% at 42 ℃ after 8-h incubation. The investigation on organic reagent resistance revealed that the immobilized enzyme can maintain strong stability in 15% DMSO. In leaching test and evaluation of storage stability, only trace amount of protein was detected in buffer of the immobilized enzyme after storage at 4 ℃ for 33 days, and the immobilized BgpA still maintained > 50% relative activity. It also demonstrated good reusability, with 76.1% relative activity remaining after fourteen successive enzymatic hydrolyses of epimedin A to sagittatoside A. The newly proposed strategy is an effective approach for the purification and immobilization of BgpA concurrently. In addition, R-SBA-15@BgpA was demonstrated to have high efficiency and stability in this application, suggesting its great feasibility and potential to produce bioactive compounds such as secondary glycosides or aglycones from natural products.Graphical

Distribution of specific prokaryotic immune systems correlates with host optimal growth temperature.

Prokaryotes encode an arsenal of highly diverse immune systems to protect themselves against invading nucleic acids such as viruses, plasmids and transposons. This includes invader-interfering systems that neutralize invaders to protect their host, and abortive-infection systems, which trigger dormancy or cell death in their host to offer population-level immunity. Most prokaryotic immune systems are found across different environments and prokaryotic phyla, but their distribution appears biased and the factors that influence their distribution are largely unknown. Here, we compared and combined the prokaryotic immune system identification tools DefenseFinder and PADLOC to obtain an expanded view of the immune system arsenal. Our results show that the number of immune systems encoded is positively correlated with genome size and that the distribution of specific immune systems is linked to phylogeny. Furthermore, we reveal that certain invader-interfering systems are more frequently encoded by hosts with a relatively high optimum growth temperature, while abortive-infection systems are generally more frequently encoded by hosts with a relatively low optimum growth temperature. Combined, our study reveals several factors that correlate with differences in the distribution of prokaryotic immune systems and extends our understanding of how prokaryotes protect themselves from invaders in different environments.

The energy-converting hydrogenase Ech2 is important for the growth of the thermophilic acetogen Thermoanaerobacter kivui on ferredoxin-dependent substrates.

Acetogens thrive by converting H2+CO2 to acetate. Under environmental conditions, this allows for only very little energy to be conserved (∆G'<-20 kJ mol-1). CO2 serves as a terminal electron acceptor in the ancient Wood-Ljungdahl pathway (WLP). Since the WLP is ATP neutral, energy conservation during growth on H2 + CO2 is dependent on the redox metabolism. Two types of acetogens can be distinguished, Rnf- and Ech-type. The function of both membrane-bound enzyme complexes is twofold-energy conversion and redox balancing. Ech couples the Fd-dependent reduction of protons to H2 to the formation of a proton gradient in the thermophilic bacterium Thermoanaerobacter kivui. This bacterium may be utilized in gas fermentation at high temperatures, due to very high conversion rates and the availability of genetic tools. The physiological function of an Ech hydrogenase in T. kivui was studied to contribute an understanding of its energy and redox metabolism, a prerequisite for future industrial applications.

Development of chia gum/alginate-polymer support for horseradish peroxidase immobilization and its application in phenolic removal

Chia gum’s molecular structure with distinctive properties as well as the alginate-based hydrogel’s three-dimensionally cross-linked structure can provide a potent matrix for immobilization of enzyme. Herein, chia gum (CG)/alginate (A)-polymeric complex was synthesized and employed as a support material for the immobilization of horseradish peroxidase (HRP). HRP was successfully immobilized on the developed ACG-polymeric support, and the highest immobilization recovery (75%) was observed at 1.0% CG and 2% A, pH 7.0, and 50 units of the enzyme. The structure, morphology, and thermal properties of the prepared ACG-HRP were demonstrated using Fourier Transform Infrared (FTIR), Scanning Electron Microscope, and Thermogravimetric (TGA) analyses. ACG-HRP showed a good reusability (60%) over ten reuses. The immobilized ACG-HRP displayed an acidic pH optimum (6.0), a higher temperature optimum (50 °C), and improved thermal stability (30–50 °C) compared to the soluble HRP at pH 7.0, 40 °C and (30–40 °C), respectively. ACG-HRP has a lower affinity for hydrogen peroxide (H2O2) and guaiacol and a higher oxidizing affinity for a number of phenolic substrates. The ACG-HRP demonstrated greater resistance to heavy metals, isopropanol, urea, Triton X-100, and urea, as well as improved efficiency for eliminating phenol and p-chlorophenol. The developed ACG-polymeric support provided improved enzyme properties, allowed the reuse of the immobilized HRP in 10 cycles, and made it promising for several biotechnological applications.

Climate acts as an environmental filter to plant pathogens.

Climate shapes the distribution of plant-associated microbes such as mycorrhizal and endophytic fungi. However, the role of climate in plant pathogen community assembly is less understood. Here, we explored the role of climate in the assembly of Phytophthora communities at >250 sites along a latitudinal gradient from Spain to northern Sweden and an altitudinal gradient from the Spanish Pyrenees to lowland areas. Communities were detected by ITS sequencing of river filtrates. Mediation analysis supported the role of climate in the biogeography of Phytophthora and ruled out other environmental factors such as geography or tree diversity. Comparisons of functional and species diversity showed that environmental filtering dominated over competitive exclusion in Europe. Temperature and precipitation acted as environmental filters at different extremes of the gradients. In northern regions, winter temperatures acted as an environmental filter on Phytophthora community assembly, selecting species adapted to survive low minimum temperatures. In southern latitudes, a hot dry climate was the main environmental filter, resulting in communities dominated by drought-tolerant Phytophthora species with thick oospore walls, a high optimum temperature for growth, and a high maximum temperature limit for growth. By taking a community ecology approach, we show that the establishment of Phytophthora plant pathogens in Europe is mainly restricted by cold temperatures.

H2/CO production via high temperature electrolysis of H2O/CO2 coupling with solar spectral splitting at a tunable cut-off wavelength

Solar driven CO2/H2O splitting is a promising path for large-scale and long-term solar energy conversion and storage. In this work, a thermodynamic model of solar driven high-temperature CO2/H2O electrolysis was established, with the full solar spectrum split at a tunable cut-off wavelength for meeting the electrical and thermal energy demands of the process at a high efficiency. Compared with the system without spectral splitting, the solar-to-H2 efficiency can be significantly improved from 36.0 % to 45.5 % by introducing spectral splitting. Besides, the optimal electrolysis temperature can be largely lowered, from 1623 K to 1323 K due to the significant reduction of the solar radiation input for electricity generation. The exergy efficiency of CO2 electrolysis was shown to be higher than that of H2O electrolysis due to the higher molar exergy of CO compared to H2, although the first-law efficiency of H2O electrolysis is higher at Tele < ∼1200 K. A high concentration ratio of the solar-thermal process can lead to both high solar-to-fuel efficiency and high optimal electrolysis temperature because of the reduction of reradiation loss. The electrolysis temperature and the use of excessive CO2/H2O were shown to have an important effect on both the efficiency and the optimal cut-off wavelength because they are closely related with the thermal and electrical energy demands of the whole process. The optimal cut-off wavelength can be as low as 540 nm at electrolysis temperature of 1873 K (Tele = 1873 K) and excessive H2O coefficient of 6 (r = 6), and increases to the working limit of the photovoltaic material (900 nm in this work) as the electrolysis temperature and excessive coefficient decrease to Tele < 1023 K and r < 2.

Biophysical and biochemical characterization of the thioredoxin system from Colwellia psychrerythraea.

The thioredoxin system is a ubiquitous oxidoreductase system consisting of the enzyme thioredoxin reductase, the protein thioredoxin, and the cofactor nicotinamide adenine dinucleotide phosphate. The system has been comprehensively studied from many organisms, such as Escherichia coli; however, structural and functional analysis of this system from psychrophilic bacteria has not been as extensive. In this study, the thioredoxin system proteins of a psychrophilic bacterium, Colwellia psychrerythraea, were characterized using biophysical and biochemical techniques. Analysis of the complete genome sequence of the C. psychrerythraea thioredoxin system suggested the presence of a putative thioredoxin reductase and at least three thioredoxin. In this study, these identified putative thioredoxin system components were cloned, overexpressed, purified, and characterized. Our studies have indicated that the thioredoxin system proteins from E. coli were more stable than those from C. psychrerythraea. Consistent with these results, kinetic assays indicated that the thioredoxin reductase from E. coli had a higher optimal temperature than that from C. psychrerythraea.

Whither the genus Caldicellulosiruptor and the order Thermoanaerobacterales: phylogeny, taxonomy, ecology, and phenotype.

The order Thermoanaerobacterales currently consists of fermentative anaerobic bacteria, including the genus Caldicellulosiruptor. Caldicellulosiruptor are represented by thirteen species; all, but one, have closed genome sequences. Interest in these extreme thermophiles has been motivated not only by their high optimal growth temperatures (≥70°C), but also by their ability to hydrolyze polysaccharides including, for some species, both xylan and microcrystalline cellulose. Caldicellulosiruptor species have been isolated from geographically diverse thermal terrestrial environments located in New Zealand, China, Russia, Iceland and North America. Evidence of their presence in other terrestrial locations is apparent from metagenomic signatures, including volcanic ash in permafrost. Here, phylogeny and taxonomy of the genus Caldicellulosiruptor was re-examined in light of new genome sequences. Based on genome analysis of 15 strains, a new order, Caldicellulosiruptorales, is proposed containing the family Caldicellulosiruptoraceae, consisting of two genera, Caldicellulosiruptor and Anaerocellum. Furthermore, the order Thermoanaerobacterales also was re-assessed, using 91 genome-sequenced strains, and should now include the family Thermoanaerobacteraceae containing the genera Thermoanaerobacter, Thermoanaerobacterium, Caldanaerobacter, the family Caldanaerobiaceae containing the genus Caldanaerobius, and the family Calorimonaceae containing the genus Calorimonas. A main outcome of ANI/AAI analysis indicates the need to reclassify several previously designated species in the Thermoanaerobacterales and Caldicellulosiruptorales by condensing them into strains of single species. Comparative genomics of carbohydrate-active enzyme inventories suggested differentiating phenotypic features, even among strains of the same species, reflecting available nutrients and ecological roles in their native biotopes.

Cross-linked phytase aggregates for improved phytate degradation at low pH in animal feed

Phytases are widely used food and feed enzymes to improve phosphate availability and reduce anti-nutritional factors. Despite the benefits, enzyme usage is restricted by the harsh conditions in a gastrointestinal tract (pH 2–6) and feed pelleting conditions at high temperatures (60–90 °C). The commercially available phytase Quantum® Blue has been immobilized as CLEAs using glutardialdehyde and soy protein resulting in a residual activity of 33%. The influence of the precipitating agent, precipitant concentration, cross-linker concentration and cross-linking time, sodium borohydride as well as the proteic feeders gluten, soy protein and bovine serum albumin (BSA) has been optimized. The best conditions were 90% (v/v) ethyl lactate as precipitating reagent, 100 mM glutardialdehyde and a soy protein concentration of 227 mg/L with a cross-linking time of 1 h. The intrinsically stable phytase remained its high thermal stability and temperature optimum. The phytase-CLEA achieved a 425% increase of residual activity under harsh acidic conditions between pH 2.2 and 3.5 compared to the free enzyme. The free and immobilized phytase were deployed in an in vitro assay simulating the acidic conditions in the gizzard of poultry at pH 2. The hydrolysis of phytate was monitored using a novel high-performance thin-layer chromatography (HPTLC) analysis and DAD scanner to study the InsPx fingerprint. All lower inositol phosphate pools (InsP1–InsP6) and free phosphate were separated and analyzed. The phytase-CLEA efficiently released 80% of the total phosphate within 180 min, whereas the free enzyme only released 6% in the same time under the same conditions.

Ultrasensitive gas sensor developed from SnS/TiO2-based memristor for dilute methanol detection at room temperature

Metal oxide-based gas sensors are widely studied and applied due to simple operation principle, small size and high stability. However, numerous gas sensors based on metals oxides usually exhibit high optimal operating temperatures and show poor response/recovery performance at room temperature (RT). To overcome the above problems, innovative device forms should be explored. Herein, we developed a memristor-based gas sensor (gasistor) for the detection of dilute methanol vapor at RT, and the resistive layer material was SnS-modified TiO2. Compared with the general film-based gas sensors, the gasistor exhibits unique resistive switching function, and the baseline resistance in the high resistance state is reduced by about 3 × 104 times, demonstrating the construction of gasistor is an effective way to solve the high resistance of metal oxides at RT. Meanwhile, the SnS/TiO2-based gasistor showed a high response of 85.2 for 1 ppm methanol, a fast response/recovery within 1.2 s for rarefied methanol (< 5 ppm), and remarkable selectivity towards methanol at RT. This indicates that the SnS/TiO2-based gasistor is a competitive candidate for methanol detection. Besides, the mechanisms of resistive switching and gas sensing were further discussed. The present work brings new inspiration to the future research of gas sensors.

A Thermostable Lipase Isolated from Brevibacillus thermoruber Strain 7 Degrades Ɛ-Polycaprolactone.

The tremendous problem with plastic waste accumulation has determined an interest in biodegradation by effective degraders and their enzymes, such as thermophilic enzymes, which are characterized by high catalytic rates, thermostability, and optimum temperatures close to the melting points of some plastics. In the present work, we report on the ability of a thermophilic lipase, by Brevibacillus thermoruber strain 7, to degrade Ɛ-polycaprolactone (PCL), as well as the enzyme purification, the characterization of its physicochemical properties, the product degradation, and its disruptive effect on the PCL surface. The pure enzyme showed the highest reported optimum temperature at 55 °C and a pH of 7.5, while its half-life at 60 °C was more than five hours. Its substrate specificity referred the enzyme to the subgroup of lipases in the esterase group. A strong inhibitory effect was observed by detergents, inhibitors, and Fe3+ while Ca2+ enhanced its activity. The monomer Ɛ-caprolactone was a main product of the enzyme degradation. Similar elution profiles of the products received after treatment with ultra-concentrate and pure enzyme were observed. The significant changes in PCL appearance comprising the formation of shallower or deeper in-folds were observed after a week of incubation. The valuable enzyme properties of the lipase from Brevibacillus thermoruber strain 7, which caused a comparatively quick degradation of PCL, suggests further possible exploration of the enzyme for effective and environment-friendly degradation of PCL wastes in the area of thermal basins, or in thermophilic remediation processes.

Multiple approaches of loop region modification for thermostability improvement of 4,6-α-glucanotransferase from Limosilactobacillus fermentum NCC 3057

4,6-α-glucanotransferase (4,6-α-GT), as a member of the glycoside hydrolase 70 (GH70) family, converts starch/maltooligosaccharides into α,1–6 bond-containing α-glucan and possesses potential applications in food, medical and related industries but does not satisfy the high-temperature requirement due to its poor thermostability. In this study, a 4,6-α-GT (ΔGtfB) from Limosilactobacillus fermentum NCC 3057 was used as a model enzyme to improve its thermostability. The loops of ΔGtfB as the target region were optimized using directed evolution, sequence alignment, and computer-aided design. A total of 11 positive mutants were obtained and iteratively combined to obtain a combined mutant CM9, with high resistance to temperature (50 °C). The activity of mutant CM9 was 2.08-fold the activity of the wild type, accompanied by a 5 °C higher optimal temperature, a 5.76 °C higher melting point (Tm, 59.46 °C), and an 11.95-fold longer half-life time (t1/2). The results showed that most of the polar residues in the loop region of ΔGtfB are mutated into rigid proline residues. Molecular dynamics simulation demonstrated that the root mean square fluctuation of CM9 significantly decreased by “Breathing” movement reduction of the loop region. This study provides a new strategy for improving the thermostability of 4,6-α-GT through rational loop region modification.

Rapid evolution of unimodal but not of linear thermal performance curves in Daphnia magna.

Species may cope with warming through both rapid evolutionary and plastic responses. While thermal performance curves (TPCs), reflecting thermal plasticity, are considered powerful tools to understand the impact of warming on ectotherms, their rapid evolution has been rarely studied for multiple traits. We capitalized on a 2-year experimental evolution trial in outdoor mesocosms that were kept at ambient temperatures or heated 4°C above ambient, by testing in a follow-up common-garden experiment, for rapid evolution of the TPCs for multiple key traits of the water flea Daphnia magna. The heat-selected Daphnia showed evolutionary shifts of the unimodal TPCs for survival, fecundity at first clutch and intrinsic population growth rate toward higher optimum temperatures, and a less pronounced downward curvature indicating a better ability to keep fitness high across a range of high temperatures. We detected no evolution of the linear TPCs for somatic growth, mass and development rate, and for the traits related to energy gain (ingestion rate) and costs (metabolic rate). As a result, also the relative thermal slope of energy gain versus energy costs did not vary. These results suggest the overall (rather than per capita) top-down impact of D. magna may increase under rapid thermal evolution.

Machine learning-based enzyme engineering of PETase for improved efficiency in plastic degradation

Globally, nearly one million plastic bottles are produced every minute. These non-biodegradable plastic products are composed of polyethylene terephthalate (PET). In 2016, researchers discovered PETase, an enzyme from the bacteria Ideonella sakaiensis that breaks down PET and nonbiodegradable plastic. Temperatures above 60 – 65 °C are optimal for PET degradation as the polymer chain fluctuates in this range, allowing water molecules to enter and weaken the chains. However, PETase has low efficiency at these temperatures, thus limiting its usage. Here, we optimized the rate of PET degradation by PETase by designing new mutant enzymes that could break down PET much faster than PETase. We used machine learning-guided directed evolution to modify PETase to have a higher optimal temperature (Topt), which would allow the enzyme to degrade PET more efficiently. First, we trained three machine learning models to predict Topt with high performance, including Logistic Regression, Linear Regression, and Random Forest. We then used Random Forest to perform machine learning-guided directed evolution. Our algorithm generated hundreds of mutants of PETase and screened them using Random Forest to select mutants with the highest Topt. After 1000 iterations, we produced a new mutant of PETase with Topt of 71.38 °C. We also produced a new mutant enzyme after 29 iterations with Topt of 61.3 °C. To ensure these mutant enzymes would remain stable, we predicted their melting temperatures using an external predictor and found the 29-iteration mutant had improved thermostability over PETase. Using this approach and novel algorithm, scientists can optimize additional enzymes for improved efficiency.

Effect of Biotic and abiotic factors on mites infestation under grain storage- A review

Mites are common pests of stored grain containing high fat and protein content. They are considered secondary invaders among the diverse range of storage pests because they are unable to infest sound grain and instead feed on fractured kernels, debris, and high moisture grain that has been already damaged by some insect pests. The high humidity and optimum temperature are determining factors for the sensitivity of the food grains for mites attack. Mites directly risk human health by contaminating food and indirectly by mycotoxin released under storage conditions. They also result in a considerable loss in grain weight and a reduction in germination ability. The losses become more severe as the mite population grows. Chemical agents have traditionally been used to prevent or manage mite infestations, as it is the simplest and most cost-effective approach to dealing with stored grain mites. This review paper discusses Distribution, pest status, host range, food grain choice, abiotic factor influence, quantitative damage, change in biochemical composition, and botanicals for mite management.

Ag nanoparticles decorated ZnSnO3 hollow cubes for enhanced formaldehyde sensing performance at low temperature

High power consumption is closely related to the high optimum operating temperature. Therefore, the exploration of sensors with excellent sensing performance at low temperature is essential in environment protection and daily life. Herein, the ZnSnO3 hollow cubes have been modified successfully by Ag nanoparticles (NPs) via a simple solution-based chemical method. A series of chemical states characterization and surface morphology analysis indicate that the Ag NPs are evenly dispersed on the surface of ZnSnO3 hollow cubes to generate the Schottky barrier-type junctions and enhance the sensing performance of sample. The gas-sensing results suggest that the sensor towards 50 ppm formaldehyde displays the excellent response value (26.7) at 100 °C, as well as fast response/recovery time (12/18 s), remarkable reproducibility and outstanding stability. Combined with ZnSnO3 hollow cubes structure and catalytic activity of Ag NPs loaded, the sensing mechanism was proposed. This work confirms that Ag NPs modification is an efficient strategy to improve the sensing propertied of ZnSnO3 based gas sensors.

Pullulanase with high temperature and low pH optima improved starch saccharification efficiency

Pullulanase, a starch debranching enzyme, is required for the preparation of high glucose/maltose syrup from starch. In order to expand its narrow reaction conditions and improve its application value, Bacillus naganoensis pullulanase (PulA) was mutated by site-directed mutagenesis and the biochemical characteristics of the mutants were studied. The mutant PulA-N3 with mutations at asparagine 467, 492 and 709 residues was obtained. It displayed the activity maximum at 60 °C and pH 4.5 and exceeded 90% activities between 45 and 60 °C and from pH 4.0 to pH 5.5, which was improved greatly compared with wild-type PulA. Its thermostability and acidic pH stability were also remarkably improved. Its catalytic rate (kcat/Vmax) was 2.76 times that of PulA. In the preparation of high glucose syrup, the DX (glucose content, %) values of glucose mediated by PulA-N3 and glucoamylase reached 96.08%, which were 0.82% higher than that of PulA. In conclusion, a new pullulanase mutant PulA-N3 was successfully developed, which has high debranching activity in a wide range of temperature and pH, thereby paving the way for highly efficient starch saccharification.

Supercritical Fluid Microcellular Foaming of High-Hardness TPU via a Pressure-Quenching Process: Restricted Foam Expansion Controlled by Matrix Modulus and Thermal Degradation.

High-hardness thermoplastic polyurethane (HD-TPU) presents a high matrix modulus, low-temperature durability, and remarkable abrasion resistance, and has been used in many advanced applications. However, the fabrication of microcellular HD-TPU foam is rarely reported in the literature. In this study, the foaming behavior of HD-TPU with a hardness of 75D was investigated via a pressure-quenching foaming process using CO2 as a blowing agent. Microcellular HD-TPU foam with a maximum expansion ratio of 3.9-fold, a cell size of 25.9 μm, and cell density of 7.8 × 108 cells/cm3 was prepared, where a high optimum foaming temperature of about 170 °C had to be applied with the aim of softening the polymer's matrix modulus. However, the foaming behavior of HD-TPU deteriorated when the foaming temperature further increased to 180 °C, characterized by the presence of coalesced cells, microcracks, and a high foam density of 1.0 g/cm3 even though the crystal domains still existed within the matrix. The cell morphology evolution of HD-TPU foam was investigated by adjusting the saturation time, and an obvious degradation occurred during the high-temperature saturation process. A cell growth mechanism of HD-TPU foams in degradation environments was proposed to explain this phenomenon based on the gas escape through the defective matrix.