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- Frenning, Göran, et al.
(author)
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An effective-medium analysis of confined compression of granular materials
- 2009
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In: Powder Technology. - : Elsevier BV. - 0032-5910 .- 1873-328X. ; 194:3, s. 228-232
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Journal article (peer-reviewed)abstract
- A simplified model for confined compression of granular materials is considered, which idealizes the collection of particles as a (central) force network. Applying an effective-medium procedure, an equation with micromechanically well-defined parameters is derived, which relates the applied pressure to the engineering strain of the powder during uniaxial compression. Despite the simplicity of the model, comparison with experimental data for mm-sized spherical granules indicates that this equation is able to satisfactorily predict the overall compression profile from single-particle data.
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| 3. |
- Frenning, Göran, et al.
(author)
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Effective Kawakita parameters for binary mixtures
- 2009
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In: Powder Technology. - : Elsevier BV. - 0032-5910 .- 1873-328X. ; 189:2, s. 270-275
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Journal article (peer-reviewed)abstract
- Despite the fact that many industrial processes use mixtures of powders with different physical and mechanical properties, most fundamental studies of powder compression have focused on single-component systems. There is thus an obvious need for an improved understanding of the compression behaviour of mixtures of particulate and granular solids. In this contribution, we show that the Kawakita equation may be particularly useful in this regard. The reason for this is that the degree of compression (i.e., the ratio between the volume reduction and the initial powder volume) for sufficiently high compression pressures P is a linear function of I I P. For ideal mixtures, for which the volume of each component may be determined independently of the others, this linearity implies that effective Kawakita parameters for the mixture may be readily expressed in terms of the parameters of its components. In order to validate the proposed approach, binary mixtures of mm-sized spherical agglomerates prepared by wet granulation followed by extrusion and spheronization were investigated experimentally. Predicted and measured Kawakita parameters were generally in good agreement, indicating that the assumption of ideal mixing behaviour is valid for this type of systems.
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| 4. |
- Johansson, Jonas, et al.
(author)
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Increased postural sway during quiet stance as a risk factor for prospective falls in community-dwelling elderly individuals
- 2017
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In: Age and Ageing. - : Oxford University Press. - 0002-0729 .- 1468-2834. ; 46:6, s. 964-970
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Journal article (peer-reviewed)abstract
- Objective: fall-related injuries constitute major health risks in older individuals, and these risks are projected to increase in parallel with increasing human longevity. Impaired postural stability is a potential risk factor related to falls, although the evidence is inconclusive, partly due to the lack of prospective studies. This study aimed to investigate how objective measures of postural sway predict incident falls.Design, setting and participants: this prospectively observational study included 1,877 community-dwelling individuals aged 70 years who participated in the Healthy Ageing Initiative between June 2012 and December 2015.Measurements: postural sway was measured during eyes-open (EO) and eyes-closed (EC) trials using the Wii Balance Board. Functional mobility, muscle strength, objective physical activity and cognitive performance were also measured. Participants reported incident falls 6 and 12 months after the examination.Results: during follow-up, 255 (14%) prospective fallers were identified. Division of centre of pressure (COP) sway lengths into quintiles revealed a nonlinear distribution of falls for EO trial data, but not EC trial data. After adjustment for multiple confounders, fall risk was increased by 75% for participants with COP sway lengths ≥400 mm during the EO trial (odds ratio [OR] 1.75, 95% confidence interval [CI] 1.09-2.79), and approximately doubled for sway lengths ≥920 mm during the EC trial (OR 1.90, 95% CI 1.12-3.22).Conclusion: objective measures of postural sway independently predict incident falls in older community-dwelling men and women. Further studies are needed to evaluate whether postural sway length is of interest for the prediction of incident falls in clinical settings.
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| 5. |
- Johansson, Jonas, 1984-, et al.
(author)
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Predicting incident falls : Relationship between postural sway and limits of stability in older adults
- 2019
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In: Human Movement Science. - : Elsevier. - 0167-9457 .- 1872-7646. ; 66, s. 117-123
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Journal article (peer-reviewed)abstract
- Background We have previously shown that objective measurements of postural sway predicts fall risk, although it is currently unknown how limits of stability (LOS) might influence these results.Research question: How integrated postural sway and LOS measurements predict the risk of incident falls in a population-based sample of older adults.Methods: The sample for this prospective observational study was drawn from the Healthy Ageing Initiative cohort and included data collected between June 2012 and December 2016 for 2396 men and women, all 70 years of age. LOS was compared to postural sway with measurements during eyes-open (EO) and eyes-closed (EC) trials, using the previously validated Wii Force Plate. Fall history was assessed during baseline examination and incident falls were collected during follow-up at 6 and 12 months. Independent predictors of incident falls and additional covariates were investigated using multiple logistic regression models.Results: During follow-up, 337 out of 2396 participants (14%) had experienced a fall. Unadjusted regression models from the EO trial revealed increased fall risk by 6% (OR 1.06, 95% CI 1.02–1.11) per each centimeter squared increase in sway area and by 16% (OR 1.16, 95% CI 1.07–1.25) per 1-unit increase in Sway-Area-to-LOS ratio. Odds ratios were generally lower when analyzing EC trials and only slightly attenuated in fully adjusted models.Significance: Integrating postural sway and LOS parameters provides valid fall risk prediction and a holistic analysis of postural stability. Future work should establish normative values and evaluate clinical utility of these measures.
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| 6. |
- Mahmoodi, Foad, et al.
(author)
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A comparison between two powder compaction parameters of plasticity : The effective medium A parameter and the Heckel 1/K parameter
- 2013
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In: International Journal of Pharmaceutics. - : Elsevier BV. - 0378-5173 .- 1873-3476. ; 453:2, s. 295-299
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Journal article (peer-reviewed)abstract
- The purpose of the research was to introduce a procedure to derive a powder compression parameter (EM A) representing particle yield stress using an effective medium equation and to compare the EM A parameter with the Heckel compression parameter (1/K). 16 pharmaceutical powders, including drugs and excipients, were compressed in a materials testing instrument and powder compression profiles were derived using the EM and Heckel equations. The compression profiles thus obtained could be sub-divided into regions among which one region was approximately linear and from this region, the compression parameters EM A and 1/K were calculated. A linear relationship between the EM A parameter and the 1/K parameter was obtained with a strong correlation. The slope of the plot was close to 1 (0.84) and the intercept of the plot was small in comparison to the range of parameter values obtained. The relationship between the theoretical EM A parameter and the 1/K parameter supports the interpretation of the empirical Heckel parameter as being a measure of yield stress. It is concluded that the combination of Heckel and EM equations represents a suitable procedure to derive a value of particle plasticity from powder compression data.
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| 10. |
- Nordström, Josefina, et al.
(author)
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Compressibility and tablet forming ability of bimodal granule mixtures : Experiments and DEM simulations
- 2018
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In: International Journal of Pharmaceutics. - : Elsevier. - 0378-5173 .- 1873-3476. ; 540:1-2, s. 120-131
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Journal article (peer-reviewed)abstract
- Compressibility and tablet forming ability (compactibility) of bimodal mixtures of differently sized granules formed from microcrystalline cellulose were studied experimentally and numerically with the discrete element method (DEM). Compression data was analysed using the Kawakita equation. A multi-body contact law that accounts for contact dependence resulting from plastic incompressibility/geometric hardening was used in the DEM simulations. The experimental Kawakita a and 1/b parameters both depended non-monotonically on composition (weight fraction of large particles). For the a parameter, this dependence was explained by variations in the porosity of the initial granule beds; for the 1/b parameter, other factors were found to be of importance as well. The numerical results generally compared favourably with the experiments, demonstrating the usefulness of the DEM at high relative densities, provided that a suitable multi-particle contact model is used. For all mixtures, the tensile strength of the formed tablets increased with increasing applied pressure. The tensile strength generally decreased with increasing fraction of large particle, and this decrease was more rapid for large differences in particle size. A possible interpretation of these findings was proposed, in terms of differences in lateral support of small particles in the vicinity of large particles.
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