External Stiffness Research Articles

On the geometrically exact formulations of finite deformable isogeometric beams

We present a set of advanced analytical formulations that facilitates the accurate analysis and efficient implementation of finite deformable thin Kirchhoff–Love beams. This paper enhances the prevailing differential geometry based large deformation beam models by producing geometrically exact formulations for initial curvatures, non-zero force tangents and external stiffness matrix contributions of spatial beams. Though it is not analytically merged in existing beam models, initial curvatures of beams have a significant influence on the integration of forces over beam cross-sections. We reveal this influence through the systematic deduction of the Jacobian in volume integrals of beam forces. Also, this paper demonstrates the applicability of follower loads on beams with necessary adjustments to the global Hessian matrix. We adopt the isogeometric analysis formalism in beam body discretisation and algorithmic implementation of the presented formulations.

Resonance phenomenon in the single stage cycloidal gearbox. Analysis of vibrations at the output shaft as a function of the external sleeves stiffness

Resonance phenomenon in the single stage cycloidal gearbox. Analysis of vibrations at the output shaft as a function of the external sleeves stiffness

Towards a Hybrid Mass Sensing System by Combining a QCM Mass Sensor With a 3-DOF Mode Localized Coupled Resonator Stiffness Sensor

This paper describes a hybrid mass sensing system comprising a QCM (quartz crystal microbalance) mass sensor operating under atmospheric pressure and a 3-DOF mode localized coupled resonator operating in vacuum. Nanoparticles as consecutive mass perturbations are added onto the QCM, the output signals with respect to the amount of mass change are then being manipulated to generate electrostatic forces. Subsequently, the electrostatic forces act on the 3-DOF mode localized coupled resonator as external stiffness perturbations. The output metrics of the hybrid system were defined as: the resonant frequency shifts, vibration amplitude changes, and the changes in resonance amplitude ratio. Measured data was analyzed for these metrics and compared. This work demonstrated that the proposed hybrid mass sensing system attained a 2.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> N(m · kg)-1 mass to stiffness transduction factor, and has the potential to be employed as a direct liquid contact biochemical transducer.

The Effect of Crank Resistance on Arm Configuration and Muscle Activation Variances in Arm Cycling Movements.

Arm cycling on an ergometer is common in sports training and rehabilitation protocols. The hand movement is constrained along a circular path, and the user is working against a resistance, maintaining a cadence. Even if the desired hand trajectory is given, there is the flexibility to choose patterns of joint coordination and muscle activation, given the kinematic redundancy of the upper limb. With changing external load, motor noise and changing joint stiffness may affect the pose of the arm even though the endpoint trajectory is unchanged. The objective of this study was to examine how the crank resistance influences the variances of joint configuration and muscle activation. Fifteen healthy participants performed arm cranking on an arm-cycle ergometer both unimanually and bimanually with a cadence of 60 rpm against three crank resistances. Joint configuration was represented in a 3-dimensional joint space defined by inter-segmental joint angles, while muscle activation in a 4-dimensional "muscle activation space" defined by EMGs of 4 arm muscles. Joint configuration variance in the course of arm cranking was not affected by crank resistance, whereas muscle activation variance was proportional to the square of muscle activation. The shape of the variance time profiles for both joint configuration and muscle activation was not affected by crank resistance. Contrary to the prevailing assumption that an increased motor noise would affect the variance of auxiliary movements, the influence of noise doesn’t appear at the joint configuration level even when the system is redundant. Our results suggest the separation of kinematic- and force-control, via mechanisms that are compensating for dynamic nonlinearities. Arm cranking may be suitable when the aim is to perform training under different load conditions, preserving stable and secure control of joint movements and muscle activations.

Stiffness of a type II external skeletal fixator and locking compression plate in a fracture gap model.

To compare the stiffness of constructs fixed with a type II external skeletal fixator (ESF) or a 3.5-mm locking compression plate (LCP) in axial compression and bending with a fracture gap model. Quasi-static four-point bending and axial compression tests. Ten LCP and 10 ESF immobilizing epoxy cylinders with a 40-mm fracture gap. Five constructs of each type were tested in nondestructive mediolateral (ML) four-point bending and then rotated and tested in nondestructive craniocaudal (CC) four-point bending. Five additional constructs of each type were tested in nondestructive axial compression. Stiffness was compared between loading modes by construct type and between construct types by loading mode. Type II ESF were stiffer than LCP in ML bending (difference, 1474 N/mm, P < .0001) and in axial compression (difference, 458 N/mm, P = .008) but not in CC bending (P = .1673). Type II ESF were stiffer in ML bending than in CC bending (difference, 999 N/m, P < .0001), while LCP were stiffer in CC bending than in ML bending (difference, 634 N/mm, P < .0001). Type II ESF generated stiffer constructs compared with LCP in ML bending and in axial compression without a difference in CC bending. External skeletal fixator and LCP bending stiffness varied by loading direction. A type II ESF should be considered in a comminuted fracture requiring increased stability in ML and axial directions.

Apathetic hyperthyroidism presenting as conversion disorder: a case report

Background: Thyroid storm is a life-threatening health condition associated with hyperthyroidism. Apathetic hyperthyroidism is an atypical presentation of the thyrotoxicosis spectrum; this atypical presentation of apathetic thyroid storm makes the diagnosis of thyroid storm more challenging. Case Presentation: A previously healthy 23-year-old female presented to the emergency department with a 3-hour history of inability to talk or move limbs. Initial clinical examination showed that patient was conscious, alert, and febrile, with no external rashes or neck stiffness but was unable to move all limbs. Increased reflexes in all four limbs with no clonus and no rigidity was found. There was no sign of stroke in computed tomography scan, and other neurological assessments were normal. Cardiac examination showed persistent sinus tachycardia reaching 160 heartbeats per minute, despite antipyretic and fluid treatment. This finding raised the suspicion of hyperthyroidism and thyrotoxicosis. Thyroid function test revealed thyroid stimulating hormone level of 0.005 μlU/ml, free thyroxine >100, and free triiodothyronine of 41. Patient was admitted and treatment of anti-thyroid medication, steroid, and beta-blocker was initiated. The condition improved and patient was discharged home. Conclusion: The diagnosis of apathetic hyperthyroidism is difficult and sometimes delayed owing to its atypical presentation. The patient could potentially quietly sink into coma, if high index of suspicion is not maintained.

Dynamics of Heaving Buoy Wave Energy Converters with a Stiffness Reactive Controller

Heaving buoy wave energy converters (WEC) are floating oscillators, commonly modeled as single-degree-of-freedom vibrating systems. As the wave frequencies change according to the sea state, these devices must be controlled to maximize energy absorption. A new short-term reactive loading control technique is proposed that maximizes power absorption. The control is realized through tuning the effective stiffness of the vibrating system; thus, adjusting its natural frequency to meet the incident waves energy frequency achieving near-to-resonance operation and maximum power absorption. This stiffness is adjusted using an external stiffness, whose value is varied by a continuously variable transmission (CVT) mechanism connected to the buoy. The system equations were derived then solved analytically to calculate the controller bandwidth. Experiments demonstrated promising results for near-resonance tuning at different input frequencies. Results show that an optimized damping value exists at which power absorption can be significantly increased. The WEC equipped with the proposed reactive controller can provide faster tuning actions than long-term techniques. It also works on longer time intervals than phase-control methods; hence, reducing the continuous demands from the PTO system.

Comparative study between close reduction internal fixation and open reduction internal fixation in lower fibula with medial mallolus fracture

Comparative study between close reduction internal fixation and open reduction internal fixation in lower fibula with medial mallolus fracture - IJOS- Print ISSN No: - 2395-1354 Online ISSN No:- 2395-1362 Article DOI No:- 10.18231/j.ijos.2020.055, Indian Journal of Orthopaedics Surgery-Indian J Orthop Surg

Biomechanical Assessment of Three Osteosynthesis Constructs by Periprosthetic Humerus Fractures.

Background Biomechanical stability assessment of 3 different constructs for proximal fixation of a locking compression plate (LCP) in treating a Worland type C periprosthetic fracture after total shoulder arthroplasty. Methods 27 Worland type C fractures after shoulder arthroplasty in synthetic humeri were treated with 14-hole LCP that is proximally fixed using the following: (1) 1 × 1.5 mm cerclage wires and 2x unicortical-locking screws, (2) 3 × 1.5 mm cerclage wires, or (3) 2x bicortical-locking attachment plates. Torsional stiffness was assessed by applying an internal rotation moment of 5 Nm and then after unloading the specimen, an external rotation moment of 5 Nm at the same rate was applied. Axial stiffness was assessed by applying a 50 N preload, and then applying a cyclic load of 250 N, then increasing the load by 50 N each time, until a maximum axial load of 2500 N was reached or specimen failure occurred. Results With regard to internal as well as external rotational stiffness, group 1 showed a mean stiffness of 0.37 Nm/deg and 0.57 Nm/deg, respectively, group 2 had a mean stiffness of 0.51 Nm/deg and 0.39 Nm/deg, respectively, while group 3 had a mean stiffness of 1.34 Nm/deg and 1.31 Nm/deg, respectively. Concerning axial stiffness, group 1 showed an average stiffness of 451.0 N/mm, group 2 had a mean stiffness of 737.5 N/mm, whereas group 3 had a mean stiffness of 715.8 N/mm. Conclusion Group 3 displayed a significantly higher torsional stiffness while a comparable axial stiffness to group 2.

Improving the performance of conventional base isolation systems by an external variable negative stiffness device under near-fault and long-period ground motions

Recent studies have shown that base-isolated objects with long fundamental natural periods are highly influenced by long-period earthquakes. These long-period waves result in large displacements for isolators, possibly leading to exceedance of the allowable displacement limits. Conventional isolation systems, in general, fail to resist such large displacements. This has prompted the need to modify conventional base isolation systems. The current work focuses on the development of an external device, comprising a unit of negative and positive springs, for improving the performance of conventional base isolation systems. This unit accelerates the change in the stiffness of the isolation system where the stiffness of the positive spring varies linearly in terms of the displacement response of the isolated objects. The target objects of the present study are small structures such as computer servers, sensitive instruments and machinery. Numerical studies show that the increase in the damping of the system and the slope of the linear function is effective in reducing the displacement response. An optimal range of damping values and slope, satisfying the stability condition and the allowable limits of both displacement and acceleration responses when the system is subjected to near-fault and long-period ground motions simultaneously, is proposed.

The mechano-sensitive response of β1 integrin promotes SRC-positive late endosome recycling and activation of Yes-associated protein

Yes-associated protein (YAP) signaling has emerged as a crucial pathway in several normal and pathological processes. Although the main upstream effectors that regulate its activity have been extensively studied, the role of the endosomal system has been far less characterized. Here, we identified the late endosomal/lysosomal adaptor MAPK and mTOR activator (LAMTOR) complex as an important regulator of YAP signaling in a preosteoblast cell line. We found that p18/LAMTOR1-mediated peripheral positioning of late endosomes allows delivery of SRC proto-oncogene, nonreceptor tyrosine kinase (SRC) to the plasma membrane and promotes activation of an SRC-dependent signaling cascade that controls YAP nuclear shuttling. Moreover, β1 integrin engagement and mechano-sensitive cues, such as external stiffness and related cell contractility, controlled LAMTOR targeting to the cell periphery and thereby late endosome recycling and had a major impact on YAP signaling. Our findings identify the late endosome recycling pathway as a key mechanism that controls YAP activity and explains YAP mechano-sensitivity.

The influence of preoperative external rotation weakness or stiffness on reverse total shoulder arthroplasty

BackgroundSome reverse total shoulder arthroplasty (rTSA) patients may have limited preoperative external rotation (ER) because of stiffness or weakness. Currently it is not known if this affects their clinical outcome or if their ER will improve after surgery.MethodsA multicenter shoulder arthroplasty database was queried to analyze patients undergoing a primary rTSA using a single prosthesis design featuring a medial glenoid–lateral humerus. Their pre- and postoperative range of motion was evaluated in addition to 5 outcome measures. Patients with limited preoperative ER due to weakness or stiffness were compared to patients with normal preoperative range of motion. The following questions were asked: (1) Does a preoperative ER deficit impact the postoperative outcome? (2) Do patients with preoperative ER deficits due to stiffness or weakness regain ER after rTSA? and (3) Does a preoperative ER lag sign predict a poor outcome?Results608 patients were included in this study. Active external rotation (preoperative/postoperative) was as follows for the 3 patient groups: Normal patients (45°/44°), Stiff (–4°/30°), and Weak (16°/32°). Weak patients had a preoperative ER lag of 30°, which improved by 16° after surgery. The clinical outcome scores for all 3 groups improved after rTSA. Stiff patients had significantly greater improvement than Weak and Normal patients. Outcome scores were equivalent for Normal and Stiff patients. Weak patients tended to have slightly lower outcome scores.ConclusionsPatients with limited preoperative ER can obtain a good clinical result with rTSA using a medial glenoid–lateral humerus prosthesis, ER range of motion can improve after rTSA, and stiff patients have a particularly good prognosis for recovery.

Vertical and Rotational Stiffness of Coracoclavicular Ligament Reconstruction: A Biomechanical Study of 3 Different Techniques

To compare the biomechanical stability of 3 different coracoclavicular reconstruction techniques under rotational and vertical loading using a cadaveric model. In total, 12 cadaveric shoulders were used for testing. The native state was first tested then followed by 3 different reconstruction configurations using suture tapes and cortical buttons: coracoid loop (CL), single-bundle (SB), and double-bundle (DB). Superior displacement was measured by cycling an inferiorly directed force of 70 N to the scapula. The rotational stiffness of the scapula was determined by cycling the scapula in rotational displacement control between 15° of internal and external rotation. The rotational stiffness of the clavicle was determined by rotating the clavicle around its long axis 20° anteriorly and 30° posteriorly in rotational displacement control. All measurements were captured over 10 cycles at a rate of 200 Hz. Both the CL and SB techniques demonstrated significantly less internal scapular rotation stiffness. (intact: 19.70 ± 9.07 cNm/deg, CL:3.70±2.63 cNm/deg, SB:4.30 ± 2.66 cNm/deg, P <.001) External scapular rotation stiffness was significantly decreased in all techniques (intact: 17.70 ± 4.43 cNm/deg, CL: 3.30 ± 1.37 cNm/deg, SB: 4.50 ± 1.56 cNm/deg, DB:4.67± 1.99 cNm/deg, P < .001). The CL and SB reconstructions were significantly less stiff with regards to posterior rotation of the clavicle (intact: 5.60 ± 1.80 cNm/deg, CL: 2.90 ± 1.10 cNm/deg, SB: 1.40 ± 0.65 cNm/deg, P < .001). Anterior rotation stiffness of the clavicle was significantly lower in all of the reconstructions (intact: 6.95 ± 1.90 cNm/deg, CL: 3.08 ± 0.84 cNm/deg, SB: 3.64 ± 0.93 cNm/deg, DB: 4.48 ± 1.21 cNm/deg, P < .001). None of the described techniques provided equivalent rotational stability in all planes compared with the native state. DB reconstruction presented stiffness characteristics closest to the native state under cyclic loading during internal scapular and posterior clavicular rotation. Additional procedures such as tendon grafting or acromioclavicular ligament reconstruction may be required to control rotational stability.

955: Novel application assessing cervical stiffness in second trimester

Cervical elastography is a novel, non-invasive assessment of tissue stiffness by transvaginal ultrasound (TVUS). which may be useful in the prediction of preterm birth. Our objective is to establish normative data of cervical elastography indices by second trimester TVUS. This is an IRB approved prospective cohort study utilizing Samsung’s E-cervix application. All patients presenting for routine cervical length (CL) by TVUS at a single academic perinatal ultrasound unit from March-July 2019 were included. Women who had a cerclage in place, or prior cervical excision procedures were excluded. The areas of the internal and external os were identified and marked with a machine defined 1 cm semi-circle and the following parameters calculated by the application (Figure 1): Internal Os Stiffness (IOS), External Os Stiffness (EOS), and Hardness Ratio (HR). Stiffness increases as the value approaches 0 and is softer as it approaches 1. Hardness Ratio (HR) is defined as % of total cervical area characterized as hard (i.e. red pixel). Measurements were obtained by RDMS certified sonographers. The average of three measurements were used for analysis and the intra-class correlation was assessed. Pearson correlation coefficients were used to determine the association between each of the E-cervix parameters and cervical length. 619 patients met criteria for analysis with a median maternal age of 32 (18-49) years. 54% were white. Median gestational age at assessment was 20 (15-29) weeks. 96% were singleton pregnancies. Prior preterm birth rate was 9.7%. All measurements except EOS had good to excellent intra-observer reliability. Normative data is provided in Table 1 with median Hardness Ratio 66.83, Internal Os Stiffness 0.24, External Os Stiffness 0.31, IOS/EOS 0.81, and CL 37.3 mm. Cervical elastography was found to be an easy and reliable tool for the evaluation of novel cervical parameters in the second trimester. Furthermore, these findings suggest cervical stiffness (function) does not correlate with CL (structure), and maybe an important assessment for preterm birth.View Large Image Figure ViewerDownload Hi-res image Download (PPT)

Hip external rotation stiffness and midfoot passive mechanical resistance are associated with lower limb movement in the frontal and transverse planes during gait

BackgroundHip external rotation stiffness, midfoot passive mechanical resistance and foot alignment may influence on ankle, knee and hip movement in the frontal and transverse planes during gait. Research questionAre hip stiffness, midfoot mechanical resistance and foot alignment associated with ankle, knee and hip kinematics during gait? MethodsHip stiffness, midfoot mechanical resistance, and foot alignment of thirty healthy participants (18 females and 12 males) with average age of 25.4 years were measured. In addition, lower limb kinematic data during the stance phase of gait were collected with the Qualisys System (Oqus 7+). Stepwise multiple linear regressions were performed to identify if hip stiffness, midfoot torque, midfoot stiffness and foot alignment were associated with hip and knee movement in the transverse plane and ankle movement in the frontal plane with α = 0.05. ResultsReduced midfoot torque was associated with higher hip range of motion (ROM) in the transverse plane (r2 = 0.18), reduced hip stiffness was associated with higher peak hip internal rotation (r2 = 0.16) and higher ROM in the frontal plane (r2 = 0.14), reduced midfoot stiffness was associated with higher peak knee internal rotation (r2 = 0.14) and increased midfoot torque and midfoot stiffness were associated with higher peak knee external rotation (r2 = 0.36). SignificanceThese findings demonstrated that individuals with reduced hip and midfoot stiffness have higher hip and knee internal rotation and higher ankle eversion during the stance phase of gait. On the other hand, individuals with increased midfoot torque and stiffness have higher knee external rotation. These relationships can be explained by the coupling between ankle movements in the frontal plane and knee and hip movements in the transverse plane. Finally, this study suggests that midfoot passive mechanical resistance and hip stiffness should be assessed in individuals presenting altered ankle, knee and hip movement during gait.

Aortic, common carotid and external iliac artery arterial wall stiffness parameters in horses: Inter-day and inter-observer and intra-observer measurement variability.

In human medicine, local and regional arterial wall stiffness (AWS) parameters are routinely used to assess the vascular health. In horses, information regarding reproducibility of ultrasonographically derived AWS parameters is lacking. To evaluate the inter-day and inter-observer and intra-observer measurement variability of both local and regional AWS parameters in horses. Experimental study. In 10 healthy, adult Warmblood horses, B-, M-mode and pulsed-wave Doppler ultrasound images were collected on two different days from aorta, cranial and caudal common carotid arteries and external iliac artery. Heart rate and noninvasive blood pressure were recorded simultaneously. From blinded data, diastolic and systolic vessel lumen areas and diameters were measured from B/M-mode images and the velocity of the pressure wave was determined by pulsed-wave Doppler spectra. From each horse, one examination was measured again by the same observer and by a second, independent observer. Local and regional AWS parameters were calculated and inter-day and inter-observer and intra-observer measurement coefficient of variation (CV) were assessed. Low CV was found for both arterial diameter and lumen area measurements. Moderate to high CV was found for local AWS parameters, while regional AWS parameters had low CV. The number of horses investigated was too low to obtain reference values. The inter-operator variability was not evaluated. Our results show good reproducibility of aortic, carotid and external iliac artery diameter and area measurements using both B- and M-mode ultrasonography. Nevertheless, the variability of the derived local AWS parameters was relatively high. Therefore, local AWS parameters might be less suitable for follow-up studies, although they might be useful for population studies. On the other hand, regional AWS parameters showed low CV, making them valuable for both follow-up and population studies.

Clinical Case: Casual Finding of Part of an Intubation Guide after its Use for Exchange and Placement of Double Light Tube

A difficult airway is defined as a clinical situation in which a trained anesthesiologist has difficulties to ventilate a patient with a face mask, difficulties for endotracheal intubation, or both. Intubation with a double lumen tube (DLT) may be more difficult than an equivalent intubation with a single-lumen tracheal tube because of its greater external diameter, stiffness and concavity, so its use is not recommended as a technique of initiation in patients with VAD.

Dual Nonlocked Plating as an Alternative to Locked Plating for Comminuted Distal Fibula Fractures: A Biomechanical Comparison Study

The purpose of this cadaveric study was to compare the biomechanical properties of dual nonlocked plating and single-locked plating using matched pairs of isolated fibula specimens. Fractures were simulated in 10 matched pairs of isolated cadaveric fibulae and plated with a single lateral locking plate for right-sided specimens, or with a one-third tubular plate and a 7-hole 2.4-mm minifragment adaption plate for left-sided specimens. An external rotation torque was applied at a rate of 1°/second, and torque at 10° was measured. Each fibula specimen was evaluated using a micro computed tomography scanner, and bone mineral density was calculated as milligrams of bone per cubic centimeter of volume. Dual nonlocked plating and locked plating specimens demonstrated torque measurements that were not significantly different at 10° of external rotation (1.48 N·m and 1.92 N·m, respectively; p = .093). The stiffness of the dual nonlocked plated and locked plating constructs were not significantly different (p = .228 and p = .543, respectively). The effect of bone mineral density on maximum torque at failure was not a reliable predictor of maximum torque in either the dual nonlocked plating or locked plating specimens (R2 = 0.548 and R2 = 0.096, respectively). We found no differences in torque at 10° of external rotation or stiffness between locking plate and dual nonlocking plate fixation constructs. This study provides evidence that dual nonlocked plating likely constitutes adequate fixation in situations in which a locking plate is being considered for comminuted distal fibula fractures.

Experimental test and hysteretic behavior of single cross-arm PCS-BRBs

An innovative buckling-restrained brace (BRB) is devised by imposing a cable stayed system (CSS) on a common BRB, namely single cross-arm pre-tensioned cable-stayed buckling-restrained brace (SPCS-BRB). Its overall external stiffness thus load-bearing capacity can be dramatically enhanced over the common BRB. A previous study conducted by the authors investigated comprehensively the elastic buckling behavior of the brace, yet experimental investigations and detailed design suggestions were not provided. Two SPCS-BRB specimens have been devised and tested. Test results revealed that both specimens possessed excellent hysteretic response with stable hysteretic curves, which fulfilled the ductility requirement specified in the AISC seismic provisions. The adopted simplified Finite Element (FE) model for experimental simulations corresponds well with the obtained test results. Moreover, additional FE analysis (FEA) has been conducted on the SPCS-BRBs subjected to monotonic and cyclic axial loads, respectively as to validate the test results further. In addition, threshold values of restraining ratios are obtained by guaranteeing the cables to remain tensioned prior to attaining ultimate failures, and ensuring maximum load resistance of the SPCS-BRBs is obtained. It is observed from the FEA that global instability failures of the SPCS-BRBs are caused by expansion of yielding at the extreme fibers of the external tube. Finally, FEA has been conducted on the load resistance of the SPCS-BRBs by considering the effect of interactive buckling and prestressing force of cables. It is found that interactive buckling could become critical in dominating the load resistance of the SPCS-BRBs when single-wave symmetric buckling mode is considered.

Matrix stiffness modulates the activity of MMP-9 and TIMP-1 in hepatic stellate cells to perpetuate fibrosis

Liver fibrosis is characterised by a dense and highly cross-linked extracellular matrix (ECM) which promotes progression of diseases such as hepatocellular carcinoma. The fibrotic microenvironment is characterised by an increased stiffness, with rigidity associated with disease progression. External stiffness is known to promote hepatic stellate cell (HSC) activation through mechanotransduction, leading to increased secretion of ECM components. HSCs are key effector cells which maintain the composition of the ECM in health and disease, not only by regulating secretion of ECM proteins such as collagen, but also ECM-degrading enzymes called matrix metalloproteinases (MMPs) and their inhibitors (TIMPs). Uninhibited MMPs degrade ECM proteins to reduce external rigidity. Using fibronectin-coated polyacrylamide gels to alter substrate rigidity without altering ligand density, we show that fibrotic rigidities downregulate MMP-9 expression and secretion, and also upregulate secretion of TIMP-1, though not its expression. Using tissue immunofluorescence studies, we also report that the expression of MMP-9 is significantly decreased in activated HSCs in fibrotic tissues associated with hepatocellular carcinoma. This suggests the presence of a mechanical network that allows HSCs to maintain a fibrotic ECM, with external rigidity providing feedback which affects MMP-9 and TIMP-1 secretion, which may become dysregulated in fibrosis.